Red Sea heterotrophic bacteria and DOM sources

In spite of their key role in biogeochemical cycling in tropical waters, heterotrophic bacterioplankton dynamics have been poorly investigated in the Red Sea. To fill that gap, we conducted 4 seasonal incubations of unfiltered (Community treatment) and predator-free seawater (Filtered treatment) at three coastal sites characterized by different dominant DOM sources: Seagrass, Mangrove and Phytoplankton. We assessed the variability of heterotrophic bacterioplankton standing stocks, growth and physiological status by flow cytometry and community composition by 16S rRNA sequencing. Chlorophyll a, bulk DOM concentration and optical properties were also measured. Bacterial abundances at the Seagrass site (6.93 ± 0.30 x 105 cells mL-1) were consistently higher than at the other sites, coincident with maximum DOC concentration (104-137µm C L-1). Growth rates were significantly lower in the Community than in the Filtered treatment regardless of the site and peaked at the Mangrove site (1.11 ± 0.09 d-1). The Filtered to Community carrying capacity ratio (0.85-1.64), showed consistently low values at the Seagrass site (1.05 ± 0.05) while removal of predators boosted growth at Mangrove and Phytoplankton sites. Since bacterial diversity was similar at the Mangrove and Seagrass sites we interpret the unexpected lack of bacterial response to predators removal to a possible antimicrobial effect of seagrasses exudates. DOM quality (using C:N ratio as a proxy) consistently affected specific growth rates, contrary to temperature. Our data confirm the hypothesis that top-down control constrained bacterial standing stocks while specific growth rates were bottom-up controlled in representative Red Sea coastal environments.