Bacterivory by autotrophic nanoflagellates in an oligotrophic coastal system

UNREIN, F.; GASOL, J. M.; MASSANA, R.

Santiago de Compostela (España)

Congreso; ASLO Summer Meeting 2005; 2005

We studied the relevance of mixotrophic flagellates as grazers of bacterioplankton in an oligotrophic coastal Mediterranean site (Blanes Bay). Grazing experiments were conducted using fluorescentely labelled bacteria (FLB) and stablished at monthly intervals during one year. Long-term (disappearance) and short-term (FLB uptake) incubations were performed in a culture chamber mimicking in situ light and temperature. Phototrophic (i.e. mixotrophic) flagellates (PF) showed lower cell-specific grazing rates than heterotrophic flagellates (HF). However, because of their higher abundance, the overall impact on the bacterial community by PF was higher, accounting on average for 56% of total bacterivory. This high impact was relatively constant throughout the year. Nevertheless, dissolved phosphorus concentrations were significantly negatively correlated with the grazing impact of mixotrophs, which suggests that flagellate algae might graze on bacteria to obtain phosphorous when this essential nutrient is limitating. On the contrary, the grazing rates of HF showed a pattern with marked seasonality, likely related to water temperature, suggesting this variable to control the rates of grazing on bacteria by the purely heterotrophic component of the flagellate assemblage.