Vertical and latitudinal distribution of bottom-up and top-down controls of heterotrophic prokaryotes in the Red Sea

Heterotrophic prokaryotes (HPs) represent the largest fraction of living biomass in the ocean. Comprehensively understanding the spatio-temporal variability of their controlling factors remains a challenge in microbial oceanography, especially in little explored low latitude regions such as the Red Sea, one of the hottest and saltiest basins on Earth. In this study, we assessed the vertical (5–1000 m) and latitudinal (16°-27° N) variations in HPs and their bottom-up (resource availability) and top-down controls (protistan grazing and viral lysis) at eight stations along the Red Sea, in three cruises carried out between 2017 and 2019. The decrease in HPs abundances with depth was less pronounced than that of heterotrophic nanoflagellates (HNFs) and viruses. We found that inorganic nutrient and dissolved organic carbon (DOC) concentrations do not vary significantly from north to south, thus suggesting a similar bottom-up control on HPs abundances along the latitudinal gradient. We found significant southward increase in the HP:HNF ratio (r = 0.56, p < 0.0001, n = 140), suggesting that HNFs have a lower impact on their HPs prey in the southern Red Sea. The preference of HNFs for larger HPs cells with depth was found only in the spring cruise. Viral abundances do not show any marked latitudinal gradient but show a significant positive relationship with HPs abundances in the water column in all seasons. The higher linear regression slope found in summer suggests that viruses are more important for HPs mortality in the warmer months. This study strengthens the importance of top-down controls in maintaining lower HPs stocks in the Red Sea and suggests that both latitudinal and seasonal variations have minor but measurable roles.