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)[2] and decrease significantly below.[3] 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.[1] They show high phylogenetic diversity[4][5] and high variability in global cell concentrations, ranging from 107 to 105liter−1.[3]
Diversity
Autotrophic picoeukaryotes commonly found in nature are members of groups such as the
Prasinophyceae[6] (a kind of
green algae) and the
Haptophyceae.[4][7] Despite their small size, these organisms have been found to contribute >10% of the total global aquatic net
primary productivity.[8] Although much less abundant than cyanobacterial
photosynthetic picoplankton, they have been shown to be as important in terms of biomass and primary production than
picocyanobacteria.[9] In more oligotrophic environments, such as
Station ALOHA, researchers believe that approximately 80% of the
chlorophyll α
biomass is due to cells in the
pico-size range.[2] and picoeukaryotes are now known to make up a large fraction of the biomass and productivity in this size fraction in open ocean environments[10] and even in exported carbon in the North Atlantic Bloom.[11]
Analysis of
rDNA sequences indicate that heterotrophic oceanic picoeukaryotes belong to lineages such as the
Alveolata,
stramenopiles,
choanoflagellates, and
Acantharea.[5] In these lineages, many groups do not have cultured representatives yet. Grazing experiments have demonstrated that novel stramenopile picoeukaryotes are bacterivorous.[4]
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.[12] Their contribution to
carbon cycling is difficult to assess because they are difficult to separate by techniques such as filtration.[13] Recent
fluorescent in situ hybridization (FISH) experiments have shown that picoeukaryotes are fairly abundant in the
deep sea.[1] Increased resolution with the development of better FISH techniques indicates that study and detection should become easier.[14] Additionally, qPCR has been a valuable approach for delineating and quantifying the different species, e.g. oceanic and coastal Bathycoccus[15] and Ostreococcus species.[16] Research has also shown that picoeukaryotes have a strong correlation with chlorophyll concentrations in both meso-autotrophic reservoirs and hypereutrophic reservoirs.[17] 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.[8]
Biological characteristics
Photosynthetic picoeukaryotes, much like other
planktonic species in the ocean photic zone, 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.[18] However, there are also many types of non-photosynthetic picoeukaryotes that extend into the deep ocean and do not have these biochemical pathways.[19]
^
abcMoreira, D.; P. Lopez-Garcia (2002). "The molecular ecology of microbial eukaryotes unveils a hidden world". Trends in Microbiology. 10 (1): 31–38.
doi:
10.1016/S0966-842X(01)02257-0.
PMID11755083.
^Worden, A. Z. et al. (2004). Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component. Limnology and Oceanography49: 168-79.
^Not, Fabrice; Gausling, Rudolf; Azam, Farooq; Heidelberg, John F.; Worden, Alexandra Z. (2007). "Vertical distribution of picoeukaryotic diversity in the Sargasso Sea". Environmental Microbiology. 9 (5): 1233–1252.
Bibcode:
2007EnvMi...9.1233N.
doi:
10.1111/j.1462-2920.2007.01247.x.
PMID17472637.
External links
MicrobeWiki A site on a biology Wiki run by Kenyon College