Oceans cover approximately 70% of Earth’s surface and play fundamental roles in processes that have global ecological and socioeconomic impacts. They are a vital component of the climate system and are suffering and partially attenuating climate change. Life originated in the oceans, which have been the main sites of evolution. Photosynthesis is a critical process that allows life on Earth, and interestingly, half the global primary production occurs in the sea, mostly by planktonic microorganisms that account for only 0.2% of global primary producer biomass. This has many consequences for the functioning of marine ecosystems, influencing carbon and energy fluxes through organisms (food webs), affecting carbon fluxes to deep waters (biological pump), and fine-tuning of all biogeochemical cycles.
Planktonic microorganisms are categorized into classes based on size for operational purposes. Initially, only prokaryotes were included in the smallest class (picoplankton: cells 0.2 to 2 μm) and microbial eukaryotes (protists) were included in the nanoplankton (2 to 20 μm) or microplankton (20 to 200 μm). However, minute eukaryotes were soon detected by epifluorescence microscopy and flow cytometry. Picoeukaryotes are now known to be ubiquitous in surface oceans and form, together with prokaryotes, an ocean’s veil above which larger protists and metazoans might bloom. They exemplify the ecological success of miniaturized cells prepared for independent life by keeping only the minimal cellular components, typically one mitochondrion, one Golgi apparatus, and optionally one chloroplast and flagellum. Molecular methods today offer new tools for studying picoplankton biogeography, activity, biological interactions, and population control mechanisms.
Eukaryotic picoplankton in surface oceans. Annu Rev Microbiol. (2011) 65: 91-110
The eukaryotic picoplankton is a heterogeneous collection of small protists 1 to 3 µm in size populating surface oceans at abundances of 10(^2) to 10(^4) cells ml(^1). Pigmented cells are important primary producers that are at the base of food webs. Colorless cells are mostly bacterivores and play key roles in channeling bacteria to higher trophic levels as well as in nutrient recycling. Mixotrophy and parasitism are relevant but less investigated trophic paths. Molecular surveys of picoeukaryotes have unveiled a large phylogenetic diversity and new lineages, and it is critical to understand the ecological and evolutionary significance of this large and novel diversity. A main goal is to assess how individuals are organized in taxonomic units and how they participate in ecological processes. Picoeukaryotes are convincingly integral members of marine ecosystems in terms of cell abundance, biomass, activity, and diversity and they play crucial roles in food webs and biogeochemical cycles.