Mucospheres produced by a mixotrophic protist impact ocean carbon cycling

Marine microbes govern ocean productivity and biogeochemistry, regulating global climate. Here the authors describe the sophisticated feeding strategy of a mixotrophic dinoflagellate and show how its behaviour impacts the vertical flux of carbon. Mixotrophic protists (unicellular eukaryotes) that engage in both phototrophy (photosynthesis) and phago-heterotrophy (engulfment of particles)-are predicted to contribute substantially to energy fluxes and marine biogeochemical cycles. However, their impact remains largely unquantified. Here we describe the sophisticated foraging strategy of a widespread mixotrophic dinoflagellate, involving the production of carbon-rich 'mucospheres' that attract, capture, and immobilise microbial prey facilitating their consumption. We provide a detailed characterisation of this previously undescribed behaviour and reveal that it represents an overlooked, yet quantitatively significant mechanism for oceanic carbon fluxes. Following feeding, the mucospheres laden with surplus prey are discarded and sink, contributing an estimated 0.17-1.24 mg m(-2) d(-1) of particulate organic carbon, or 0.02-0.15 Gt to the biological pump annually, which represents 0.1-0.7% of the estimated total export from the euphotic zone. These findings demonstrate how the complex foraging behaviour of a single species of mixotrophic protist can disproportionally contribute to the vertical flux of carbon in the ocean.

Automated summary

The authors describe the sophisticated feeding strategy of a mixotrophic dinoflagellate and its impact on the vertical flux of carbon. The mixotrophic dinoflagellate produces carbon-rich 'mucospheres' that attract and capture microbial prey, contributing significantly to the oceanic carbon flux. This study highlights the importance of complex foraging behavior in the vertical transport of carbon in the ocean.