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. Microbial consumption of phytoplankton-derived organiccarbon in the pelagic food web is an important component of the global Ccycle. We studied C cycling in two phytoplankton–bacteria systems(non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in twocomplementary experiments. In the first experiment we grew phytoplankton andbacteria in nutrient-replete conditions and followed C processing at earlyexponential growth phase and twice later when the community had growndenser. Cell-specific primary production and total community respirationwere up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based onthe optical signals, accumulating dissolved organic C (DOC) was degradedmore in the R. marina treatments, and the rate of bacterial production to primaryproduction was higher. Thus, the flow of C from phytoplankton to bacteriawas relatively higher in R. marina treatments than in A. malmogiense treatments, which was furthersupported by faster 14C transfer from phytoplankton to bacterialbiomass. In the second experiment we investigated consumption of thephytoplankton-derived DOC by bacteria. DOC consumption and transformation,bacterial production, and bacterial respiration were all higher in R. marinatreatments. In both experiments A. malmogiense supported a bacterial communitypredominated by bacteria specialized in the utilization of less labile DOC(class Bacteroidia), whereas R. marina supported a community predominated bycopiotrophic Alpha- and Gammaproteobacteria. Our findings suggest that largedinoflagellates cycle relatively more C between phytoplankton biomass andthe inorganic C pool, whereas small cryptophytes direct relatively more C tothe microbial loop.

Contrasting patterns of carbon cycling and dissolved organic matter processing in two phytoplankton–bacteria communities