Euglenophyta
Euglenophyta and Dinophyta
• In euglenophytes, a single class, Euglenophyceae, is recognized.
• It includes about 800 species, most of which are freshwater species,
but some are estuarine and intertidal.
• Predominantly unicellular, flagellate.
• Only a third of the species have chloroplasts, and the rest are
heterotrophic (both facultative and obligate).
• The ability of Euglena to grow heterotrophically in the absence of
plastids is unique amongst the algae.
• Autotrophic euglenophytes contain chlorophylls a and b and, thus,
have been related to Chlorophyta, but this is the only shared
character.
• Carotenoid include β-carotene and diadinoxanthin, other xanthophylls
less prominent.
• The energy storage product is paramylon, a polysaccharide found
outside the chloroplast in the cytoplasm. Chrysolaminarin can also
accumulate in a liquid form in vacuoles in the cytoplasm.
Algae with one membrane of
chloroplast endoplasmic
reticulum
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Eyespot, paraflagellar swelling, and phototaxis
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The eyespot (stigma) (E) is a collection
of orange-red lipid droplets, independent
of the chloroplast.
The eyespot is in the anterior part of the
cell, curving to ensheath the neck of the
reservoir on the dorsal side.
The eyespot has been reported to contain
α-carotene and seven xanthophylls,
mainly β-carotene, or a β-carotene
derivative, echineone.
The base of the longer flagellum bears a
thickening (P), believed to be a
photoreceptor, close to the stigma.
The swelling is composed of a crystalline
body next to the axoneme and inside the
flagellar membrane.
All euglenoid species with an eyespot and
flagellar swelling exhibit phototaxis,
usually swimming away from bright light
(negative phototaxis) and away from
darkness toward subdued light (positive
photoaxis) to accumulate in a region of
low light intensity.
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A semidiagrammatic drawing of the fine structure of the
anterior part of a Euglena cell. (E) eyespot; (LF) long
flagellum; (P) paraflagellar swelling; (R) reservoir; (SF)
short flagellum.
•The strips are arranged in parallel, are characteristic of the species.
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The cell shows an anterior bottle-shaped
invagination called the reservoir (R) . At the base
of the reservoir, there is a contractile vesicle
(CV) as well as two flagella, but only one of them
emerges through the neck.
Euglenoid cells have two basal bodies and one
or two emergent flagella. The flagella having a
paraxonemal rod (paraxial rod) that runs the
length of the flagellum inside the flagellar
membrane.
The one emergent flagellum in Euglena has
helically arranged fibrillar hairs (no microtubules)
attached along the length of the flagellar
membrane.
Reproduction is asexual, sexual reproduction
has not yet been observed.
In order to survive when external conditions are
unfavorable, some species produce resistant
cysts surrounded by a thick mucilaginous sheath
that they produce.
A semidiagrammatic drawing of the fine structure of the
anterior part of a Euglena cell. (C) Canal; (CER)
chloroplast endoplasmic reticulum; (CV) contractile
vacuole; (E) eyespot; (LF) long flagellum; (M)
mastigonemes; (MB) muciferous body; (Mt)
microtubules; (N) nucleus; (P) paraflagellar swelling;
(Pa) paramylon; (PG) pellicle groove; (Pl)
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plasmalemma; (PS) pellicle strip; (Py) pyrenoid; (R)
(A) Axoneme of flagellum; (B) paraxonemal rod of flagellum reservoir; (SF) short flagellum.
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Euglenoid cells are not
surrounded by cell wall, but a
pellicle that containing
structural protein.
The pellicle has four main
components:
– the plasma membrane,
– repeating proteinaceous units
called strips,
– subtending microtubules, and
– tubular cisternae of
endoplasmic reticulum.
Scanning electron micrographs showing metaboly in different euglenids: (B) Distigma, (C) Eutreptia and (D) Euglena
Scanning electron micrographs of euglenoids showing the helical pellicle of Phacus triqueter (lower left),
Euglena acus (middle), and Lepocincilis ovata (upper right).
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Euglena showing pellicle
• The locomotory flagellum or flagella emerge from an anterior
invagination of the cell, which consists of a narrow tubular rigid
portion, the canal, and a spherical or pyriform easily changes shape
chamber, the reservoir.
• The pellicle lines the canal but not the reservoir, the reservoir being
the only part of the cell covered solely by the plasmalemma.
• The contractile vacuole, in the anterior part of the cell next to the
reservoir, has an osmoregulatory function, expelling excessive water
taken into the cell.
• It empties into the reservoir, from which the water is carried out
through the canal.
• There are two basic types of flagellar movement in the class.
• The first group (including the Eutreptiales and Euglenales) has the
flagellum continually motile from base to apex, resulting in cell
gyration with the anterior end of the cell tracing a wide circle.
• The second group (including Peranema, Entosiphon, and
Sphenomonas) has the flagellum held out straight in front of the cell
with just the tip motile, resulting in smooth swimming or gliding
locomotion in contact with the substratum or air–water interface.
• Mitochondria are of typical algal type. Colorless euglenoids always
have more mitochondria than do equivalent-sized green ones.
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• Euglena gracilis changes its shape two times per day when grown
under the synchronizing effect of a daily light–dark cycle.
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Muciferous bodies (MB) (or mucocysts) occur under the pellicle strips and
contain a water-soluble mucopolysaccharide. The muciferous bodies open to
the outside through pores between the strips of the pellicle. The muciferous
bodies function in the formation of the stalk in Colacium, lorica formation in
Trachelomonas, cyst formation and lubrication during euglenoid movement.
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Some euglenoids have a flexible pellicle that allows the cells to undergo a
flowing movement known as euglenoid movement. This type of movement
occurs only when the cells are not swimming and results from the movement
of the pellicle strips.
– At the beginning of the light period, when photosynthetic capacity is low
(as measured by the ability of the cells to evolve oxygen), the population
of cells is largely spherical.
– The mean cell length of the population increases to a maximum in the
middle of the light period when photosynthetic capacity is greatest, and
then decreases for the remainder of the 24-hour period.
– The population becomes spherical by the end of the 24-hour period
when the cycle reinitiates.
• These changes are also observed under dim light conditions and are
therefore controlled by a biological clock and represent a circadian
rhythm in cell shape.
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Surviving unfavorable periods.
• The cell rounds off and secretes a thick sheath of mucilage that
survives for months until the cell emerges by cracking the cyst.
• In conditions of partial desiccation or excessive light, the slime sheath
sometimes acts as a temporary cyst, cells emerging from the sheath
as soon as conditions improve.
• In certain genera (Euglena and Eutreptia), cell division within the slime
layer leads to the formation of a palmelloid colony, which may form
extensive sheets of cells covering many square feet of mud surface.
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Nucleus and nuclear division
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The euglenoid nucleus is of the mesokaryotic
type, having
– chromosomes that are permanently condensed
during the mitotic cycle,
– a nucleolus (endosome) that does not disperse
during nuclear division,
– no microtubules from chromosomes to pole
spindles, and
– a nuclear envelope that is intact during nuclear
division.
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Scanning electron micrographs of the encystment of Eutreptiella gymnastica. The vegetative cell (a)
loses its flagella, forms a large number of paramylon grains, and begins to round up (b). The cell swells
and produces a mucilaginous covering (c).
Euglena gracilis. (a)–(d). Sequence of shape changes photographed at 5-second intervals
of a cell undergoing euglenoid movements.
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The chromosome number is usually high, and
polyploidy probably occurs in some genera.
Mitosis in euglenoids begins during early
prophase with the nucleus migrating from the
center of the cell to an anterior position.
Microtubules appear in the nucleus, but they do
not attach to the chromosomes.
At metaphase, bundles of microtubules are among
the chromosomes, and the nucleolus has started
to elongate along the division axis.
In anaphase, the intact nuclear envelope
elongates along the division axis, the nucleolus
divides, and the daughter chromosomes disperse
into the two daughter nuclei.
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A semidiagrammatic drawing of an
anaphase nucleus of Euglena with
nuclear envelope intact, the nucleolus
pinching in two, and the chromosomes
not attached to spindle microtubules.
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Paramylon granules which
are often visible as colorless
or white particles in light
microscopy are composed
of a carbohydrate that
although isomeric with
starch (amylon), was not
stained with iodine. For this
reason, they were termed
paramylon granules.
They have since been
shown to be composed of a
β-1,3 linked glucan.
The liquid storage product,
chrysolaminarin, can be an
alternative storage product
in some Euglenophyceae.
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Chloroplasts and storage products
(a) Euglena gracilis. (b) Astasia klebsii. (c) Eutreptiella
marina. (d) Trachelomonas grandis. (e) Phacus triqueter.
(C) Chloroplast; (Ca) canal; (CV) contractile vacuole; (E)
eyespot; (Ev) envelope; (F) emergent flagellum; (FS)
flagellar swelling; (M) mitochondrion; (N) nucleus; (P)
paramylon grains or paramylon sheath around
chloroplast; (R) reservoir.
Nutrition
• The Euglenophyceae have a number of modes of nutrition,
depending on the species involved.
• No euglenoid has yet been demonstrated to be fully
photoautotrophic – capable of living on a medium devoid of all
organic compounds (including vitamins).
• All green euglenoid flagellates so far studied are photoauxotrophic
–needing at least one vitamin. Euglena gracilis has an absolute
requirement for vitamin B12.
• Vitamin B12-starved cells increase in cell volume, sometimes to
10 times the size of control organisms, the cells in the final stage of
vitamin B12 starvation often being poly-lobed, poly-nucleate, and
containing more than the normal number of chloroplasts per cell.
• During vitamin B12 starvation, total cellular RNA and protein
increase 400% to 500% compared with controls.
• During vitamin B12 starvation the protein increase 400% to 500%
and the total DNA increases about 180%.
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• The euglenoid chloroplasts are
surrounded by two membranes of the
chloroplast envelope plus one membrane
of chloroplast endoplasmic reticulum; the
latter membrane is not continuous with the
nuclear membrane.
• The chloroplasts are usually discoid or
plate-like with a central pyrenoid.
• The thylakoids are grouped into bands of
three, with two thylakoid bands traversing
the pyrenoid.
• A shield of paramylon grains surrounds
the pyrenoid, but outside the chloroplast,
in phototropically grown cells.
• Paramylon granules are distributed
throughout the cytoplasm in
heterotrophically grown cells in the dark.
• Their shape is often characteristic of the
Euglena species that produces them.
Euglena rustica. Transmission electron micrograph
of a mid sagittal section of a cell. (Arrow) Paramylon
granule; (cp) chloroplast; (Py) pyrenoid; (Nu) nucleus
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importance of Paramylon
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Paramylon belongs to a group of naturally occurring polysaccharides which
are considered bioactive compounds.
The main interest in β-glucans stems from their ability to act as nonspecific
immune system stimulants, by binding to a specific site on
monocytes/macrophages and granulocytes.
They have been successfully used in aquaculture to strengthen the
nonspecific defense of many important species of fishes and shrimps by
injection, immersion, or in the feed.
Sulfated derivatives of Euglena paramylon, in particular, have shown antiHIV (human immunodeficiency virus) activity.
Moreover, it has been suggested that this polysaccharide has a cholesterollowering effect when incorporated in the diet of either humans or animals,
and moderates the postprandial blood glucose and insulin response in
humans.
Since Euglena gracilis can accumulate large quantities of paramylon when
grown in the presence of an utilizable carbon source, it could represent an
alternative source of this compound to Saccharomyces cerevisiae (baker’s
yeast), which is currently exploited industrially for its extraction.
Moreover, Euglena has been investigated as a potential protein source, and
a promising dietary supplement due to its content of highly nutritious
proteins and PUFAs, and to the simultaneous production of antioxidant
compounds, such as β-carotene, vitamin C, and vitamin E.
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Classification
• Three orders of euglenoids are presented here.
• Order 1 Heteronematales: two emergent flagella, the longer
flagellum directed anteriorly and the shorter one directed posteriorly
during swimming; cells have special ingestion organelle.
– Peranema trichophorum is a euglenoid that ingests other cells and
detritus. Here the colorless cells have a special ingestion organelle, and
are phagocytic, taking up food particles whole and digesting them in
food vesicles.
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Peranema trichophorum. (a) General cell structure. (b),(c) Two stages in the ingestion of a cell of
Euglena (stippled cell). (c) Canal; (cv) contractile vacuoles; (cy) rim of cytosome; (fv) food vesicle;
(g) Golgi; (ir) ingestion rods; (lf) leading flagellum; (m) mitochondrion; (n) nucleus; (p) paramylon;
(tf) trailing flagellum
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As Euglena cells age, they become immobile and spherical, with a tendency
to form enlarged “giant” cells and to accumulate orange to black pigment
bodies.
Aging also results in the formation of larger numbers of lysosomes and
microbodies with an increase in the degradation of organelles.
The older cells undergo a shift from carbohydrate to fat oxidation.
The euglenoids belong to the osmotrophic acetate flagellates, having the
ability to grow heterotrophically in the dark by utilizing substrates such as
acetic (CH3COOH) and butryic acids (CH3CH2CH2-COOH) and the
corresponding alcohols (e.g., ethanol). The two most commonly used
substrates are acetate and ethanol.
• Order 3 Euglenales: two flagella, only one of which emerges from
the canal; no special ingestion organelle.
• Common genera in the order are the green photosynthetic Euglena,
Trachelomonas, and Phacus, as well as the colorless osmotrophic
Astasia.
• Colacium libellee is a member of this order that establishes itself in
the rectum of damselfly nymphs during the winter in colder lakes.
During the warm summer months the damselfly nymphs and C.
libellee live separately.
• Order 2 Eutreptiales: two emergent flagella, one directed anteriorly
and the other laterally or posteriorly during swimming; no special
ingestion organelle.
– Eutreptia and Eutreptiella are estuarine or marine genera, while
Distigma is characteristic of acid freshwaters.
Eutreptia
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(a) Larva of the damselfly Ischnura verticalis with a plug of
Colacium libellee in the rectum. (b), (c) Colony and single
swimming cell of Colacium vesiculosum.
Distigma
Eutreptiella
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• Dinoflagellates are unicellular, biflagellate organisms.
• The term "dinoflagellate" means "whirling flagella".
• These organisms are important members of the plankton in both
fresh and marine waters, although a much greater variety of forms is
found in marine members.
• Dinoflagellates are less important in the colder polar waters than in
warmer waters.
Dinophyta
(Pyrrophyta)
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• The exact number and arrangement of thecal plates in armored
dinoflagellates are characteristic of the particular genus, therefore
they are used for taxonomic differentiation.
• Also, the form of the thecal plates varies from spherical forms
like Peridinium to elongate horn-like forms such as Ceratium.
• Dinoflagellates are surrounded by a complex covering called
the amphiesma, which consists of the plasma membrane and an
underlying single layer of flattened vesicles (amphiesmal vesicles =
"thecal vesicles").
• Based on their amphiesma, dinoflagellates are separated into two
major subgroups
– In thecate, or ‘armoured’, forms, the amphiesmal vesicles contain cellulosic
plates (= thecal plates), resulting in a rigid and inflexible wall.
– In athecate, or ‘unarmoured’ (naked) dinoflagellates, the cells have empty
amphiesmal vesicles (without plate-like structures) and may be fragile, easily
distorted and difficult to preserve.
(a ventral view of Cachonina
niei showing the arrangement
of thecal plates:
(b) Gymnodinium neglectum
(b) Dorsal and ventral views showing
the thecal plates of Ceratium
• In many dinoflagellates, cell division usually involves sharing of the
mother cell thecal plates between the daughter cells, with the
daughter cells producing the new thecal plates that they lack.
• However, in certain genera the theca is completely shed (ecdysis)
at cell division, followed by the formation of a thickened pellicle
around the cell to form an ecdysal cyst.
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Representative drawings of different types of
amphiesmal arrangements in the Dinophyceae. The
outer plasma membrane (Pl). The cellulosic thecal plate
(T). vesicle (V).
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AV, amphiesmal (thecal) vesicle; TP, thecal
plate within an amphiesmal vesicle; .
• A typical motile dinoflagellate consists of an epicone (epitheca) and
hypocone (hypotheca) divided by the transverse girdle or
cingulum.
• There is a longitudinal sulcus running perpendicular to the girdle.
• The longitudinal and transverse flagella emerge through the thecal
plates in the area where the girdle and sulcus meet.
• In most dinophyceae, outside of the plasma membrane there is a
glycocalyx composed of acidic polysaccharides.
Scales
• Although relatively rare, scales occur outside the plasma membrane
in some Dinophyceae
Flagella
• Generally, dinoflagellates have a transverse flagellum that fits into
the transverse girdle and a longitudinal flagellum that projects out
from the longitudinal sulcus.
Dinoflagellate scales. (a) Oxyrrhis marina. (b) Heterocapsa niei. (c) Katodinium rotundatum.
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A representation of thecal plates on a
typical armored dinoflagellate .
Scrippsiella trochoidea.Scanning electron micrographs
showing the thecal plates in ventral (a) and dorsal (b)
view.
The transverse flagellum is thought to be responsible for
driving the cell forward and it also brings about rotation,
while the longitudinal flagellum is the propelling and
steering flagellum.
The axoneme of transverse flagellum is a left-handed
screw; waves propagated from the attached end toward
the free end.
The flagellum causes a direct forward movement while at
the same time causing the cell to rotate.
The flagellar beat always proceeds counterclockwise
when seen from the cell apex.
The longitudinal flagellum swings in a narrow orbit acting
as a rudder.
The longitudinal flagellum can also reverse the swimming
direction: it stops beating, points in a different direction
by bending, and then resumes beating.
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– (1) an axoneme whose form
approximates a helix,
– (2) a striated strand that runs parallel to
the longitudinal axis of the axoneme but
outside the loops of the coil, and
– (3) a flagellar sheath that encloses both
axoneme and striated strand.
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Fibrillar hairs may cover the entire
longitudinal flagellum.
The transverse flagellum has a helical
shape and attached to the cell body
except for the tip .
The transverse flagellum consists of
On one side of the axoneme, a unilateral
row of fibrillar hairs is attached to the
flagellar sheath.
Contraction of the striated strand leads to
supercoiling of the axoneme.
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Diagram of part of the transverse flagellum of
Peridinium cinctum. Only a portion of the fibrillar hairs
have been drawn in.
Pusule
• A pusule (P) is a sac-like structure that opens by means of a pore
into the flagellar canal (Fc) and probably has an osmoregulatory
function similar to that of a contractile vacuole.
(a) Drawing of Amphidinium carteri. The small
epicone is separated from the
hypocone by the encircling girdle,
which contains the transverse
flagellum. The peripheral chloroplast
(C) is shown only at the sides of the
cell so the positions of the nucleus
(N), mitochondria (M), pyrenoid
(Py), and the pusule (P) associated
with the flagellar canals can be seen.
(b) Longitudinal section through a
pusule showing the flagellar pore
constriction (C), the pusule vesicles
(V), and the flagellum (F).
(c) Transverse section of a pusule
showing the flagellum within the
flagellar canal (Fc) and the pusule
vesicles (V) opening into the canal.
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• The dinoflagellates are one of the fastest swimmers
among the algae
• Marine dinoflagellates frequently move into deeper,
nutrient-rich waters at night and better illuminated waters
near the surface during the day.
• The diel (over a 24-hour period) migrations of
dinoflagellates are 5 to 10 m in relatively quiet waters.
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Phototaxis and eyespots
• Less than 5% of the Dinophyceae contain eyespots (e),
and those that do are mostly freshwater species; yet the
eyespots are among the most complex in the algae.
• An eyespot is not necessary for a phototactic response.
Woloszynskia tenuissima. Ventral view
showing the two flagella, the girdle (g), and the
eyespot (e).
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(a) A ventral view of a cell of Glenodinium foliaceum showing
the location of the eyespot (e). (b) Threedimensional diagram
of the eyespot area. The two flagella arise just above the
lamellar body (l). The eyespot (e) lies beneath the sulcus. (mt)
Microtubular roots; (b) banded root; (t) thecal plate.
Trichocysts
• Trichocysts (T) are membrane-bound
organelles containing proteinaceous
threadlike structures found in a
number of species.
• In these species, the cell wall is
perforated by pores (P) through which
trichocysts are discharged or
projected from the cell under
stimulation, often hundreds per cell.
• The actual benefit of trichocysts to the
cell (if any) is still obscure.
– They could be a mechanism for quick
escape as the cells move sharply in
the opposite direction of discharge,
– or they have a protective or evasive
function since they could be able to
directly “spear” a naked intruder.
Light and electron microscopical drawings of Peridinium
Accumulation body
• This is a large vesicle containing the
remains of digested organelles.
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sp., a dinoflagellate showing many of the features of the
class. (C) Chromosome; (CE) chloroplast envelope;
(CER) chloroplast endoplasmic reticulum; (E) epicone;
(G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid
globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P)
trichocyst pore; (S) starch; (T) trichocyst; (TF)
transverse flagellum; (TP) thecal plate.
• The process of encystment or resting spore formation is
regulated by a complex interaction of
– day length,
– temperature, and
– nutrient concentration
• Melatonin levels increase by several orders of magnitude
during encystment and may function in preventing
oxidation of the lipids in the cyst
• Cysts are recognizable because they lack
chromatophores and have a microgranular brown
cytoplasm and a red eyespot (if the organism normally
has an eyespot).
• Calcification of cysts in some genera occurs by the
deposition of calcium carbonate crystals in the narrow
space between the cell wall and the plasma membrane.
• The cysts of Ceratium hirundinella contain an outer
silicon layer.
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Chloroplasts and pigments
• The cells can be photosynthetic or
colorless and heterotrophic.
• Photosynthetic organisms have
chloroplasts surrounded by one membrane
of chloroplast E.R., which is not continuous
with the outer membrane of the nuclear
envelope.
• Chlorophylls a and c2 are present in the
chloroplasts, with peridinin and
neoperidinin being the main carotenoids.
• About half of the Dinophyceae have
pyrenoids in the chloroplasts.
• The storage product is starch, similar to
the starch of higher plants, which is found
in the cytoplasm.
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Light and electron microscopical drawings of Peridinium
sp., a dinoflagellate showing many of the features of the
class. (C) Chromosome; (CE) chloroplast envelope;
(CER) chloroplast endoplasmic reticulum; (E) epicone;
(G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid
globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P)
trichocyst pore; (S) starch; (T) trichocyst; (TF)
transverse flagellum; (TP) thecal plate.
Nucleus
The nucleus of the dinoflagellata is unusual where is has
many odd features unique to the group.
• The DNA does not seem to have histones (basic
proteins which the DNA coils around).
• Dinoflagellates have more DNA in their nucleus than
other eukaryotes, so much so that the nucleus often
fills half the volume of the cell .This implies that a
large amount of the DNA is genetically inactive
(structural DNA) in dinoflagellates.
• Chromosomes in its nucleus are not scattered, but are
attached to the nuclear membrane.
• Upon division, the nuclear membrane does not break
down as in plants and animals.
• The chromosomes remain condensed during mitosis
and even during interphase, though they do unwind
for replication of the DNA.
• Such permanently condensed organization of
chromatin (chromosomes) is different from either
prokarytoic or eukaryotic cells.
• This unusual nuclear situation is
termed mesokaryotic or dinokaryotic in the belief
that it was transitional between prokaryotic and
eukaryotic structure, but it is now thought to be a
uniquely derived feature of the Dinogflagellata .
A dinoflagellate nucleus. Note the
Condensed chromosomes
with characteristic banding pattern
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Resting spores or cysts or hypnospores and fossil Dinophyceae
• The resting spore or cyst of most dinoflagellates is morphologically
distinct from the parent cell.
• The cell walls of the cysts are highly resistant to decay and contain
dinosporin, a chemical similar to sporopollenin in the pollen of
higher plants.
• The cysts are extremely resistant and are preserved in ancient
sediments where they are of value in paleoecological studies.
• Many extant resting spores are identical to fossilized cysts.
٣٥ Dinoflagellate cysts. (a) Alexandrium catenella. (b)Calciodinellum operosum. (c) Scrippsiella
trophoidea. (d) Gonyaulax grindleyi. (e) Polykrikos schwartzii. (f) Gymnodinium catenatum.
Toxins have been divided into different classes based on the syndromes
associated with exposure to them, such as
1. Diarrhetic shellfish poisoning. This occurs primarily in temperate regions
and is caused by species of the planktonic dinoflagellates Exuviaella,
Dinophysis, and Prorocentrum.
Toxins
• Some Dinophyceae have the ability to produce very potent toxins
which cause the death of fish and shellfish during red tides when
there are dinoflagellate blooms that color the water red.
• The dinoflagellates become lodged in the gills of the shellfish, and
when shellfish are eaten by humans or animals, poisoning results.
• All of the dinoflagellates that have been demonstrated to produce
toxins contain chloroplasts, indicating that the ability to produce
toxins may have been derived from endosymbiotic cyanobacteria.
Prorocentrum
Fig. 7.30 Scanning electron micrographs of dinoflagellates that cause diarrhetic shellfish poisoning. (a)
Dinophysis acuminata. (b) Dinophysis fortii. (c) Prorocentrum lima with the arrows pointing to pores in the theca.
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2 Ciguatera fish poisoning. This occurs primarily in tropical regions with
the common causative agent being Gambierdiscus.
• The dinoflagellate contains gambieric acids, ciguatoxins, and maitotoxins,
that are very potent Ca2+ channel (pathways for calcium entry into cells)
activators that result in breakdown of the cell membrane.
• Gambierdiscus is epyphytic on macroalgae that are eaten by herbivorous
fish and shellfish, which, in turn, are eaten by humans. In French
Polynesia alone, approximately 1000 cases of ciguatera fish poisoning
are reported every year.
• The term ciguatera is derived from the Spanish term “cigua” for the turban
shell, which was commonly eaten before the illness developed.
• The typical course of ciguatera fish poisoning is diarrhea for two days,
followed by general weakness for one to two days.
• Occasionally the condition is fatal.
•
•
•
•
•
Diarrhetic shellfish poisoning is caused by the polyether carboxylic acids
okadaic acid, macrolide toxins, and yessotoxin.
The polyether carboxylic acid okadaic acid is initially formed inside the
dinoflagellate cell as dinophysistoxin-4. This weakly’sulfated derivative of
okadaic acid is not toxic to the dinoflagellate cell.
Dinophysistoxin-4 is either excreted by the dinoflagellate cell, or is released
on death of the cell.
Dinophysistoxin- 4 is hydrolyzed in the medium to okadaic acid diol ester,
which is lipid soluble and can pass through cell membranes. Thus, okadaic
acid diol ester can be taken up by cells which further hydrolyze the
compound to okadaic acid.
Research has shown that okadaic acid with a free acid moiety is the toxic
form of the compound.
turban shell
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•
•
•
•
•
•
•
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Alexandrium excavatum is a toxic red-tide
dinoflagellate.
The vegetative cells divide to produce
motile gametes that subsequently fuse.
Fusing pairs swim poorly and settle in the
water.
The motile quadriflagellate planozygotes
swim for a few days before losing their
flagella and thecal plates, and encyst to
form resting cysts (hypnospores) that can
survive for 5 to 15 years.
The hypnospores contain storage products
and have a thick, three-layer wall.
Cyst germination is controlled by a
biological clock with a 12 month maturation
period.
Each hypnospore undergoes meiosis with
two cell divisions to produce haploid
vegetative cells, completing the life cycle.
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Left: Hypnospores (resting spores) of Alexandrium. (a)
Light micrograph of a hypnospore surrounded by mucus.
(b) Scanning electron micrograph showing the smooth
surface of a hypnospore.
• 3 Paralytic shellfish poisoning. This is caused by species of
Alexandrium (A. catanella, A. acatenella, A. excavatum, A.
tamarensis), Pyrodinium bahamense, and Gymnodinium catenatum.
• These dinoflagellates produce a group of toxins that are derivatives
of saxitoxin.
• Saxitoxins are potent neurotoxins acting upon voltage-gated Na+ channels, preventing influx of Na+, thereby preventing the
generation of an action potential.
٤١
Alexandrium catanella
Pyrodinium bahamense
Gymnodinium catenatum
A number of factors have been suggested as the cause of red tides:
1 High surface-water temperatures:
Dinoflagellates favor warm water, and are generally more abundant near the
surface. This does not necessarily mean that they occur only in warm seas,
because the surface of the sea in normally cool areas may be warmed up
during periods of hot, calm weather.
• The amount of toxin in cells of Alexandrium is
relatively low when nutrients are in ready supply.
• The toxin concentration is highest under
conditions of phosphorus deficiency, possibly
because free amino acids (precursors of toxins)
accumulate under conditions of phosphorus
deficiency.
• Cells of dinoflagellates that produce toxins are
avoided by grazing copepods.
2 Wind: A strong, offshore wind aids upwelling, whereas a gentle onshore wind
concentrates the bloom near the coast. On the other hand, heavy weather
and strong winds disperse the bloom. Storms also result in the death of
dinoflagellates and can prevent the development of red tides.
3 Light intensity: There is usually a period of bright, sunny, calm weather
before outbreaks.
4 Nutrients: Red tides usually occur after an upwelling has stopped, but the
nutrients brought to the surface do not, themselves, appear to be the direct
cause of these blooms. It is thought that preceding blooms of diatoms may
impoverish the water and reduce one or more of the inorganic nutrients to a
level favorable for the growth of dinoflagellates (but too low for the diatoms),
and also allow the production of organic nutrients such as vitamin B12,
which are important for their growth.
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Bioluminescence
• Bioluminescence (chemiluminescence), in which energy from an
exergonic chemical reaction is transformed into light energy
• Many marine, but no freshwater dinoflagellates are capable of
bioluminescence.
• The Dinophyceae are the main contributors to marine bioluminescence,
emitting a bluish-green (maximum wavelength at 474 nm) flash of light of
0.1-second duration when the cells are stimulated.
• The luminescent wake of a moving ship is usually caused primarily by
Dinophyceae.
Dinoflagellates and oil and coal deposits
• Blooms of dinoflagellates have most likely been
responsible for some of the oil deposits of the world,
including the North Sea oil deposit.
• Petroleum deposits and ancient sediments contain 4αmethylsteroidal hydrocarbons, which probably originated
from 4α-methylsterols in dinoflagellates
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٤٥
• Associated with dinoflagellate luciferin is a luciferin-binding
protein (LBP) which sequesters luciferin at alkaline pH and
releases it under acidic conditions.
• It has been postulated that the flash of bioluminescent light may
occur simply by a lowering of the pH from 8.0 to 6.5.
• Agitation of cells depolarizes the vacuolar membrane, allowing a flux
of protons (H+) and acidification of the peripheral cytoplasm.
• Lowering the pH causes two pH-dependent reactions to occur:
(1) release of luciferin from its binding protein at acidic pH, and
(2) activation of luciferase followed by emission of a photon of blue-green
light.
•
The compound responsible for bioluminescence is luciferin, which is
oxidized with the aid of the enzyme luciferase, resulting in the emissions of
light.
•
Luciferin and luciferase are terms for a general class of compounds, and not
of a specific chemical structure.
Bioluminescence occurs in many organisms in many different phyla, ranging
from bacteria to dinoflagellates to jellyfish and brittle stars to worms, fireflies,
molluscs, and fish.
In bacteria, luciferin is a reduced flavin; in insects it is a (benzo)thiazole
nucleus; and in dinoflagellates it is a tetrapyrrole.
•
•
Dinoflagellates can emit light in three modes:
(1) they can flash when stimulated mechanically, chemically, or electrically;
(2) they can flash spontaneously; and
(3) late at night they can glow dimly.
A possible partial structure of dinoflagellate luciferin.
•
Likewise, luciferase has different structures, although all luciferases share the
feature of being oxygenases (enzymes that add oxygen to compounds).
In the basic reaction of bioluminescence, a luciferin is oxidized by a
luciferase, resulting in an electronically excited product (P)* which emits a
photon (hv) on decomposition:
•
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Rhythms
Many Dinophyceae exhibit rhythmic processes, with circadian (which
means literally about (circa) a day (diem)) characteristics.
1. It produces light via bioluminescence if the cells are stimulated by
shaking or stirring.
• The greatest luminescence will be produced in the middle of the dark
period, whereas toward morning, flashes will gradually become
smaller and a greater stimulus will be required
• The rhythm is circadian, as shown by the persistence of changes in
brightness of luminescence when the cells are kept in the dark or in
continuous light.
2. The photosynthesis, measured, either as oxygen production or carbon
dioxide fixation is also found to be rhythmic and the rhythm is
circadian and continues under conditions of continuous light.
• The maximum rate of photosynthesis occurs, as one would expect, in
the middle of the day.
3. A third rhythm with a circadian period is that of cell division; all cell
division occurring during 30 minutes when cultures are in a light–dark
cycle.
• When the light–dark cycle is 12 : 12, then this 30 minutes spans
٥٠ “dawn.”
• In Lingulodinium polyedrum, there exists a control over luminescence,
photosynthesis, and cell division, so that each process reaches a
maximum and then declines in an orderly fashion during each 24
hours (Fig. 7.45).
• A biological clock, may control all of these processes.
• It appears that the part of the cell that may be the controlling agent is
the plasma membrane because there is a rhythmic reorganization of
the plasma membrane over a 24-hour period in synchronized cells
• There are two theories concerning the adaptive value of
bioluminescence to dinoflagellates, both of which relate to nighttime
grazing of the dinoflagellates:
1 “Burglar alarm” hypothesis. In this hypothesis dinoflagellates generate a
signal identifying the location of invertebrate grazers (e.g. copepods)
to individuals two levels up the food chain from the dinoflagellates.
Bioluminescence generated by dinoflagellates serves to attract
predators of the grazers of the dinoflagellates.
2 “Startle” hypothesis. In this hypothesis, mechanical stimulation of a
bioluminescent dinoflagellate by a grazer produces a flash of light that
startles an invertebrate grazer, such as a copepod, and causes the
copepod to swim away.
• Whichever theory is correct, experiments have shown that copepods
consume only half as many dinoflagellates at night, indicating that the
bioluminescence is serving as a deterrent to grazing.
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4. A fourth type of rhythm involves the vertical migration of
dinoflagellate cells in the water column.
• Before dawn, the cells rise to the surface, where they
form dense clouds (aggregations), and before night fall,
they again sink to lower depths
• In the marine environment, this vertical migration
exposes the cells to several gradients:
– (1) Nutrients are more concentrated at lower depths while
surface waters are often practically devoid of nutrients.
– (2) Temperatures at the surface exceed those in deeper waters.
– (3) Variations in light intensities.
– (4) differences in washout by the tidal waters in shallow waters,.
Fig. 7.45 Circadian rhythmicity has been identified in at least four distinct biological processes of the
unicellular Lingulodinium polyedrum. Although the four rhythms peak at different times of the 24-hour day, as
represented here diagrammatically, they are all synchronized with the circadian clock. Perturbations that
٥٢
reset the clock will phase-shift all four rhythms simultaneously, suggesting that all of these overt rhythms are
driven by a single pacemaker.
– (2) pallium feeding where the prey is engulfed by a
cytoplasmic veil – the palium – with digestion of the
food taking place outside of the dinoflagellate cell;
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Heterotrophic dinoflagellates
• An estimated half of the more than 2000 living dinoflagellate species
lack chloroplasts and are exclusively heterotrophic.
• In addition, many dinoflagellates that contain chloroplasts are
capable of mixotrophy where a portion of their nutrients is obtained
heterotrophically, particularly when nutrient conditions are low in the
environment
• The different modes of heterotrophy are:
– (1) phagotrophy through the direct engulfment of prey;
tentacle
The heterotrophic dinoflagellate Protoperidinium conicum feeding on the diatom Corethron hystria.
Initially the dinoflagellate attaches to the prey by a long thin filament (a). Next a pseudopod extends along
the filament (b) and engulfs the prey (c), which is digested.
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Drawing of the ingestion of food organisms (other algae, bacteria) by Noctiluca. (a) The tentacle (T) is
in an extended configuration. Any food organisms (FO) that collide with the mucus-covered tentacle tip,
stick
٥٣ to the tentacle. (N) Nucleus; (OP) oral pouch. (b) The tentacle bends back toward the oral pouch. (c)
The cytosome (C) at the base of the oral pouch opens, the tentacle tip is inserted into the cytosome, and
the food organisms are swept into a food vacuole.
– (3) peduncle feeding (myzocytosis) involving the uptake of
intracellular material of the prey through a cytoplasmic extension
– the peduncle – leaving the plasma membrane and extracellular
material of the prey behind;
– (4) osmotrophy or the uptake of dissolved substances
Symbiotic dinoflagellates
•
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•
Symbiotic dinoflagellates (zooxanthellae) occur
in almost all species of tropical and reef-building
corals, jellyfish, and sea anemones (Cnidaria).
The dinoflagellates are coccoid spheres in the
symbiotic state and have been assigned to the
genus Symbiodinium.
The host exerts strong control over the
translocation of metabolites from the
dinoflagellate endosymbiont, resulting in 98% of
the carbon fixed by the endosymbiont being
released to the host.
The host animal cells secrete the amino acid
taurine which causes the dinoflagellate to release
photosynthate outside the cell for absorption by
the animal cells.
•
In the flatworm Amphiscolops langerhansi the
symbiotic dinoflagellate is Amphidinium klebsii
•
In addition to Dinophyceae living symbiotically
inside other organisms, there are other
organisms that live inside dinoflagellate cells.
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• Order 2 Dinophysiales: cell wall divided vertically into two halves,
cells with elaborate extensions of the theca.
• One of the more complex organisms in this order is Ornithocercus
(Fig. 7.56(d), (e)).
(d),(e) Ornithoceros magnificus, a righthand view (d) and an “exploded” view (e) of the theca. (DA) Dorsal
accessory moiety of the left sulcal list; (LLG) left lower girdle list; (LS(am)) anterior moiety of the left sulcal
list; (LS(pm)) posterior moiety of the left sulcal list; (LUG) left upper girdle list; (RS) right sulcal list; (RLG and
RUG) right lower and upper girdle list; (g) girdle; (p) pore; (w) wing.
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• Order 4 Gymnodiniales: motile cells with an epicone and hypocone
separated by a girdle; theca thin or reduced to empty vesicles.
• The life cycle of Gymnodinium pseudopalustre is a representative of
the order.
(a) Light micrograph of the dinoflagellate Gymnodinium fungiforme (G) ingesting the protoplasm of
Dunaliella salina (D). The peduncle (P) of G. fungiforme has attached to D. salina with the protoplasm of
D. salina passing through the enlarged and extended peduncle into the dinoflagellate. (b) Scanning
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electron micrograph of a zoospore of Pfiesteria pisciicida showing the peduncle.
Classification
• There is a single class in the Dinophyta, the Dinophyceae.
• Four orders are considered here.
• Order 1 Prorocentrales: cell wall divided vertically into two halves;
no girdle; two flagella borne at cell apex.
• Prorocentrum is an example of the order
Scanning electron micrographs of Prorocentrum
hoffmanianum.
(b),(c) Prorocentrum micans, side (b) and front (c) views.
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• Order 3 Peridiniales: motile cells with an epicone and hypocone
separated by a girdle, relatively thick theca.
• The algae in this order have the classic dinoflagellate structure with
an epicone and hypocone and two furrows, the transverse girdle and
the longitudinal sulcus.
• Ceratium is widely distributed genus.
Scrippsiella trochoidea.Scanning electron micrographs showing
the thecal plates in ventral (a) and dorsal (b) view
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Ceratium tripos