Different coupling of dissolved amino acid, protein, and carbohydrate turnover to heterotrophic picoplankton production in the Southern Ocean in austral summer and fall
Limnol. Oceanogr., 52(1), 2007, 85-95 | DOI: 10.4319/lo.2007.52.1.0085
ABSTRACT: We assessed growth dynamics of heterotrophic picoplankton and concentrations and turnover of dissolved protein, amino acids, and neutral monosaccharides in the Atlantic sector of the Southern Ocean in austral summer (December-January) and fall (March-May). Phytoplankton biomass (chlorophyll a) and biomass production of heterotrophic picoplankton in summer was twice as high as in fall. This difference was also reflected in protein turnover rate constants and in concentrations of dissolved combined neutral polysaccharides (DCCHO). Turnover rate constants of dissolved free amino acids (DFAA) were in the same range in both seasons, but turnover rate constants of glucose were higher in fall as compared to summer. In summer, dissolved protein was the major substrate for growth of heterotrophic picoplankton, followed by DFAA and dissolved free neutral monosaccharides (DFCHO). During summer, heterotrophic picoplankton production was closely correlated to concentrations and incorporation of dissolved protein. In fall, heterotrophic picoplankton production was only significantly correlated to glucose turnover rate constants. The latter were also inversely correlated to DCCHO concentrations in fall. The reduced supply of organic substrates by phytoplankton in fall not only resulted in an equal reduction of heterotrophic picoplankton production but also in a shift of the supply in dissolved protein, DFAA, and DFCHO to heterotrophic picoplankton. Dissolved protein was the major substrate for heterotrophic picoplankton growth in summer, but in fall, when supply of dissolved protein was reduced, DFAA and DFCHO were relatively more important substrates.