Are fish and bacteria like Fred and Barney?

Dissolved organic carbon (DOC) is receiving increasing research interest because of its contribution to carbon sequestration (CS). Microbes enhance CS through primary production and the microbial carbon pump (i.e. transformation of bioavailable into persistent DOC). In a mesocosm experiment, Limberger et al. (2019) found that sticklebacks increase microbial production and reprocessing of DOC. This idea was supported by positive effects on the biomass of phytoplankton and the bacterial classes Cytophagia and Alphaproteobacteria, which are competent degraders. Chlorophyll-a was used as a proxy for phytoplankton biomass. Bacteria were counted using flowcytometry and identified through Illumina sequencing of the 16S rRNA gene’s V3 region. Freshness of DOC was estimated using spectrophotometry.

This research presents the possibility for trophic cascades to increase microbial biomass, which in turn affects the composition of the DOC pool. In other words, predation may be important in the global carbon cycle. However, it is questionable whether the results are meaningful for global CS. Firstly, Limberger et al. (2019) only studied one species, from which it is difficult to make predictions about fish in general. Secondly, there was actually no measurable increase in DOC in the presence of fish (only the freshness of DOC was significantly higher)!  

Limberger, R., Birtel, J., Peter, H., Catalán, N., da Silva Farias, D., Best, R. J., Brodersen, J., Bürgmann, H. & Matthews, B. (2019). Predator‐induced changes in dissolved organic carbon dynamics. Oikos, 128(3), 430-440.