Fluxes between phytoplankton and heterotrophic bacteria maintain aquatic carbon cycles, but mechanistic understanding and quantitative determinations of fluxes remain scarce. We applied the mechanistic inference method FLUXNET to a seven-year time-series of the Western English Channel (WEC), and estimated time-variable carbon fluxes between 135 phytoplankton operational taxonomical units (OTUs, 18S) and 157 bacterial amplicon sequences variants (ASVs, 16S), as well as bacterial utilization patterns of DOC species for phytoplankton spring and summer blooms, as well as bacterial summer blooms. In spring blooms, fluxes from phytoplankton to bacteria dominated, and a succession from phytoplankton to consecutive bacterial ASVs occurred. Vice versa, successions of abundant bacterial ASVs in bacterial summer blooms were followed by an increase of phytoplankton, culminating in phytoplankton summer blooms. Both types of summer blooms, however, were characterized by high bacteria to bacteria carbon fluxes, with the same magnitude as fluxes from phytoplankton to bacteria in bacterial summer blooms. Utilization patterns of hypothetical DOC species suggest that carbon processing in periods with high internal heterotrophic recycling are dominated by generalist utilization, whereas in periods with high phytoplankton to bacteria fluxes, specialist strategies dominate. In conclusion, we propose that the mostly neglected carbon flux between different heterotrophic bacteria periodically exceeds the flux between phytoplankton and bacteria, and substantially contributes to the WEC carbon cycle.

Primary Presenter: Falk Eigemann, Technical University Berlin (falkeigemann@gmail.com)

Authors:

Falk Eigemann, Technical University of Berlin  (eigemann@tu-berlin.de)

Karen Tait, Plymouth Marine Laboratory  (ktait@pml.ac.uk)

Ben Temperton, University of Exeter  (b.temperton@exeter.ac.uk)

Ferdi Hellweger, Technical University Berlin  (ferdi.hellweger@tu-berlin.de)