Microbial bioenergetics of coral-algal interactions

Abstract

Human impacts are causing ecosystem phase shifts from coral- to algal-dominated reef systems on a global scale. As these ecosystems undergo transition, there is an increased incidence of coral-macroalgal interactions. Mounting evidence indicates that the outcome of these interaction events is, in part, governed by microbially mediated dynamics. The allocation of available energy through different trophic levels, including the microbial food web, determines the outcome of these interactions and ultimately shapes the benthic community structure. However, little is known about the underlying thermodynamic mechanisms involved in these trophic energy transfers. This study utilizes a novel combination of methods including calorimetry, flow cytometry, and optical oxygen measurements, to provide a bioenergetic analysis of coral-macroalgal interactions in a controlled aquarium setting. We demonstrate that the energetic demands of microbial communities at the coral-algal interaction interface are higher than in the communities associated with either of the macroorganisms alone. This was evident through higher microbial power output (energy use per unit time) and lower oxygen concentrations at interaction zones compared to areas distal from the interface. Increases in microbial power output and lower oxygen concentrations were significantly correlated with the ratio of heterotrophic to autotrophic microbes but not the total microbial abundance. These results suggest that coral-algal interfaces harbor higher proportions of heterotrophic microbes that are optimizing maximal power output, as opposed to yield. This yield to power shift offers a possible thermodynamic mechanism underlying the transition from coral- to algal-dominated reef ecosystems currently being observed worldwide. As changes in the power output of an ecosystem are a significant indicator of the current state of the system, this analysis provides a novel and insightful means to quantify microbial impacts on reef health.

Rehabilitation of coral reefs through removal of macroalgae: state of knowledge and considerations for management and implementation

“…Arrows indicate flow of feedback loop, linking microbial coral–algal interactions with increased macroalgae cover promoting macroalgae dominated regimes. Adapted from Roach et al ().…”

Evidence for a biological source of widespread, reproducible nighttime oxygen spikes in tropical reef ecosystems has implications for coral health

“…Increased respiration in at least 7 2 7 one species of coral an hour after exposure to hypoxia has been reported, followed by respiration 7 2 8 decrease after longer exposure (Dodds et al, 2007). Many studies of the effects of hypoxia on 7 2 9 corals have used environmental conditions at less than 50% air saturation, more in line with the reef health due to rising temperatures, eutrophication and competition with fleshy macroalgae 7 3 7 (Barott et al, 2009;Haas et al, 2013b,a;Sugden, 2017;Roach et al, 2017;Gajdzik & DeCarlo, 7 3 8 2017; Breitburg et al, 2018;Nelson & Altieri, 2019). H 2 O 2 in excess has also been linked to 7 3 9 coral bleaching (Downs et al, 2002).…”

Climate change alterations to ecosystem dominance: how might sponge‐dominated reefs function?

“…Furthermore, it will be important to address the role of microbes in these competitive interactions, since recent studies have identified the important roles played by microorganisms in mediating coral–algal interactions (Roach et al. ).…”

“…Looking forward, it will be imperative to address uncertainty in the outcomes of competitive interactions between non-bioeroding sponges and algae, particularly in regions where they co-occur in high abundance. Furthermore, it will be important to address the role of microbes in these competitive interactions, since recent studies have identified the important roles played by microorganisms in mediating coral-algal interactions (Roach et al 2017).…”