Contribution of different bacterial groups in the carbon flow through the microbial food web

Bacteria greatly contributes to the transfer of dissolved organic carbon to the upper trophic levels. Nevertheless, little is known about the contribution of specific bacterial groups. Here, we conducted three seasonal experiments (both, microcosm and mesocosm) in the coastal area of the central Adriatic with the aim of determining the contribution of selected bacterial groups to carbon flow through the microbial food web. We assessed the growth rates of four bacterial groups (Bacteroidota, SAR11, Rhodobacteraceae, and Gammaproteobacteria), as well as aerobic anoxygenic phototrophs, and their contributions to biomass production and carbon transfer to upper trophic levels. For the first time, we reported a significant contribution of aerobic anoxygenic phototrophs to the carbon flow, especially during the summer.

Under initial winter conditions, SAR11 contributed the most to bacterial biomass production, while Gammaproteobacteria were the primary contributors to grazing loss and served as the preferred prey. As temperature increased, the contribution of Gammaproteobacteria and Bacteroidota to bacterial biomass production also increased. Regarding grazing loss, SAR11 and Bacteroidota were the dominant contributors in spring, whereas Gammaproteobacteria and Rhodobacteraceae played the most significant role during summer. Under nutrient-enriched conditions, SAR11 contributed the most to both bacterial biomass production and grazing loss during winter. However, with rising temperatures, Gammaproteobacteria, Rhodobacteraceae and Bacteroidetes became the primary contributors to bacterial biomass production and Gammaproteobacteria and Bacteroidetes in grazing loss. Our estimates suggest that Gammaproteobacteria and the SAR11 clade were more significantly impacted by grazing, whereas Bacteroidota and Rhodobacteraceae were equally influenced by both nutrient availability and grazing pressure.

Therefore, the results obtained in this study are useful for evaluating the potential contributions of diverse bacterial groups to carbon cycling in marine ecosystems under changing environmental conditions, particularly during shifts in seawater temperature. We further analyzed the dynamics of bacterial community composition in relation to varying environmental factors.