Grazing of a heterotrophic nanoflagellate on prokaryote and eukaryote prey: ingestion rates and gross growth efficiency

Abstract

Heterotrophic nanoflagellates (HNANs) play a pivotal role as consumers of picoplankton, remineralizers and carbon vectors, yet knowledge on how prey quantity and quality affect HNAN physiology remains limited. In a series of grazing experiments using an uncharacterized member of the HNAN assemblage, we found that growth (μ) and ingestion rate (IR) varied when offering heterotrophic bacteria (HB), Synechococcus spp. (Syn), Ostreococcus lucimarinus (Ost) or a combination of all 3 prey types. Highest average μ rates (1.8 d-1) were detected on HB at densities of ~106 cells ml-1 and maximum IR on Syn (485 pg C d-1) at ~106 cells ml-1. Independent of prey type, flagellate μ increased with IR up to ~50 pg C d-1. A relatively low P-content in Ost was linked to shifts in C:N:P ratios of the HNAN in the single-prey experiment and when Ost was offered as part of the mixed assemblage. Presented with a mixed diet, the highest contribution to daily C intake came from Ost with 50%, followed by HB with 46% and Syn with only 4%. C-based gross growth efficiencies (GGEs) were higher when feeding on HB and mixed prey, compared to both picophototrophs, while N- and P-based GGEs in mixed prey treatments markedly exceeded those when feeding on any single prey. The findings in this study corroborate the importance of investigating the biogeochemical role of HNANs in relation to prey availability and quality to refine estimates of energy transfer within the microbial loop.