MEPS 588:43-57 (2018)  -  DOI: https://doi.org/10.3354/meps12435

ABSTRACT: The ability to combine autotrophic and heterotrophic nutritional modes by planktonic protists may significantly influence nutrient dynamics in the surface ocean. The extent and significance of organic resource utilization by small photosynthetic eukaryotes remain poorly explored. In order to evaluate the importance of autotrophy and heterotrophy within the most abundant pico-nanoeukaryotic phytoplankton groups in surface waters of a coastal upwelling area of NW of Spain, we combined catalyzed reported deposition-fluorescence in situ hybridization and microautoradiography with radiolabeled bicarbonate or amino acids. Over an annual cycle in 2015, picoplanktonic taxa represented more than 55% of the pico-nanoeukaryotic cells. The genus Ostreococcus dominated both in terms of abundance and activity during the nutrient-limiting spring to summer stratification period, when the utilization of amino acids was confirmed for dinoflagellates (22 to 33% of total dinoflagellate cells), Haptophyta (23 to 88% of total Haptophyta cells) and the picoeukaryotic genera Micromonas (9 to 38% of total Micromonas cells), Ostreococcus (10 to 38% of total Ostreococcus cells) and Bathycoccus (19 to 22% of total Bathycoccus cells). Our data suggest the adoption of different nutritional strategies among the smallest plastidic protists, which likely implies contrasting functional roles and competitive abilities within the picoeukaryotic phytoplankton.

KEY WORDS: Eukaryotic picophytoplankton · Autotrophy · Heterotrophy · CARD-FISH · Microautoradiography