Oceanic Protists

extraordinary species diversity and variety of interactions of protists in the sea are only now being fully appreciated. Figure 1 shows representative examples of marine protists, and of methods used to visualize these microbes. Protists can be autotrophic or heterotrophic. The former, also called microscopic algae, contain chloroplasts, thrive by photosynthesis, and are at the base of all oceanic food webs, with the exception of deep-sea chemosynthetic ecosystems. There is a general trend for > 20-μm sized phytoplankton (microplankton), such as diatoms and dinoflagellates, to dominate episodically in coastal waters, while in the open ocean 2–20-μm sized cells (nanoplankton), such as coccolithophorids, sporadically form massive blooms that can be seen from space. However, during nonbloom seasons, even smaller cells (picoplankton—prokaryotes and eukaryotes less than a few micrometers in size) typically dominate phytoplankton biomass and production (Li, 2002; Worden et al.,