Suspended particulate matter (SPM) is a ubiquitous and important component of rivers and es-tuaries. It mainly consists of terrestrial and (in estuarine stretches) also of marine organic matter (OM). Aggregates, as part of SPM, consist of mineral particles, non-living OM (detritus), phytoplankton, bacteria, archaea and other heterotrophic microbes. Due to the co-occurring mixtures of labile and recalcitrant OM, aggregates are hotspots of C turnover in aquatic ecosystems. In analogy to rhizospheres in terrestrial ecosystems, the microzones with intense C turnover around phytoplankton and aggregates are recognized as phycospheres. Understanding microbial OM degradation in phycospheres is challenging since abiotic conditions within aggregates often differ from the surrounding water following steep redox gradients. High community respiration rates in phycospheres due to grazing by zooplankton may substantially contribute to the formation of sub- to anoxic microzones and profoundly affect C turnover and GHG emissions.

While aggregate formation and microbial colonization along estuarine gradients have been relatively well studied, little is known on biotic interactions in phycospheres, on the role of redox gradients in aggregate processes and on implications of aggregate processes and trophic C transfer for C cycling and GHG fluxes in estuarine waters.

Doctoral Projects within RT B will address the following research questions:

• How do phytoplankton-microbe interactions in the phycosphere control C turnover?
• How are trophic interactions affecting C cycling and GHG dynamics?
• How do phytoplankton-microbe interactions and trophic interactions affect C cycling on different scales?