Pigmented and heterotrophic nanoflagellates: Abundance and grazing on prokaryotic picoplankton in the ultra-oligotrophic Eastern Mediterranean Sea

Abstract

The abundance and grazing effect of small (<5 μm) pigmented (PNF) and heterotrophic (HNF) nanoflagellates on prokaryotic picoplankton stock (i.e. heterotrophic bacteria (HB) and Synechococcus) were assessed during April 2016 at four stations along a longitudinal transect in the ultra-oligotrophic Eastern Mediterranean Sea and at two selected depths with varying nutrient and light conditions; the 5 m depth representing surface water and the 75 m depth representing the deeper euphotic layer, targeting the Deep Chlorophyll Maximum (DCM) zone at the sampling stations. Cells of size <3 μm dominated the small nanoflagellate community and accounted for 84 ± 8.2% and 73 ± 12.2% (mean ± sd) of total HNF and PNF, respectively. HNF abundance and their grazing effect on prokaryotic picoplankton did not differ between the two depths, while PNF exhibited higher abundance and grazing effect on prokaryotic picoplankton at 75 m compared to the surface (5 m). HNF dominated prokaryotic picoplankton consumption at 5 m accounting for 67–92% and 71–90% of the HB and Synechococcus consumption, respectively. On the other hand, PNF were the dominant grazers of prokaryotic picoplankton at 75 m accounting for 72–80% of the HB consumption and 60–74% of the Synechococcus consumption. HNF and PNF together consumed 4.4 ± 3.2% d−1 (mean ± sd) of the HB standing stock and 19.7 ± 16% d−1 (mean ± sd) of the Synechococcus standing stock. Moreover, a negative relationship between phosphate concentration and ingestion rates of PNF on prokaryotic picoplankton was observed. Pigment analysis showed that the major PNF groups were Prymnesiophytes followed by Pelagophytes/Chrysophytes. Prymnesiophytes was the group mostly associated with mixotrophy. Overall, our results pinpoint the role of small nanoflagellates as prokaryotic picoplankton consumers and the importance of mixotrophy as a trophic mode for PNF in the ultra-oligotrophic E. Mediterranean Sea.