Picoeukaryote

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 Mamiellophyceae, 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 10⁷ to 10⁵ liter⁻¹.

Diversity

Autotrophic picoeukaryotes commonly found in nature are members of groups such as the Prasinophyceae (a kind of green algae) and the Haptophyceae. Despite their small size, these organisms have been found to contribute >10% of the total global aquatic net primary productivity. 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. 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. and picoeukaryotes are now known to make up a large fraction of the biomass and productivity in this size fraction in open ocean environments and even in exported carbon in the North Atlantic Bloom. 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.