Picoeukaryotes are
picoplanktonic eukaryotic organisms that range in size from 0.2
– 2.0 µm. They are distributed throughout the world’s
marine and
freshwater ecosystems
and constitute a significant contribution to
autotrophic communities. Though the
SI prefix
pico- might imply an
organism smaller than
atomic size, the
term was likely used to avoid confusion with existing
size classifications of
plankton.
Characteristics
Cell structure
Picoeukaryotes can be either
autotrophic
and
heterotrophic, and usually contain
a minimal number of
organelles. For
example,
Ostreococcus tauri,
an autotrophic picoeukaryote belonging to the class
Prasinophyceae, contains only the
nucleus, one
mitochondrion and one
chloroplast, tightly packed within a
cell membrane. Members of a heterotrophic
class, the
Bicosoecida, similarly
contain only two mitochondria, one food
vacuole and a nucleus.
Distributions
These organisms are found throughout the water columns. Autotrophic
picoeukaryotes are restricted to the upper 100-200 m (the layer
that receives light) and are often characterized by a sharp cell
maximum near the Deep Chlorophyll Maximum Layer (DCML) and decrease
significantly below.
Heterotrophic groups are found at greater
depths and for example, in the Pacific Ocean
, they have been found in the vicinity of hydrothermal vents at depths up to
2000-2550 m. Some heterotrophic lineages are found,
unstratified, at all depths from the surface down to 3000 m. They
show high phylogenetic diversity and high variability in global
cell concentrations, ranging from 10
7 to 10
5
liter-1.
Diversity
Autotrophic picoeukaryotes are members
of groups such as the
Prasinophyceae
(a kind of
green algae) and the
Haptophyceae. Regardless of their small size,
these organisms have been found to contribute at least 10% of the
total global aquatic net
primary
productivity. In more oligotrophic environments, such around
the Hawaiian station of ALOHA, researchers believe that
approximately 80% of the
chlorophyll α
biomass is due to cells in the
pico-size range.Analysis of
rDNA
sequences indicate that heterotrophic oceanic picoeukaryotes belong
to lineages such as the
Alveolata,
stramenopiles,
choanoflagellates, and
Acantharea. In these lineages, many groups do not
have cultured representatives yet. Grazing experiments have
demonstrated that novel stramenopile picoeukaryotes are
bacterivorous.
Ecology
Since the size of these organisms determines how they interact with
their environment, it is no surprise that they are not known to
form significant sinking organic aggregates. Their contribution to
carbon cycling is difficult to assess
because they are difficult to separate by techniques such as
filtration. Recent
fluorescent in situ
hybridization (FISH) experiments have shown that picoeukaryotes
are fairly abundant in the
deep sea.
Increased resolution with the development of better FISH techniques
indicates that study and detection should become easier. Research
has also shown that picoeukaryotes have a strong correlation with
chlorophyll concentrations in both meso-autotrophic reservoirs and
hypereutrophic reservoirs. Moreover, nitrogen enrichment
experiments suggest that picoeukaryotes have an advantage over
larger cells when it comes to acquiring nutrients because of their
large surface area per unit volume. They have exhibited more
effectiveness in the uptake of photons and nutrient from
low-resource environments.
Biological characteristics
Picoeukaryotes, much like other
planktonic species, are exposed to light
variations during the diel cycle and due to vertical displacement
in the mixed layer of the water column. They have specialized
biological reactions to help them deal with excessive densities of
light, such as the
Xanthophyll
cycle.
See also
Notes
External links
- MicrobeWiki A site on a biology Wiki run by
Kenyon College
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