Role of diatom-attached choanoilagellates of the genus Salpingoeca as pelagic bacterivores

We studied the ecological role of choanoflagellates of the genus Salpingoeca (possibly S. amphoridium) attached to the colonial diatoms Asterionella formosa and Fragilaria crotonensis in the Rimov reservoir (South Bohemia). The choanoflagellate was found only on these 2 species of diatoms (0 to 35 choanoflagellate cells per diatom colony). It is clearly identifiable in natural samples, and is a highly efficient planktonic bacterivore. In 2 field experiments (Expt 1 in June 2000 and Expt 2 in September 2002) we estimated in situ rates of bacterivory using fluorescently labeled bacteria (FLB) as tracers. There was a linear increase in the number of FLB per choanoflagellate during the first 30 min of incubation, with only 8 to 16 % of individuals within the natural populations displaying no ingestion of FLB. Taxon-specific bacteria uptake rate of Salpingoeca spp. was 35 to 65 bacteria cell(-1) h(-1). In Expt 2, by manipulating phosphorus (P) and organic carbon availability (glucose), we also examined how alga host and bacterial prey abundances can affect the dynamics of Salpingoeca and of other bacterivorous heterotrophic nanoflagellates (HNF). P additions increased the growth of F. crotonensis and subsequently also that of the attached Salpingoeca sp. In contrast, glucose addition produced a marked development of small, free-swimming chrysomonads which competed with the choanoflagellate for the bacteria. Overall, across all experimental treatments Salpingoeca spp. grazing accounted for 11 to 64 % of the total protistan grazing and for 6 to 49% of the daily removal rate of the bacterial standing stock. Our data indicate that given a sufficient abundance of suitable host algae, which possibly provide a refuge from potential grazers, and efficient grazing of free-swimming HNF by zooplankton, this single choanoflagellate taxon can temporarily constitute the most important pelagic bacterivore in the reservoir.