Heterotrophic bacterioplankton responses in coral- and algae-dominated Red Sea reefs show they might benefit from future regime shift.

In coral reefs, dissolved organic matter (DOM) cycling is a critical process for sustaining ecosystem functioning. However, global and local stressors have caused persistent shifts from coral- to algae-dominated benthic communities. The influence of such phase shifts on DOM nature and its utilization by heterotrophic bacterioplankton remains poorly studied. Every second month for one year, we retrieved seawater samples enriched in DOM produced by coral- and algae-dominated benthic communities in a central Red Sea reef during a full annual cycle. Seawater incubations were conducted in the laboratory under in situ temperature and light conditions by inoculating enriched DOM samples with bacterial assemblages collected in the surrounding waters. Dissolved organic carbon (DOC) concentrations were higher in the warmer months (May-September) in both communities, resulting in higher specific growth rates and bacterial growth efficiencies (BGE). However, these high summer values were significantly enhanced in algal-DOM relative to coral-DOM, suggesting the potential for bacterioplankton biomass increase in reefs with algae replacing healthy coral cover under warmer conditions. The potential exacerbation of heterotrophic bacterial activity in the ongoing widespread regime shift from coral- to algae-dominated communities may have detrimental consequences for the overall health of tropical coral reefs.

Heterotrophic Bacterioplankton Growth and Physiological Properties in Red Sea Tropical Shallow Ecosystems With Different Dissolved Organic Matter Sources.

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.

Low Abundances but High Growth Rates of Coastal Heterotrophic Bacteria in the Red Sea.

Characterized by some of the highest naturally occurring sea surface temperatures, the Red Sea remains unexplored regarding the dynamics of heterotrophic prokaryotes. Over 16 months, we used flow cytometry to characterize the abundance and growth of four physiological groups of heterotrophic bacteria: membrane-intact (Live), high and low nucleic acid content (HNA and LNA) and actively respiring (CTC+) cells in shallow coastal waters. Chlorophyll a, dissolved organic matter (DOC and DON) concentrations, and their fluorescent properties were also measured as proxies of bottom-up control. We performed short-term incubations (6 days) with the whole microbial community (Community treatment), and with the bacterial community only after removing predators by filtration (Filtered treatment). Initial bacterial abundances ranged from 1.46 to 4.80 × 105 cells mL-1. Total specific growth rates in the Filtered treatment ranged from 0.76 to 2.02 d-1. Live and HNA cells displayed similar seasonal patterns, with higher values during late summer and fall (2.13 and 2.33 d-1, respectively) and lower in late spring (1.02 and 1.01 d-1, respectively). LNA cells were outgrown by the other physiological groups (0.33-1.08 d-1) while CTC+ cells (0.28-1.85 d-1) showed weaker seasonality. The Filtered treatment yielded higher bacterial abundances than the Community treatment in all but 2 of the incubations, and carrying capacities peaked in November 2016 (1.04 × 106 cells mL-1), with minimum values (3.61 × 105 cells mL-1) observed in May 2017. The high temperatures experienced from May through October 2016 (33.4 ± 0.4°C) did not constrain the growth of heterotrophic bacteria. Indeed, bacterial growth efficiencies were positively correlated with environmental temperature, reflecting the presence of more labile compounds (high DON concentrations resulting in lower C:N ratios) in summer. The overall high specific growth rates and the consistently higher carrying capacities in the Filtered treatment suggest that strong top-down control by protistan grazers was the likely cause for the low heterotrophic bacteria abundances.

Importance of sampling frequency when collecting diatoms.

There has been increasing interest in diatom-based bio-assessment but we still lack a comprehensive understanding of how to capture diatoms' temporal dynamics with an appropriate sampling frequency (ASF). To cover this research gap, we collected and analyzed daily riverine diatom samples over a 1-year period (25 April 2013-30 April 2014) at the outlet of a German lowland river. The samples were classified into five clusters (1-5) by a Kohonen Self-Organizing Map (SOM) method based on similarity between species compositions over time. ASFs were determined to be 25 days at Cluster 2 (June-July 2013) and 13 days at Cluster 5 (February-April 2014), whereas no specific ASFs were found at Cluster 1 (April-May 2013), 3 (August-November 2013) (>30 days) and Cluster 4 (December 2013 - January 2014) (<1 day). ASFs showed dramatic seasonality and were negatively related to hydrological wetness conditions, suggesting that sampling interval should be reduced with increasing catchment wetness. A key implication of our findings for freshwater management is that long-term bio-monitoring protocols should be developed with the knowledge of tracking algal temporal dynamics with an appropriate sampling frequency.