The Role of Mixotrophs in Oceanic Carbon Accumulation

Mixotrophic plankton - organisms capable of combining photosynthesis with the ingestion of prey - absorb and sequester more carbon in the Ocean than organisms that rely on only one of these nutritional strategies. This increase, however, over very long timescales, may be partially offset by interactions with Ocean circulation, particularly in the North Atlantic.

These findings emerge from a study recently published in Biogeosciences by an international research group involving OGS, the School of Ocean and Earth Science (University of Southampton, UK), and the Earth, Ocean and Ecological Sciences Department (University of Liverpool, UK). The team of scientists set out to investigate the effects of mixotrophy on the Ocean’s carbon storage capacity by considering a much longer time period than in previous studies.

Earlier research had shown that the activity of mixotrophic organisms contributes to increasing both their average size and the export of carbon to the Ocean interior: the additional resources provided by prey enable mixotrophs to better sustain photosynthesis, under the same availability of limiting inorganic nutrients, compared with models that include only autotrophs and heterotrophs. However, until now these simulations had always been limited to timescales on the order of decades.

By using low-resolution mathematical modeling software, it was instead possible to analyze the impact of mixotrophs over a 10,000-year timespan. The results showed that the increase in carbon transfer to the Ocean interior driven by these organisms occurs very rapidly and remains stable over the long term. Over thousands of years, however, an opposing effect also emerges: the amount of carbon and nutrients stored in the deep Ocean by surface waters sinking in the North Atlantic decreases.

This does not occur because the carbon uptake efficiency of mixotrophs declines, but because, over longer timescales, the responses of a complex system such as ocean circulation come into play, partially counterbalancing the enhanced carbon uptake.

Image: Pexels/Pixabay