Abstract
Despite the key role of heterotrophic bacterioplankton in the biogeochemistry of tropical coastal waters, their dynamics have been poorly investigated in relation to the different dissolved organic matter (DOM) pools usually available. In this study we conducted four seasonal incubations of unfiltered and predator-free seawater (Community and Filtered treatment, respectively) at three Red Sea coastal sites characterized by different dominant DOM sources: Seagrass, Mangrove, and Phytoplankton. Bacterial abundance, growth and physiological status were assessed by flow cytometry and community composition by 16S rRNA gene amplicons. The Seagrass site showed the highest initial abundances (6.93 ± 0.30 × 105 cells mL–1), coincident with maximum DOC concentrations (>100 μmol C L–1), while growth rates peaked at the Mangrove site (1.11 ± 0.09 d–1) and were consistently higher in the Filtered treatment. The ratio between the Filtered and Community maximum bacterial abundance (a proxy for top-down control by protistan grazers) showed minimum values at the Seagrass site (1.05 ± 0.05) and maximum at the Phytoplankton site (1.24 ± 0.30), suggesting protistan grazing was higher in open waters, especially in the first half of the year. Since the Mangrove and Seagrass sites shared a similar bacterial diversity, the unexpected lack of bacterial response to predators removal at the latter site should be explained by differences in DOM characteristics. Nitrogen-rich DOM and fluorescent protein-like components were significantly associated with enhanced specific growth rates along the inshore-offshore gradient. Our study confirms the hypotheses that top–down factors control bacterial standing stocks while specific growth rates are bottom-up controlled in representative Red Sea shallow, oligotrophic ecosystems.
Highlights
In the tropical, oligotrophic oceans the role of small autotrophic and heterotrophic prokaryotes is fundamental for carbon budgets and fluxes (Bock et al, 2018; Richardson, 2019)
In order to improve our knowledge about how different autochthonous dissolved organic matter (DOM) sources affect bacterioplankton metabolism in the central Red Sea, we conducted four short-term (4–6 days) incubations of epipelagic samples from three sites, each dominated by one of the major DOM sources found in tropical regions, covering a full seasonal cycle, from February 2016 to December 2017
Total chlorophyll a concentrations were low to moderate (0.10–1.35 μg L−1), with the maximum observed at the Seagrass site in June and the lowest values systematically detected at the Phytoplankton site (Table 1)
Summary
Oligotrophic oceans the role of small autotrophic (mostly cyanobacteria) and heterotrophic (bacteria and archaea) prokaryotes is fundamental for carbon budgets and fluxes (Bock et al, 2018; Richardson, 2019). Heterotrophic prokaryotes [hereafter heterotrophic bacteria since archaea represent a minor contribution in surface waters (Karner et al, 2001)] are the main recycling organisms of dissolved organic matter (DOM) Through their uptake of DOM, derived from primary producers and their subsequent consumption by protistan grazers, bacteria are an important link for higher organisms in marine food webs, with consequences for biogeochemical cycling (Azam et al, 1983; Azam and Malfatti, 2007; Pomeroy et al, 2007; Kirchman et al, 2009). DOM can be transported, and autochthonous sources can work as allochthonous sources in nearby regions (Alongi and Mukhopadhyay, 2015) In this regard, changes in the relative contribution of the different autochthonous (phytoplankton or macrophytes) sources of DOM (Stepanauskas et al, 2000; Säwström et al, 2016) can affect the carbon flux in tropical marine food webs, especially through microbes (Lønborg et al, 2018)
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