Grazing rates and functional diversity of uncultured heterotrophic flagellates

MASSANA, R.; UNREIN, F.; RODRÍGUEZ-MARTÍNEZ, R.; FORN, I.; LEFORT, T.; PINHASSI, J.; NOT, F.

ISME JOURNAL

NATURE PUBLISHING GROUP

Lugar: Londres; Año: 2009 vol. 3 p. 588 - 596

1751-7362

Aquatic assemblages of heterotrophic protists are very diverse and formed primarily by organismsthat remain uncultured. Thus, a critical issue is assigning a functional role to this unknown biota.Here we measured grazing rates of uncultured protists in natural assemblages (detected byfluorescent in situ hybridization (FISH)), and investigated their prey preference over several bacterialtracers in short-term ingestion experiments. These included fluorescently labeled bacteria (FLB) andtwo strains of the Roseobacter lineage and the family Flavobacteriaceae, of various cell sizes, whichwere offered alive and detected by catalyzed reporter deposition-FISH after the ingestion. Weobtained grazing rates of the globally distributed and uncultured marine stramenopiles groups 4 and1 (MAST-4 and MAST-1C) flagellates. Using FLB, the grazing rate of MAST-4 was somewhat lowerthan whole community rates, consistent with its small size. MAST-4 preferred live bacteria, andclearance rates with these tracers were up to 2 nl per predator per h. On the other hand, grazing ratesof MAST-1C differed strongly depending on the tracer prey used, and these differences could not beexplained by cell viability. Highest rates were obtained using FLB whereas the flavobacteria strainwas hardly ingested. Possible explanations would be that the small flavobacteria cells were outsidethe effective size range of edible prey, or that MAST-1C selects against this particular strain. Ouroriginal dual FISH protocol applied to grazing experiments reveals important functional differencesbetween distinct uncultured protists and offers the possibility to disentangle the complexity ofmicrobial food webs.