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 theDeep Chlorophyll Maximum Layer (DCML)[2] and decrease significantly below.[3] Heterotrophic groups are found at greater depths and for example, in thePacific Ocean, they have been found in the vicinity ofhydrothermal 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]
Autotrophic picoeukaryotes commonly found in nature are members of groups such as thePrasinophyceae[6] (a kind ofgreen algae) and theHaptophyceae.[4][7] Despite their small size, these organisms have been found to contribute >10% of the total global aquatic netprimary productivity.[8] Although much less abundant than cyanobacterialphotosynthetic picoplankton, they have been shown to be as important in terms of biomass and primary production thanpicocyanobacteria.[9] In more oligotrophic environments, such asStation ALOHA, researchers believe that approximately 80% of thechlorophyll αbiomass is due to cells in thepico-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 ofrDNA sequences indicate that heterotrophic oceanic picoeukaryotes belong to lineages such as theAlveolata,stramenopiles,choanoflagellates, andAcantharea.[5] In these lineages, many groups do not have cultured representatives yet. Grazing experiments have demonstrated that novel stramenopile picoeukaryotes are bacterivorous.[4]
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 tocarbon cycling is difficult to assess because they are difficult to separate by techniques such as filtration.[13] Recentfluorescent in situ hybridization (FISH) experiments have shown that picoeukaryotes are fairly abundant in thedeep 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 coastalBathycoccus[15] andOstreococcus 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]
Photosynthetic picoeukaryotes, much like otherplanktonic 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 theXanthophyll 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.
