INFLUENCE
OF MARINE
PROTOZOA
REGENERATION1
ON NUTRIENT
R. E. Johannes
University
of Georgia
Marine
Institute,
Sapelo Island, Georgia
ABSTRACT
Per unit weight, marine protozoa excrete dissolved phosphorus
one to two orders of
magnitude faster than marine microcrustaceans
and several orders of magnitude faster than
marine macrofauna.
Protozoa may therefore be responsible for a major fraction of faunal
nutrient excretion even though present only as a minor fraction of the faunal biomass.
Regeneration
of dissolved inorganic phosphate from organic detritus proceeds faster and
more completely in the presence of bacteria and ciliates or colorless flagellates than in the
presence of bacteria alone. An increased consumption of organic detritus in the presence of
protozoa
is performed
by bacteria,
which
are kept in a prolonged
state of “physiological
youth”
by grazing protozoa.
Present results indicate
that it is the protozoa
that produce, via excretion, the greater regeneration
of nutrients from organic detritus. By
constituting
a nutrient source for protozoa, bacteria are indirectly
involved in this increased
regeneration.
INTRODUCTION
High concentrations of ciliates and other
protozoa are characteristic of decomposing
sewage. Several investigato,rs have Found
that the purification of organic sewage, as
measured by the rate of oxygen consumption accompanying its decomposition, occurs more ra,pidly and proceeds further in
the presence of ciliates and bacteria than
in the presence of bacteria alone (Butterfield, Purdy, and Theriault
1931; Javornick9 and Prok&ov& 1963).
Any body of water containing living organisms produces a natural “sewage” in the
form of dead organisms, feces and dissolved mctabolites. The biological processes governing the decomposition of these
substances may be similar to those occurring
in domestic sewage. If this is tho case,
marine protozoa can play an important
role in the regeneration of nutrients.
An investigation of the influence of some
marine ciliates and heterotrophic
microflagellates on phosphorus regeneration relative to the influence of other marine
animals and of marine bacteria was undertaken. The work was supported by National
Science Foundation
Grant 1040 and by
grants from the Sapelo Island Research
Foundation. I wish to thank Drs. L. R.
Pomeroy
and K. L. Webb for critically
of
reading the manuscript.
434
Protozoa are an abundant,
ubiquitous
component of the marine fauna. Tintinnids
and colorless flagellates may be present in
large numbers in the plankton (Lohmann
1908; Kofoid and Swezy 1921; Wood 1963a,).
Ciliates, colorless flagcllatcs, and sarcodines
often abound in benthic environments
(Lackey 1936; Marc 1942; Borror 1962;
Wood 1963b), but because of their small
size and fragility their numbers are often
ignored by marine ecologists. Many species
arc not retained quantitatively
in plankton
nets, and they are seldom recognizable in
the stomach contents of larger animals.
They may be destroyed by fixatives, the
pressure of a coverslip, or even by coming
in contact with the air-water interface
( Borror 1963; Faur&Frcmict
1950). Conscqucntly, their role in the biological metabolism of the sea has received little attention.
Several studies have indicated that animal excretions are an important source of
plant nutrients in the sea (Harris 1959;
Pomeroy, Mathews, and Min 1963; Ketchum 1962 ) . Planktonic crustaceans were
the main objects of investigation in these
studies; excretion of nutrients by marine
protozoa was not considered.
1 Contribution
Georgia Marine
No. 85 from
Jnstitutc.
the
University
MARINE
PROTOZOA
AND
METHODS
Organisms and cultwe methods
Bacteria-eating ciliates wore isolated from
mud flats, tidal creeks, and sand bcachcs
at Sapelo Island, Gclorgia. Thres species
were identified
by Dr. A. C. Borror as
Euplotes crassus Dujardin, E, vannus Muller, and E. trisulcatus Kahl. No attempt
was made to identify o,thcr species. Several
microlflagellatcs
species of heterotrophic
were isolated from similar habitats and, in
one instance, from a seawater sample taken
at the water surface on the continental
shelf 25 km east of Sapelo Island.
The ciliates wore maintained in cottonplugged flasks using Pan-Mede bacteria
nutrient medium ( 10 mg/100 ml membrane
filtered seawater) as a nutrient source for
the bacteria on which they fed. Heterotrophic microflagellates
( 1-4 p diam) were
present along with bacteria in all cultures.
An initial bloom of scvcral hundred thousand flagellates per ml preceded the growth
of ciliates. The; flagellates were not eaten
but always died out within about four
days after fresh cultures were started. The
ciliates lived for several weeks or months.
Repeated attempts to culture two species of
ciliates ( E, vannus and E. trisulcatus ) in
cultures free of these flagellates failed; the
ciliates multiplied slowly or not at all. The
initial growth of flagcllatcs was apparently
necessary to condition the medium in some
way that facilitated ciliate growth.
The cultured flagellates grew luxuriantly
in seawater containing Pan-Mede ( 10 mg/
100 ml). Although none of the species was
identified,
they could bc! differentiated
from each other on the basis of number
and placement of flagella and cell shape.
Their average diamctcr was about 2 p, but
several species achieved their greatest size in
freshly inoculated
cultures and became
progressively smaller as the cultures aged
(range l-4 p) . All grew well in the dark.
A mixed culture of marine bacteria taken
from stock ciliate and flagellate cultures
was grown in a seawater-nutrient
medium
( Johannes 1964a).
NUTRIENT
435
REGENERATION
Counting and awlytical
methods
Ciliates were counted using a ScdgwickRaf tcr counting
cell. Flagellates
were
counted using a hemacytometer. Bacteria
counts were made using the stained membrane filter prolcedure of Jannasch ( 1958)
as slightly modified by Johanncs ( 1964~).
The mean weights of four species of
ciliates were dotorminod as follows: The
length, width and depth of 10 spccimess of
each species were measured at sevelral
points with an ocular microImote!r, and a
scale model plaster cast of the organism
was made using the means of these measurements. The volumes wcse calculate;d from
the volumes of wate,r displaced by the
models, Dry weights were calcula,ted assuming the ciliates ha.d a specific gravity
of 1.0 and that dry weight was 25% of wet
weight. Erro,rs involvad in this kind of
calculation may be rathclr large. The estima,tad weight is perhaps only 2 50% of the
true weight. Tho error thus introduced is
insignificant,
however, when the excretion
rates oE those ciliates are co’mpared with
those of animals several orders of magnitude larger.
Phosphate analyses were performed according to the one-solution ascorbic acid
method developed by J. P. Riley and described by J. D. II. Strickland (unpublished
manuscript ) .
Radioactivity of samples was determined
with a 150 pg/crn” micromil end-window
gas flow Geiger counter.
Uptuke and emretion of phosphorus
by cili4ztes
Uptuke. To daterminc whether the ciliates obtained their pho’sphorus primarily
from their food (ba.ctcria) or from solution,
their ““P uptake rates in the prosence and
absence of bacteria were colmpared. Cultures of ciliates were placed on a fairly
coarse filter (Whatman #541 or #44) and
washed free of bacteria in artificial seawater2 ( ASW) containing no phosphorus.
’ The ASW composition was similar to that dcscribed
by Provasoli,
McLaughlin,
and Droop
( 1957) ; the main cliffercncc was use of a calcium
436
R. E.
JOHANNES
They were then resuspended in 50 ml of
this medium.
A 25-ml aliquot of this ciliate suspension
was added to 25 ml of sterile ASW containing 25 PC/liter of “P and 3 pg-at/liter
of dissolved inorganic phosphate ( DIP).
Another 25-ml aliquot of the ciliate suspension was added to 25 ml of a mixed species
culture of marine bacteria, The bacterial
cells
contained the same concentration of
31Pand “P per unit volume of medium as
the bacteria-free
solution in which the
first aliquot of ciliates was suspended; that
is, bacterial cells contained 25 PC “P and 3
pg-at. DI”‘P per liter of medium. Less than
0.001 pg-at/liter
of phosphorus was in solution.
Duplicate
5.0-ml samples were taken
from each experimental culture periodically
for 3 hr. The samples were filtere-d through
a Whatman #44 or #541 filtelr. The filters
wore washed to remove any remaining
bacteria by drawing through two 5-ml
portions of ASW without P. The radioactivity of the ciliates on the filter was detcrmined.
The ciliates were: always filtered at a
vacuum pressure of 20 Torr. Initial cxpcriments showed that they could withstand
pressures of over 40 Torr without injury.
Initial tests also showed that over 99% of
the phospholrus in bacteria cells passed
through the filter during filtration
and
washing.
Excretion. Dissolved phosphorus excretion rates of the ciliates were measured
using a radio,active tracer method similar
to that used with a benthic amphipod by
Johannes ( 1964b ) .
The ciliates wero grown in culture media
containing 125 PC ““P/liter with which they
became labeled. They were used in the
following
experiments while still in the
logarithmic
growth phase. The ciliates
were filtered on a Whatman #44 or #541
filter, then washed by drawing 10 ml of
phosphorus-free
ASW through the filter.
They were then suspended in 5 ml of phos-
An experiment was pe:rfomed to determine the influence of ciliates on the regeneration of phosphorus from decomposing marsh grass, Spartina alterniflora Loisel.
Frosh leaves were cut into pieces about 1
cm square, then added to each of three 2liter beakers containing 1.5 liters of artificial seawater without phosphorus. These
cultures were stirred and irradiated with
ultraviolet light (2537A peak energy) for
10 min to kill extraneous protozoa that
might have been introduced with the ‘leaves.
A drop of a culture of Euplotes vanrws was
added to each of two beakers. A drosp of a
mixed species bacteria culture was added
to the third. The cultures were covered and
kept in the dark at room temperature.
Samples for DIP analysis were taken by
withdrawing
50-ml aliquots from the cultures with sterile pipettes. The aliquots
were centrifuged and the top 25 ml was
drawn off and filtered through an acidwashed HA Millipore
filter. Two) lo-ml
samples were preserved with a drop of
chloroform and frozen until analyzr:d.
Experiments were also carried out to
determine the influence of ciliates and
flagellates on the regeneration of’ phosphorus contained in marino bacteria,.
Four flasks were set up containmg 200
concentration
similar
water ( 0.4%).
3 Registered
tion, Bedford,
to that found
in natural
sea-
phorus-free
ASW. The suspension was
poured into a second filter holder containing an HA Millipore@ filter and held for
2.5 to 4.5 min. The medium was then drawn
through the filter and the time noted (to
the second) during which the ciliates had
been in contact with the filtrate (the length
of time between when the, ciliates were
suspended and when the filter went dry).
Two l-ml samples of the filtrate were withdrawn and evaporated on an aluminum
planchet. The filter was cut into quarters
and these were placed on planchets, The
radioactivity
of the filtrate, containing excreted 32P,and of the filters containing the
?? in the ciliates, was measured.
Phosphorus regeneration
trademark, ~4illipore
Massachusetts.
by protozoa
Filter
Corpora-
MARINE
PROTOZOA
AND
TABLE
30,000-
2
25,000
-
20,000
z
P
-
15,000
-
10,000
-
1.
excretion
Phosphorus
body-equivalent
times of marine ciliates
BEET
Mcnn dry weight
Euplotes crassus
Euplotes uannus
Euplotes trisulcatus
Uronema sp.
=:
r-
437
REGENERATION
Ciliate
!j
G
NUTRIENT
30
8
(min)
x 10-3pg
160 (137-202)*
x lo-” /-Lg 20( 19- 21)
2.2 x lo-” /Lb’
0.4 x 10-3/Lg
43( 36- 50)
14 ( 12- 17)
z
2
* Figurcs
nations.
in parenthcscs
are the range of three dctermi-
5,000,$
0
I
1
I
2
TIME
I
3
(HAS)
FIG. 1. Uptake of a2P by E. vannus from solution and from ingested bacteria ( e-uptake
from
bacteria; O-uptake
from solution).
ml of RSW, 50 mg/liter
of phosphorusfree dissolved organic bacteria nutrient
mixture ( Johannes 1964(l), 25 PC/liter ““P
and 7.0 pg-at./litor of DIP. ToI all four cultures was added a drop oi seawater containing marine bacteria isolated from ciliate
and flagellate cultures. After 24 hr bacteria
reached a density of 50,000,OOO cells/ml
and had taken up over 99% of the phosphorus present.
At this point, three of the cultures were
inoculated with a drop of proltozoan culture. Two cultures then contained ciliates
( E. vannus and E. trisulcatus respectively )
and one culture contained a heterotrophic
microflagellate.
The fourth culture contained only bacteria.
Periodically,
IO-ml samples from each
culture were filtered through an HA Millipolre filter. (Samples frolm the flagellate
culture were first centrifuged to sediment
the flagellates which otherwise ruptured
during filtration).
Duplicate l-ml portions
of the filtrate were evaporated on a planchet and counted for ““P.
REXJLTS
Phosphorus uptake and excretion by ciliates
Eupbfes vamus took up ““P 4.6 times
faster by ingesting ““P-labeled bacteria than
they did from the same amount of 32P of
the same specific activity in solution (Fig.
1). Tests with the other three species of
ciliates yielded similar results.
Previous investigations on a marine amphipod (Johannes 1964b) showed that the rate
of excretion of dissolved ““P began to drop
about 1/2hr after the animal was deprived
of food, coincidentally
with the emptying
of the gut of undigested food. In the prescnt expcrimcnts, the much smaller size of
the test o,rganisms and their coasequent
higher
metabolic
rates suggested that
their dissolved pholsphorus, excretion rates
would drop even sooner after they were
deprived of food. The present excretion expcrimcnts therefore lasted only 3-5 min.
The phosphorus excretion rate was calculatcd in terms of the time it would take
ths ciliates toi release an amount of dissollved phosphorus ( measured as ““P ) equal
to their own phosphorus co,ntent. This will
be referred toI as the body-equivalent
oxcretion time ( BEET)4.
BEET = F
x
D
where
P, = ““P in the ciliates 0; the filter,
p.3= ““P in the filtrate, and
D = duration of the test.
The BEE 2’ values of four species are
shown in Table 1. Further tests showed
that the ciliates’ BEET values began to
increase o,nce the organisms had passed the
logarithmic growth phase.
Tha dissolved organic ““P fraction in the
excretions of E. vannus was measured by
4 BEET is not synonymous with turnover time,
The latter, although variously defined, is usually a
measure of the flow of a substance through an organism irrespective
of whether this substance is in
dissolved or particulate
form. BEET, on the other
hand, is cz measure of the release of dissolved substances only- it is an index of nutrient regeneration.
438
R. E.
JOHANNES
50 -
BACTERIA
0 BACTERIA+
l
40 -
K
Y
<
30ii!!
c;m
I
g
20-
10 -
A
&dl--2 4
7
10
TIME
”
19
21
24
27
30
(DAYS)
2. Influence of ciliates, E. uannus, on regeneration of phosphorus from dead Spmtina. Arrow indicates Spartina ncklition.
FIG.
the method described by Johannes ( 1964b)
and constituted 26% of the total dissolved
phosphorus.
Protozoa and phosphorus regeneration
The influence of Euplotes vannus on regeneration of phosphate from Spartina is
shown in Fig. 2. Initially the vessds contained 2 g ( fresh weight) of chopped Spartina leaves. A measurable release of DIP
was first noted on the fourth day. By the sevcnth day, phosphorus concentrations in the
ciliate cultures were significantly
higher
than in the control culture containing only
bacteria. Net release of DIP had ceased in
all three cultures by the 10th day.
On the 21st day, an additional 10 g of
chopped Spartina were added to oath culture. A rcpotition of the fo’rmer pattern of
DIP rcleasc followed; that is, DIP rcgencration occurred more rapidly and moire completely in the cultures containing ciliates.
The final concentration of DIP averaged
43% higher in cultures containing ciliates
after the first phase of the experiment and
73% higher after the secoad phase.
Although no bacterial counts were possible because of the large amount of particulate material present, microscopic
cxamination of water samples indicated that
suspended bacteria were always denser by
an order of magnitude or more in the control culture than in the ciliate cultures, after
the first three days.
In the first phase of the cxpcrimont, ciliates reached a maximum density of 4OO/ml
by the sixth day. They reached a density of
10,000 per ml seven days after Spartina
was added the second time. Their numbers
did not begin to drop until sommetimeaftelr
the 30th day o’f the experiment.
Heterotrophic microflagellates developed
in the ciliate cultures initially but died out
within 5 days. They did not raappoas when
Spartina was added the second time.
In the three protozoa-bacteria cultures in
which bacteria initially
contained all the
“P, net release of dissolved phosphorus began within 24 hr and continued throughout
the experiment ( Fig. 3). DIP cone cntrations rose to become 31% to 85% of the total
phosphorus concentration in from 9 to 10
days. Less than 1% of the phosphorus in
the culture containing only bacteria was
released during this time.
Euplotes vanrms reached a maximum
density of 55/ml on the sixth day and E.
trisulcatus reached 5OO/ml on the eighth
day. Both ciliates maintained their maximum densities for the duration of the experiment. Flagellates died out of the ciliate
cultures by the fifth day. In the flagellate
culture, flagellates attained densities of
600,000 cells/ml within
three days and
maintained this density until the ninth day.
By the third day, bacterial concentrations
in the protozoa cultures were significantly
lower than in the bacterial culture. Subsequent development in the protozoa cultures of mucous particles containing many
bacteria made further bacteria counts unreliable.
A similar series of experiments using 8
instaad of 7 pug-at/liter of dissolvod inorganic phosphate shotwed that initially,
MARINE
PROTOZOA
AND
NUTRIENT
TABLE 2.
2
P
!i
100
I RACTERlA
. “A(‘TER,A+ ELIPLOTESTRISULCATUS
I
1
0 Bh(‘7ERlA+
2
EUPLOTFS
3
439
REGENERATION
&zgeneration
of phosphorus
and bacteria*
by protozoa
“ANNUS
4
5
6
7
9
10
TIME (DAYS)
FIG. 3. Regeneration
by protozoa.
of bacterial
phosphorus
about 98% of the phosphorus was assimilated by bacteria (Table 2). The DIP in
these cultures was measured spectrolphotometrically after 1 day and after 7 days. Regeneration occurred in all cultures. After 7
days, the DIP concentration in the control
bacterial culture was less than 30% of the
next lowest DIP concentratioa in the protozoa-bacteria cultures.
DISCUSSdON
Dissolved phosphorus excretion in ciliates
The rate of release of dissolved phosphorus by an organism in an initially phosphorus-free medium is a valid measure of
its phosphorus regeneration rate oinly if this
phosphorus is obtained from particulate
food. If excreted phosphorus was derived
by ciliates from phosphorus, in solution,
this release could not be colnsidered to be
regeneration becanse it would not constitute a net release of disso,lvcd pholsphorus.
The much faster uptake of ““P by ciliates
in the presence of ““P-containing bacteria
than from ASW indicates that they obtain
the bulk of their phosphorus from ingosted
particulate food. The net uptake1 of dissolved pho8spho,rus, is pro,bably even less
important than the expcsiments appear to
indicate. The uptake! of ““1’ by a ““P‘lP exchange reaction involving
no net
transfer of phosphorus is well-known
in
living cells ( Rice 1953; and others ) and part
of the uptake of dissolved ““P by ciliates is
probably due to this mechanism.
Owing to ths unkno,wn magnitude of
Organism
One day
Seven days
Euplotes vannus
no. 11 ciliate
no. 13 ciliate
mixed ciliates?
no. 2 flagellate
no. 4 flagellate
no. 6 flagellate
no. 14 flagellate
bacteria control
0.18
0.11
0.11
0.21
0.08
0.23
0.17
0.16
0.19
3.1
2.0
2.3
4.0
46:;
3.8
2.1
0.G
* Numbers are fig-&./liter
dissolved inorganic phosphate.
-1 Seven species were nddecl, and three of them became
numerous.
this isotopic exchange, the net rate of uptake
of DI”“P in these experiments cannolt be
accurately estimated. As dissolved phospho,rus represents, at mo’st, a secondary
source of phophorus, all the phosphorus
excreted by ciliates will be considered to
be derived from particulate food for the
purposes of computation. That is, the rate
of excretion of phosphorus by these organisms is taken to be synonymous with their
phosphorus regeneration rate.
Johannes (1964c) has shown that, per
unit weight, dissolved phospholrus cxcrction rates of marine animals’ increase with
decreasing animal size. The high metabolic
rates of ciliates, in accordance with their
small size, results in very sho,rt plzospholrus
Z?EET values compared with those of
larger marinc animals. The four ciliates in
the present experiments had BEET values
averaging less than 1 hr (Table 1).
The phospho,rus BEET value of zooplankton caught in a nol. 2 net varies between 1.5 and 3 days (Pomeroy, Mathews,
and Min 1963; Satomi 1964). The BEET
value of a 0.6 mg amphipod was 31 hr
( calculated from data. of Johannes 1964h).
Horse mussels, Mod&&s demissus, of 0.55
g mean dry weight without shells, have
a mean BEET value of 1,540 hr (data kindly provided by Dr. IL J. Kucnzlcr). It can
be seen that the contribution of ciliates to
the process of phospholrus: regeneration will
greatly omutweigh their biolmass in magnitudc relative to larger marinc animals.
440
R. IL
t
D~s~olvcd
JOHANN%
t
Phosohorus
FIG. 4. Pathways
of detrital
phosphorus
regeneration examined in the present experiments.
Regmeration
of pholsphorus by marine
protozoa
Spartina alterniflora
is a major source
of detritus in United States east coast
ostuaries (Odum 3.961; Teal 1962). I have
observed numerous ciliates associated with
Spartina detritus ( including the two species
used in the present experiment on phosphorus regeneration from Spartina). The
abundance of marine and freshwater ciliates associated with detritus has also be,en
reported by Burkholder
( 1959)) Mare
( 1942), Gellert and Tamas ( 1961)) Lackey
( 1936)) and others.
One of the functions of this group apparently is hastening the regeneratioa of
nutrients from organic detritus ( Fig, 2). It
has been known for many years that a
significant increase in the rate of decomposition of sewage occurs when ciliates arc
present, and most workers agree that this
increase is achieved indirectly.
Grazing
ciliates prevent bacteria from reaching
self-limiting
numbers; the bacterial populations are thus kept in a prolonged state of
“physiological youth,” and their rate of assimilation of organic ma.torials is greatly
increased.
A seemingly logical co’rollary is that increased nutrient regeneratioa from detritus
in the presence of ciliates is the direct result of the increased activity of marine
bacteria. On the contrary, however, the
experiment shown in Fig. 3 indicates that
it is the ciliates that are directly responsible
for increased nutrient regeneration. All the
phospholrus in these cultures was initially
bacterial phosphorus. Increased bacterial
activity could not result in regeneration of
phosphorus from the substrate because no
phosphorus was present in the substrate.
The high phosphosrus excretion rates, of
ciliates has, been described above (Table
1). It must be concluded that ciliates increase the rate of regeneration of phosphorus from detritus by the ingestion of
bacteria and subsequent excretion of dissolved phosphorus. The two1 pathwayjs of
phosphorus regeneration examined in the
present experiments aro shown diagramatitally in Fig. 4. (No attempt was made to
differentiate
between bacterial and autolytic release of dissolved phosphorus from
Spartina in these experiments. )
Like ciliates, colorless marine flagellates
are present in large numbers in a variety
of marine habitats (Lackey 1936; Mare
1942; Wood 1963b). I have observed numerous colorless microflagellates
in and
on decomposing copepods, moribund diatoms, benthic crustacean feces, and itt the
mud-water interface on the Sap&
salt
marshes. When seawater o’r substrate samples, taken either offshore on the conltinental shelf at any depth olr in the intertidal
zone at Sap& Island, were enriched with
bacteria nutrient medium, colorless microflagellates invariably
developed densities
of several hundred thousand/ml within 36
hr.
Bacterial numbers are reduced somewhat
by these flagellates, but not to the extent
they arc reduced by ciliates (see Javornickji
and Prolk&ov& 1963; Butterfield,
Purdy,
and Therianlt 1931). Unlike ciliates, colorless flagellates have been. reported tcl’ exert
only a minor effect oln the rate’ of decommposition of sewage, as measured by cbxygon
consumption, Nevertheless, the present results (Fig. 3 and Table 2) indicate that
heterotrophic flagellates in the presence of
bacteria strongly stimulate the regeneration
of phosphorus.
The importance of prostoazoa as nutrient
regenerato’rs may also extend to the plankton. The abundance of planktonic protozoa,
particularly
heterotrophic
flagella1 es, is
described by Wood ( 1963a, b ), Kofoid
and Swezy ( 1921)) and others. Flagellates
MARINE
PROTOZOA
AND
which hastened pho’sphate regeneratioa in
the present experiments
included
one
planktonic species (flagellate no;. 14, Table
2).
Some protolzoa, including many of the
larger planktonic forms,, feed on algal cells
or miorofauna ( Kofoid and Swezy 1921).
In these cases, phosphorus-containming substrates (that is, algae, microfauna) are directly linked to protozoa and phosphorus is
regenerated without the intermediate agency of bacteria.
Bacteria and nutrient wgmeration
An increasing number of observations
suggasts tha,t bacteria are not responsible
for the bulk of nutrient regeneration in the
soa. Roughly 25 to1 75% of the dissolved
phosphorus, released frolm dead marine
microcrustaceans and unicellular plants appears to be duel toI their autolysis, (IIoffman 1956; Bruce and Hood 1959; Satomi
1961; Mars,hall and Orr 1961; Goltermann
1964; Johannes 1964a ) . Various studies
have indicated that animal excretions outweigh bacterial activities in the regeneration of nutrients. in the plankton (Harris
1959; Ketchum 1962; and others ).
Tha present results suggest tha,t a significant fractio,n of benthic regeneration of
phospholrus fro,m organic detritus, often attributed to bacteria, may in fact be attributable to protozoa. Although no information is available oln nutrient excretion by
marine nemato8des, their vast numbers and
high metabolic rates (Wiesm and Kanwisher 1961; Wieser 1960) s.uggest that
these animals may share with protozoa a
major role in benthic nutrient regenera,tioa,.
Larger benthic animals also) play solme part
in this process, (Kuenzler 1961; Pomeroy
and Bush 1959; Johannes 1964a).
While bacteria are thus apparently not
directly responsible for the bulk of nutrient
regeneration in the sea,, their indirect involvement in the process is important,
Bacterial activities greatly influence the @t3
oE scdime,nts and of boldies of wate,r such
as shallow lagoons where circulation
is
restricted. The pH in turn determines
whether certain excreted nutrients such as
NUTRIENT
REGENERATION
441
pho,sphorus and iron remain in solution or
are precipitated. Bacteria also use a wide
spectrum of organic colmpounds, some of
which other heterotrophs cannot assimilate
and in which nutrients would otherwise be
locked up. After converting dissolved metabolites and detritus into energy-rich cellu1a.rm,aterial, they become foo’d for protozoa
and other bacteria-feeders whose role in
nutrient regeneration is thus made possible.
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