Taming the smallest predators of the oceans

Abstract

Protists (unicellular eukaryotes) arguably account for most eukaryotic diversity and are central players of the biosphere. Known protist diversity and biology is largely based on cultured strains. Yet, environmental molecular surveys have unveiled entirely novel lineages that, as their prokaryotic counterparts, are essentially uncultured. Culture bias is an important drawback for any microbe-related science and is particularly severe for heterotrophic protists, which depend on organic food sources for growth. Here, we show how ecologically significant bacterivorous protists have been brought into culture by mimicking in situ conditions. Single cells sorted by serial dilution or flow cytometry were inoculated into seawater amended with natural bacterial assemblage at nearly in situ abundances. Strains belonging to lineages only known so far from environmental sequencing were isolated. Among them, Minorisa minuta gen. nov. sp. nov. forms a novel branch within Rhizaria, holding a key evolutionary position, and with an average size of 1.4 μm represents one of the smallest bacterial grazers known to date. It has a worldwide planktonic distribution and can account for 5% of heterotrophic protists communities in coastal waters. Physiological features of this strain can partly explain its success in the environment. Culturing ecologically relevant but elusive protists provide invaluable material for ecophysiology, genomics, ecosystem modeling and evolutionary issues.

Figure 1

(a) ML phylogenetic tree with complete 18S rDNA sequences showing the position of Minorisa minuta within the Cercozoa. The scale bar indicates 0.1 substitutions per position. The coverage of the specific probe CRN02 is shown in grey. (b) SEM image of a Minorisa minuta cell of 1.6 μm in size and possessing a single flagellum. (c) Growth of Minorisa minuta with natural bacteria as prey in a batch culture.

Figure 2

(a) Abundances of Minorisa minuta cells at various sampling sites as estimated by TSA-FISH counts. (b) Abundances of Minorisa minuta cells during a temporal study at Blanes Bay (year 2007), together with the abundance of heterotrophic flagellates, estimated by epifluorescence (log scale). Bars represent the contribution (in %) of Minorisa minuta cells to heterotrophic flagellates.

Figure 3

(a) Correlation between the abundances of Minorisa minuta and natural bacteria during the seasonal study (year 2007) at Blanes Bay. (b) Cell size distribution of Minorisa minuta in these samples (1148 cells measured by TSA-FISH).