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

<p>Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton–bacteria systems (non-axenic cultures of a dinoflagellate <i>Apocalathium malmogiense</i> and a cryptophyte <i>Rhodomonas marina</i>) in two complementary experiments. In the first experiment we grew phytoplankton and bacteria in nutrient-replete conditions and followed C processing at early exponential growth phase and twice later when the community had grown denser. Cell-specific primary production and total community respiration were up to 4 and 7 times higher, respectively, in the <i>A. malmogiense</i> treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the <i>R. marina</i> treatments, and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in <i>R. marina</i> treatments than in <i>A. malmogiense</i> treatments, which was further supported by faster <span class="inline-formula">¹⁴</span>C transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production, and bacterial respiration were all higher in <i>R. marina</i> treatments. In both experiments <i>A. malmogiense</i> supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia), whereas <i>R. marina</i> supported a community predominated by copiotrophic Alpha- and Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool, whereas small cryptophytes direct relatively more C to the microbial loop.</p>

Abstract licence: CC BY

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