Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

Abstract

Grazing rate estimates indicate that approximately half of the bacterivory in oligotrophic oceans is due to mixotrophic flagellates. However, most estimations have considered algae as a single group. Here we aimed at opening the black-box of the phytoflagellates (PF) <20 µm. Haptophytes, chlorophytes, cryptophytes and pigmented dinoflagellates were identified using fluorescent in situ hybridization or by standard DAPI-staining. Their fluctuations in abundance, cell-size, biomass, and bacterivory rates were measured through an annual cycle in an oligotrophic coastal system. On average, we were able to assign to these groups 37% of the total pico-phytoflagellates and 65% of the nano-phytoflagellates composition. Chlorophytes were mostly picoplanktonic and they never ingested fluorescently labelled bacteria. About 50% of the PF <20 µm biomass was represented by mixotrophic algae. Pigmented dinoflagellates were the least abundant group with little impact on bacterioplankton. Cryptophytes were quantitatively important during the coldest periods and explained about 4% of total bacterivory. Haptophytes were the most important mixotrophic group: (i) they were mostly represented by cells 3-5 µm in size present year-round; (ii) cell-specific grazing rates were comparable to other bacterivorous non-photosynthetic organisms, regardless of the in situ nutrient availability conditions; (iii) this group could acquire a significant portion of their carbon by ingesting bacteria; and (iv) haptophytes explained on average 40% of the bacterivory exerted by mixotrophic flagellates and were responsible for 9-27% of total bacterivory. Our results, when considered alongside the widespread distribution of haptophytes in the ocean, indicate that they play a key role as bacterivores in marine ecosystems.