Ultramicrobacteria
Secondary article
Article Contents
Ricardo Cavicchioli, University of New South Wales, Sydney, Australia
Martin Ostrowski, University of New South Wales, Sydney, Australia
. Introduction
. UMB: an Ecological Perspective
Ultramicrobacteria are microorganisms with a cell volume of less than 0.1 mm3 which have
been cultivated from aquatic and soil environments. Nan(n)obacteria are reported to be
even smaller than ultramicrobacteria and have been described in deep subsurface,
meteorite and clinical samples. The size of these tiny microorganisms challenges views
about the minimal size of a living cell.
. The Proliferation of Terms
. Marine UMB
. Clinical UMB and NB
. Soil UMB
. Subsurface NB
. Minimum Cell Size
Introduction
The size of microorganisms varies considerably. Even
amongst the prokaryotes (Archaea and Bacteria) cells with
volumes of 0.02–180 000 000 mm3 have been isolated. At
the smallest end of this size spectrum are the ultramicrobacteria (UMB). UMB are highly abundant in most
natural aquatic and terrestrial environments and for years
microbial ecologists have recognized that they play
important roles in the biological cycling of nutrients and
in the formation of biomass. More recently, nan(n)obacteria (NB) have been reported in human kidney stones,
deep subsurface samples and as fossil remains in meteorites. Accompanying these new findings has been intense
discussion debating the minimum size for a living cell. It is
clear that UMB and NB impact on fields of study as diverse
as microbial ecology, genomics and astrobiology. This
article examines the enthusiasm for research on UMB and
NB. It focuses on many of the fascinating discoveries in the
field, and attempts to separate fact from fiction, and
demystify the jargon which has proliferated.
UMB: an Ecological Perspective
The term ‘ultramicrobacteria’ was first adopted by Torella
and Morita (1981) to describe extremely small bacteria
(less than 0.3 mm diameter) isolated from seawater that
formed ‘ultramicrocolonies’ on agar plates, retained their
small cell size when growing on agar plates, and grew very
slowly in the presence of high concentrations of nutrients.
MacDonell and Hood (1982) modified this description to
include isolates from an estuary obtained by filtration
through a 0.2-mm membrane and which could form
normal-sized colonies on low-nutrient agar. In their
review, Schut et al. (1997) further modified the description
of UMB to include microorganisms which have a cell
volume of less than 0.1 mm3, and which retain this volume
irrespective of growth conditions. This description, using
volume as the defining criterion, is particularly useful for
studies of natural communities as a range of cell shapes is
often encountered, and volume provides a measurement of
size that is independent of shape. A list of criteria defining
UMB is also described by Velimirov (2001).
There are two distinct types of cells from the environment which have small volumes: UMB and ultramicrocells
(UMC). UMC are characterized by a larger sized (greater
than 0.1 mm3) reproductive form, and a miniature form
that may have a volume of less than 0.1 mm3. Unlike UMB,
which retain a volume of less than 0.1 mm3 when growing,
the miniature form of UMC is a dormant, stress-resistant
cell, similar to a spore in differentiating bacteria. The
presence of distinct classes of tiny microorganisms is
consistent with studies of natural aquatic and soil
communities. Microscopy observations of environmental
samples reveal cells with volumes of 0.02–0.12 mm3 (Schut
et al., 1997). However, once the environmental samples
have been cultured on agar plates, cells typically have
volumes of the order of 0.34–6.4 mm3. Many of the larger
cells are UMC and in practice UMC are more easily
isolated than UMB. However, since the discovery of true
UMB from marine and soil environments, a key area of
research in contemporary microbial ecology has been to
determine the proportion and physiological state of UMB
and UMC, and to determine the contribution of each class
in the ecology of their respective environments.
The Proliferation of Terms
Marine ecologists describe a size-graded series of plankton
with dimensions ranging from 0.02 mm to 200 cm (Sieburth
et al., 1978). The terms femto-, pico-, nano-and microplankton are used to distinguish size classes with 10-fold
increments between 0.02 mm and 200 mm, respectively
(Table 1). According to these descriptions, UMB most
closely correlate with femtoplankton (or femtobacterioplankton).
A range of other terms have been coined to describe
small microorganisms (Table 1). The prefix recently adopted in a number of fields is ‘nano’; e.g. nan(n)obacteria
(NB), nanobe, nanocell and nanosize. The use of ‘nano’ in
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Ultramicrobacteria
Table 1 Description of terms relating to very small microorganisms, or structures thought to be microorganisms or of
microbial origin
Ultramicrobacteria (UMB) Microorganisms with a cell volume of less than 0.1 mm3 that maintain their size with only minor
changes, irrespective of growth conditions. Observed by light microscopy
Ultramicrocells (UMC)
Smaller forms (usually starved) of microorganisms that are larger when actively growing.
Usually associated with reductive cell division during starvation. Observed by light
microscopy
Nan(n)obacteria (NB)
Possible synonym for UMB. In the literature usually associated with structures in geological
samples with sizes ranging from 0.01 to 0.1 mm. Usually associated with uncultured and
unsubstantiated descriptions of microorganisms. Observed by electron microscopy
Microfossils
Mineral structures with morphologies and dimensions that resemble microbial cells. May be
formed as a result of biomineralization of cell surfaces. Observed by electron microscopy
Femtoplankton
Marine microorganisms 0.02–0.2 mm
Picoplankton
Marine microorganisms 0.2–2.0 mm
Nanoplankton
Marine microorganisms 2.0–20 mm
Microplankton
Marine microorganisms 20–200 mm
Other related terms
Dwarf cells/bacteria, lilliputanian cells, femtobacterioplankton, miniature cells/bacteria,
nanocells, nanosized, nanobe, nano-organisms, nannograins, nanofossils
this context is intended to refer to a size range much smaller
(e.g. tens of nanometers) than in the definition of
nanoplankton (0.2–2 mm). The use of the term ‘nanobacteria’ may derive from Morita (1988) where it was
described as a synonym for UMB. This usage may be
consistent with that of Hamilton (2000), who described NB
as ‘extremely small cellular forms’, and Uwins et al. (1998)
who described nanobes as having a ‘significantly different
size to bacteria and archaea’. However, the term is being
increasingly equated with sizes of objects that may not be
large enough to support life (reviewed in Velimirov, 2001;
Trevors and Psenner, 2001). Most of the recently described
NB come from geological or mineral samples, and none of
these have been conclusively proven to be biological life
forms. Moreover, the term has been used to describe microfossils, for which proof of origin is even more difficult to
establish. In view of this, Southam and Donald (1999)
argue that NB should no longer be used to describe
geomicrobiological formations. Irrespective of personal
view and convention, it is likely that the term ‘nanobacteria’ and other associated ‘nano’ terms will continue to
evolve and continue to be used. This is well illustrated by
the description of the first ‘nano’ archaeal isolate which has
been formerly proposed as Nanoarchaeum equitans, within
the new archaeal kingdom Nanoarchaeota (Huber et al.,
2002).
Marine UMB
UMB appear to be most prevalent in oligotrophic
environments, such as the open ocean, where the concentration of microorganisms is of the order of 105 –
106 cells mL 2 1 (Schut et al., 1997). At least one-third of
the ocean is oligotrophic and the total number of
2
microorganisms in the ocean is predicted to be approximately 1030 cells (reviewed in Cavicchioli et al., 2003). This
implies that UMB make an enormous contribution to
microbial biomass and are likely to play a key role in
biogeochemical cycling of organic and inorganic matter.
According to the definition of a UMB (volume less than
0.1 mm3) only one marine strain has been isolated and
extensively studied: Sphingopyxis alaskensis (formerly
Sphingomonas alaskensis) (Table 2). However, a number
of other marine bacteria that are only marginally larger
have also been isolated, including the photosynthetic
cyanobacterium Prochlorococcus (Table 2). A range of
morphological, physiological and ecological characteristics of UMB and small microorganisms are summarized in
Table 3.
The fact that UMB are often associated with oligotrophic conditions is consistent with these small-sized cells
having a high surface area to volume ratio, which enhances
the opportunity for cells to uptake nutrients from the
environment. By containing less mass, UMB also require
less nutrients than larger cells to produce progeny. UMB
may also be less subject to grazing pressure by larger
predators (e.g. marine protozoa). S. alaskensis fulfils many
of the criteria expected for a model oligotroph (Table 3 and
reviewed in Cavicchioli et al., 2003). These properties are
consistent with a UMB that is adapted to growth under
nutrient limitation and not simply a dormant member of
the population. These physiological properties are corroborated by ecological data demonstrating the presence of
S. alaskensis as one of the most numerically abundant
microorganisms from a number of ocean sites (reviewed in
Cavicchioli et al., 2003).
There are many examples of UMC from the marine
environment. Changes in cell volume for UMC are
associated with growth phase and culture conditions. For
example, the cell volume of Vibrio sp. ANT 300 may vary
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Ultramicrobacteria
by more than an order of magnitude, from 0.048 to
5.9 mm3. The small cells are typically produced by the
process of reductive cell division during periods of nutrient
deprivation. In contrast to UMC, growth and media
conditions have a minimal effect ( 2-fold) on the cell
volume of the UMB S. alaskensis (reviewed in Cavicchioli
et al., 2003).
An important hurdle in this field is the isolation of UMB
from the environment. Insight into overcoming this
problem can be gained from the isolation attempts which
yielded S. alaskensis. Strains from waters near Alaska, the
North Sea and Japan were obtained using an extinction
dilution method. This relies on diluting samples in low
nutrient media until the final dilution tube contains the
most abundant members of the population which are able
to grow in the media. The next phase in the isolation is
storage of cells at low temperature in low-nutrient liquid
medium for up to 12 months, with monthly culturing on to
solid medium. A consistent finding for S. alaskensis was
initial growth of cells only in liquid medium, with the
eventual formation of colonies on solid media. It is
important to establish if these methods can be successfully
used to isolate UMB other than Sphingomonads from
these environments.
Filtration has also been employed, although unsuccessfully, to isolate UMB from the ocean (reviewed in
Cavicchioli et al., 2003; Velimirov, 2001). Isolates obtained
from the filtrates of 0.2-mm filters have been larger than
UMB and are likely to be UMC. This method, however, may
be useful if the filtrates are initially grown in low-nutrient
liquid medium, and then processed in a similar way to that
described for the extinction dilution cultures.
Unique to the oligotrophic waters that have yielded S.
alaskensis, are the remarkable hydrothermal vent systems.
Within the vent fluid a diverse range of thermally adapted
(hyperthermophilic) microorganisms have been isolated.
Most of these are archaea and many of them have unusual
cell shapes and develop a large variation of cell sizes
(Workshop, 1999). Species such as Thermodiscus produce
flat disks with a thickness of only 0.1–0.2 mm, however
their diameter is 0.2–3.0 mm. A variable cell size appears to
be characteristic of these archaea and growth conditions
producing batches with homogeneous cell sizes have not
been established. In effect these archaea may be equated to
extreme UMC. In contrast to the size-variable hyperthermophiles, the recently described archaeal symbiont Nanoarchaeum equitans appears to consistently form coccoidshaped cells with a diameter of 0.4 mm (Huber et al., 2002).
The cells require an actively growing archaeal host (a
species of Ignicoccus) and grow at 70–988C. During late
exponential growth phase, the N. equitans cells detach from
Ignicoccus. In this form they contain a small genome of
approximately 0.5 Mb. While this UMB is not free-living,
it provides a unique opportunity to examine the growth
and survival strategies of an archaeal UMB.
Clinical UMB and NB
Clinically important and obligately parasitic UMB and
UMC have been isolated that belong to a few distinct
taxonomic groups. Most of the intracellular parasites and
vertebrate pathogens are members of the Bdellovibrio,
Brucella, Mycoplasma, Rickettsia and Chlamydia genera.
The latter four groups are parasitic or pathogenic in
humans and animals while the bdellovibrios are parasites
of Gram-negative bacteria. The pathogenic UMB are
fastidious and have a strict host dependency. They have
incomplete cell structures and metabolic pathways, and
possess extremely small genomes (as small as 0.5 Mb). It
will be interesting to compare the minimal genomes of
these parasites with that of the archaeal symbiont N.
equitans. Genes in common will represent those that are
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Ultramicrobacteria
Table 3 Characteristics of selected UMB and small microorganisms
Marine isolates
Characteristics
Sphingopyxis alaskensis
Isolated as an abundant species from ocean waters near Alaska and in the North
Sea and the oligotrophic North Pacific Ocean
High-affinity, broad-specificity nutrient-uptake systems predicted to enable
successful competition in oligotrophic waters
3.2 Mb genome
Single copy of rRNA operon, maximum 2000 ribosomes cell 2 1, minimum 200
ribosomes cell 2 1
High level of regulation of proteome and rates of ribosome, protein and RNA
synthesis
Intrinsically resistant (no starvation cross-protection) to oxidative stress, UV,
ethanol, high temperature and antibiotics. Even higher levels of resistance to
hydrogen peroxide during low, nutrient-limited growth
Most abundant phototrophic organism
Global distribution between 408N and 408S and 0–200 m depth
Smallest known phototroph
1.8 Mb genome
Accounts for more than 50% of chlorophyll and contributes 30–80% of the total
photosynthesis in the oligotrophic oceans
Isolated from same location as S. alaskensis from Resurrection Bay, Alaska
Dilute cytoplasm
3 Mb genome
Utilizes only a few aromatic hydrocarbons and acetate as growth substrates
Kinetic constants for uptake compatible with growth on ambient concentrations
of nutrients in seawater
Hyperthermophilic archaeon
Isolated from a hydrothermal vent
Symbiosis with archaeal host, Ignicoccus sp.
Represents a previously unknown phylum of Archaea
Harbours the smallest archaeal genome of 0.5 Mb
Prochlorococcus marinus
Cycloclasticus oligotrophus
Nanoarchaeum equitans
Soil isolates
Verrucomicrobiales lineage
Pseudomonas sp. & Xanthomonas sp.
Clinical isolates
Mycoplasma genitalium
Chlamydia
Anaerobic heterotroph isolated from anoxic rice paddy soil
Abundant isolate
Aerobic copiotrophic heterotrophs isolated from polluted soil in Japan
Pathogen of animals
Fastidious and host dependent for growth
Anaerobic or facultatively anaerobic
Devoid of cell walls and surrounded by only a plasma membrane
Smallest genome (0.58 Mb) completely sequenced
468 predicted protein encoding genes
Obligate intracellular parasite of animals and insects
1.0–1.2 Mb genome
important for a microorganism with a minimal genome,
irrespective of the cell’s environment.
Mycoplasma and Chlamydia form ‘elementary bodies’ as
small as 0.13 mm (Morowitz, 1967). These forms may
survive out of their host and remain infectious, however
they do not represent the main growing forms of the cells.
For example, actively growing Chlamydia are typically
4
0.6–1.5 mm in diameter. This changeable cell size conforms
to the description of UMC.
NB have been reported from human and cow blood and
commercial cell culture serum (Kajander and Ciftcioglu,
1998). The isolated NB are reported to produce biogenic
apatite on their cell envelope and be responsible for
pathogenic calcification observed in a number of human
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Ultramicrobacteria
diseases, including kidney stone formation. The organisms
causing this are reported to be coccoid with a cell size of
0.2–0.5 mm (0.1–0.7 mm3) when growing. However, smaller
forms (0.05–0.2 mm diameter) have also been described and
these are believed to account for the ability of the NB to
pass through 0.1-mm membrane filters and establish
growing cultures. There is no evidence provided to
substantiate that the smaller forms are complete, live cells.
It is possible that the smaller forms are staining artefacts,
or are UMC that re-establish growth after filtration,
similar to those observed for the marine strains. Alternative explanations for the nanobacterial-induced biomineralization have been provided by Cisar et al. (2000). They
isolated mineralized particles from fetal blood serum,
human saliva and dental plaque. The particles possessed all
of the properties of the NB, including the ability to initiate
biomineralization. However, the authors could find no
evidence that the particles were living entities and
suggested that the biomineralization was dependent on
the nucleating activities of nonliving biological macromolecules. In addition they provided evidence that
Nanobacterium sanguineum, which was implicated as the
aetiological agent of kidney stone formation, was caused
by a problem of experimental design (i.e. polymerase chain
reaction (PCR) artefact or a contaminant organism).
Soil UMB
Cells with volumes conforming to the definition of UMB
have routinely been observed in soil samples. However,
similar to the situation for UMB from aquatic environments, it has been difficult to establish whether the small
cells represent UMB or UMC. In 1997, Janssen et al.
reported the isolation of three UMB from rice paddy soil.
The isolates belong to the order Verrucomicrobiales, are
anaerobic heterotrophs and retain their ultramicro-size
(0.03–0.04 mm3) even when grown in rich media. The
isolation of these strains demonstrates that UMB and not
just UMC are present in the soil. Furthermore, the isolates
were obtained from a high dilution culture series, indicating that they were abundant in the original soil sample.
Iizuka et al. (1998) isolated a number of aerobic,
heterotrophic bacteria with cell volumes ranging from
0.07 to 0.22 mm3, from polluted soil in Japan. Three isolates
with volumes of 0.07–0.08 mm3 and one isolate with a
volume of 0.12 mm3 were reported to be closely related to
the species Pseudomonas lemoignei. Two additional isolates with volumes of 0.11 mm3 were closely related to
Xanthomonas campestris. The bacteria were obtained by
plating soil suspensions directly on to nutrient-rich media
(tryptic soy agar). As a result the strains were described as
copiotrophic (as opposed to oligotrophic). The strains
similar to X. campestris grew rapidly (0.67 h 2 1) in tryptic
soy broth, whereas two of the strains similar to P. lemoignei
grew slowly (less than 0.10 h 2 1) in the rich media but more
rapidly in a defined medium (0.17 h 2 1). The strains
isolated by Janssen et al. (1997) and Iizuka et al. (1998)
provide excellent resources for probing the physiology of
soil UMB.
Subsurface NB
Preliminary evidence has been found for the existence of
NB or nanobes as small as 0.01 mm in diameter in a variety
of geological samples from the deep subsurface. In one
publication by Uwins et al. (1998), filamentous forms
0.020–1.0 mm in diameter were identified from sandstone
samples taken from a depth of 3400–5100 m below the sea
bed where the pressure is 2000 atm and the temperature
115–1708C. They subsequently identified, on freshly
fractured rocks, similar structures that appeared to grow
spontaneously by forming colonies after 2–3 weeks in the
air at 228C. Similar formations also appeared on copper,
polystyrene and glass surfaces. The structures in the newly
formed colonies were filaments with diameters of 0.05–
0.1 mm. The nanobes were described as exhibiting structures similar to fungi with membranes and a cell wall. They
were also reported to be composed mainly of carbon,
nitrogen and oxygen and to contain DNA. Similar deep
subsurface NB have been reported by Hamilton (2000),
however no independent publications have appeared to
verify the existence of the NB from either of these groups.
Folk (1999) reports the existence of NB in a broad
variety of locations from diverse geological to exotic sites
including birdbaths and groundwater pipes. While the
work by Folk and colleagues does not include a search for
biological markers (other than morphology) they provide
hypotheses for how the formations may occur. These
include carbonate precipitations as a result of heterotrophic growth, or bacteria acting as nucleating centres.
The reporting of NB in easily accessible and available
samples (e.g. water pipes) provides the opportunity to
substantiate the existence of NB by growth and other
studies, but to date this has not occurred.
Contributing to the controversy surrounding the existence
of NB as living entities is the fact that the term has been used
to describe microfossils that are thought to be the remains of
NB. Deducing a biological origin for microfossils is clearly a
difficult task as the causative agent may never be cultured. As
a consequence, a range of circumstantial evidence must be
used to derive a hypothesis. This evidence must include more
than just structural morphology as it has been clearly shown
that the inorganic processes may produce structures that
look indistinguishable from biological cells (Southam and
Donald, 1999; Ruiz et al., 2002). Recently, Raman spectroscopy, chemical composition and morphology was combined to determine whether 3.5-billion-year-old microfossils
were of biological origin. Despite the use of these new
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Ultramicrobacteria
methods, an editorial in the journal Nature (2002, 417: 782–
784) described reports from two independent groups which
reached opposite conclusions about the same microfossils. A
similar level of controversy exists for the structures observed
in the Martian meteorite ALH84001 (McKay et al., 1996),
where, for example, the structure of magnetite crystals was
used to infer a biological origin (Thomas-Keprta et al., 2001)
or contradict a biological origin (Barber and Scott, 2002).
It is important to establish whether the NB in geological
samples include live biological forms as this will substantiate the novelty of claims and may guide studies of
microfossils. Moreover, it will clarify the targets for
searches of life on Earth and in extraterrestrial environments (Cavicchioli, 2002).
Minimum Cell Size
Estimates for minimum cell sizes have been proposed
based on observations of the smallest sizes of cells in
environmental and clinical samples, estimations of the
volume occupied by the minimum number of macromolecules required for a living cell, or based primarily on
estimations of the minimum size of a genome. Using these
types of approaches, most reports estimate the minimum
size of a cell to be in the range 0.1–0.3 mm in diameter.
In 1967, Morowitz examined the physical and biochemical factors that may limit the ability of a biological entity
to replicate. He deduced a physical limit for the size of a cell
by calculating the number of atoms present within a
defined size. For example, a cell with a diameter of 0.2 mm
could contain about 4 108 atoms. Underlying principles
for a living cell were then examined and ten generalizations
were derived (Table 4). These included the requirement for a
membrane, and a minimum set of organic molecules. The
average size of essential components (e.g. a molecular
weight of 40 000 for an enzyme) were combined to derive a
formula for the minimum radius of a cell. The formula also
incorporated a factor (n) for biochemical complexity,
based on the number of enzymes required to fulfil cellular
function. Where n is 45, Morowitz calculated the minimum
size to be a sphere with a diameter of about 0.1 mm. When
considering 45 as a minimum level of complexity,
Morowitz noted that additional complexity should be
incorporated to account for the fidelity of replication and
adaptation to environmental changes.
‘Complexity’ described by Morowitz (1967) may be
equated to the number of protein-encoding genes in a
genome. Mycoplasma genitalium has one of the smallest
genomes known and is predicted to encode 468 proteinencoding genes. Using M. genitalium as a model, the
minimum number of genes for a viable genome has been
calculated as 250–350 (Mushegian and Koonin, 1996).
Using this level of complexity and the formula derived by
Morowitz (1967) the minimum cell diameter calculated is
0.16 mm or 0.17 mm, respectively. Genomic approaches
have helped to clarify the minimum number of genes within
known functional classes (e.g. energy generation, translation; reviewed in Trevors and Psenner, 2001). Interestingly,
genomics has also led to the identification of a large
number of uncategorized genes which appear to be
essential for cell viability. Until the role of these
hypothetical yet essential genes are determined in a wide
variety of cell types it will limit the accuracy with which
predictions may be made about the requirements of a cell
with a minimum genome.
The announcement of ovoid features resembling NB in
the Martian meteorite ALH84001 (McKay et al., 1996) has
invigorated debate about the minimum size of cells
(Maniloff et al., 1997). This has also led to a broader
debate in astrobiology, including discussions on the abiotic
conditions tolerated by biological life forms; in particular,
by extremophiles (Cavicchioli, 2002). In October 1998 the
US National Academy of Sciences (NAS) held a workshop
to discuss the size limits of very small microorganisms
(Workshop, 1999). Conclusions from the workshop
included a minimum viable diameter of 0.25–0.3 mm, and
minimum genome size of 250–450 genes. An important
factor highlighted for determining cell size was the number
of ribosomes. This is due to the significant size of a
Table 4 General principles of conventional biological self-replicating systems (derived from Morowitz, 1967)
1. Biological information is structural
2. Functioning biological systems are cellular in nature
3. There is a universal type of membrane structure utilized in all biological systems
4. All populations of replicating biological systems give rise to mutant phenotypes
5. There is a ubiquitous and restricted set of small organic molecules which constitute a very large fraction of the total mass of
all cellular systems
6. Biological energy utilization is accompanied by the hydrolysis of phosphate bonds, usually those of ATP
7. All replicating cells have a genome made of DNA which stores the genetic information of the cell that may be read out in
sequences of RNA and translated into polypeptides
8. All growing cells have ribosomes which are the site of protein synthesis
9. The translation of information from nucleotide language to amino acid language takes place through specific activating
enzymes and tRNA
10. The major component of all cellular systems is water
6
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ribosome which has a diameter of approximately 0.02 mm
and is predicted to be 0.057 mm in diameter when
surrounded by a membrane and cell wall.
The NAS meeting also identified two environments that
may harbour cells with minimum cell sizes. Consistent with
earlier views of Morowitz (1967), nutrient-rich (eutrophic)
environments with low levels of abiotic fluctuations were
thought to allow the evolution of cell types with reduced
genetic and physiological capacity; examples are parasitic
and symbiotic archaea and bacteria (Table 2). Alternatively,
oligotrophic environments may enrich for microorganisms
that maximize nutrient uptake ability by increasing their
surface to volume ratio; examples include the marine
UMB.
The most comprehensive understanding of the physiology of a free-living UMB is the marine bacterium S.
alaskensis from oligotrophic marine environments
(Table 3). Some of the studies describing this UMB may
shed light on the discussion of minimum cell size. S.
alaskensis maintains a maximum of 2000 ribosomes per cell
(reviewed in Cavicchioli et al., 2003). However, with just
200 ribosomes per cell it is able to respond to the addition
of nutrients and reach maximum rates of growth without
any growth lag. In fact, the ribosome content appears to be
excessive at certain stages of growth and ribosomes may
perform functions in addition to protein synthesis. To date,
S. alaskensis sets an upper-limit for the minimum size of a
cell (0.024 mm3) and the minimum number of ribosomes
(200).
The genome size for S. alaskensis is 3.2 Mb (reviewed in
Cavicchioli et al., 2003) which may be larger than expected
for a ‘minimalist’ bacterium. The fact that it maintains a
genome of this size implies that it provides a competitive
advantage enabling it to remain numerically abundant in
the world’s oceans. Its genomic capacity may relate to its
ability to mount significant changes in gene expression,
maintain intrinsically high levels of stress resistance,
immediately respond to nutrient availability, and scavenge
and utilize oligotrophic levels of nutrients. It is also
noteworthy that in the context of the theoretical predictions of minimum cell size (Morowitz, 1967), a complexity
level of 3200 results in the calculation of a diameter of
0.34 mm and volume of 0.021 mm3. This predicted
volume is equivalent to the minimum size observed for
these rod-shaped cells (Table 2; reviewed in Cavicchioli
et al., 2003).
Acknowledgements
Research conducted in the lab of RC which is presented in
this chapter was supported by the Australian Research
Council.
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Further Reading
Folk RL and Lynch LF (2001) Organic matter, putative nannobacteria
and the formation of ooids and hardgrounds. Sedimentology 48: 215–
229.
Hutchison CA, Peterson SN, Steven GR, et al. (1999) Global transposon
mutagenesis and a minimal Mycoplasma genome. Science 286: 2165–
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Genome Biology 2: 2002.1–2002.8.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2003 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net