Our research project investigates the mechanism by which the increased growth of algae or seaweeds contributes to the mortality of stony corals in reef ecosystems. Previous research has provided evidence that the microbial communities inhabiting the surface of corals and the water column play a role in the degradation of coral reefs. Our overall hypothesis states that: Reef degradation and the associated phase-shifts from coral to algal dominance, leads to elevated levels of algal-derived organic exudates (i.e., carbon energy sources such as sugars) in the water column, which influences the microbial community dynamics in the water column. These conditions result in: 1) higher microbial numbers and biomass, 2) a community shift to 'super-heterotrophs', which are potential coral pathogens, and 3) lower standing stock concentrations of dissolved organic carbon. Our results provide strong evidence that human activities, such as fishing and pollution, provide algae with a competitive advantage over corals by facilitating a favorable environment for bacterial growth and resulting in microbially-mediated coral mortality. During two field campaigns to Moorea, French Polynesia, our group consisting of four labs from SDSU, SIO, and UCSB collaborated to elucidate the mechanism for microbially mediated mortality on coral reefs. First, we characterized organic exudate (referred to a dissolved organic carbon, DOC) release by the dominant benthic primary producers present on the reef as well as the utilization of this organic energy source by the resident microbes (Haas et al., 2011). The results from this study showed that all of the species of algae that we measured, but not the coral, exuded significant amounts of labile DOC into their surrounding environment. The exudates released by algae also yielded the greatest bacterial growth; whereby turf algae produced nearly twice as much DOC per unit surface area as the other benthic producers, stimulating rapid bacterial growth and concomitant oxygen drawdown. These experiments have allowed us to generate predictive models of microbial dynamics on coral reefs. Figure 1 depicts a model generated from the microbial activity data from the Haas et al., 2011 study. Here, we predicted concentrations of organic exudates (DOC release) by different theoretical assemblages of reef primary producers and the subsequent bacterial cell growth yielded from these exudates. The benthic assemblages measured on three coral reefs are also shown. These types of models will be very important for generating predictions of reefs on a global scale and also for generating testable hypotheses. To complement the above study on microbial activity, the coral and algal-derived organic exudates were further characterized to determine the composition of sugars released by different species of primary producer. We found that the fleshy macroalgal exudates were enriched in the sugar components fucose and galactose whereas coral and coralline algal exudates were enriched the total concentration of sugars but in the same component proportions as ambient seawater (Figure 2). Rates of bacterial growth and carbon utilization were significantly higher in algal exudate treatments than in coral exudate and control incubations with each bacterial community selectively removing different sugar components. We also compared the bacterial community profiles that respond to exudates from different algal species by sequencing the 16S rRNA gene (Nelson et al., 2013). Coral exudates engendered the smallest shift in overall bacterial community structure, maintained high diversity, and enriched taxa from Alphaproteobacteria lineages containing cultured representatives with relatively few virulence factors (Hyphomonadaceae, Erythrobacteraceae). In contrast, macroalgal exudates selected for less diverse communities heavily enriched in copiotrophic Gammaproteobacteria lineages containing cultured pathogens with increased virulence factors (Vibr...
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