Near‐future ocean acidification causes differences in microbial associations within diverse coral reef taxa

Abstract

Microorganisms form symbiotic partnerships with a diverse range of marine organisms and can be critical to the health and survival of their hosts. Despite the importance of these relationships, the sensitivity of symbiotic microbes to ocean acidification (OA) is largely unknown and this needs to be redressed to adequately predict marine ecosystem resilience in a changing climate. We adopted a profiling approach to explore the sensitivity of microbes associated with coral reef biofilms and representatives of three ecologically important calcifying invertebrate phyla [corals, foraminifera and crustose coralline algae (CCA)] to OA. The experimental design for this study comprised four pHs consistent with current IPCC predictions for the next few centuries (pHNIST 8.1, 7.9, 7.7, 7.5); these pH/pCO₂ conditions were produced in flow-through aquaria using CO₂ bubbling. All reduced pH/increased pCO₂ treatments caused clear differences in the microbial communities associated with coral, foraminifera, CCA and reef biofilms over 6 weeks, while no visible signs of host stress were detected over this period. The microbial communities of coral, foraminifera, CCA and biofilms were significantly different between pH 8.1 (pCO₂ = 464 μatm) and pH 7.9 (pCO₂ = 822 μatm), a concentration likely to be exceeded by the end of the present century. This trend continued at lower pHs/higher pCO₂. 16S rRNA gene sequencing revealed variable and species-specific changes in the microbial communities with no microbial taxa consistently present or absent from specific pH treatments. The high sensitivity of coral, foraminifera, CCA and biofilm microbes to OA conditions projected to occur by 2100 is a concern for reef ecosystems and highlights the need for urgent research to assess the implications of microbial shifts for host health and coral reef processes.

Responses of Coral-Associated Bacterial Communities to Local and Global Stressors

“…For example, exposure of corals to lower pH resulted in microbiome communities reminiscent of those associated with diseased and stressed corals as they contained more Vibrionaceae and Altermonadaceae (Vega Thurber et al, 2009;Meron et al, 2011). Additionally, reduced pH significantly changed the microbial communities in A. millepora with the loss of Proteobacteria sequences typically associated with healthy corals while Gammaproteobacteria, which are often associated with diseased and stressed corals, increased (Webster et al, 2012).…”

Benthic N2 fixation in coral reefs and the potential effects of human‐induced environmental change

“…Microorganisms comprise the largest diversity and biomass of all marine biota, yet how they may be affected by ocean acidification (OA) remains uncertain (Joint et al 2011). Recent findings emphasize their high sensitivity to expected near-future pH changes and highlight the importance of assessing implications of microbial shifts for host health and coral reef processes (Webster et al 2012). First results looking at the coral-associated microbes also suggest a drastic change in microbial composition in the coral mucus, tissue, and skeleton under OA conditions (Meron et al 2011).…”

Variability in Microbial Community Composition and Function Between Different Niches Within a Coral Reef

“…At the scale of an entire reef, microbes drive chemical cycling processes that ultimately control the productivity and biogeochemical function of the reef ecosystem [3][4][5][6][7]. At finer scales, microbes form both mutualistic and pathogenic relationships with a variety of benthic organisms including corals, sponges, clams, ascidians and macroalgae [8][9][10][11][12]. At the scale of the individual coral colony, the abundance, taxonomic composition and metabolic potential of bacterial communities inhabiting the gastrodermal cavity, skeleton and surface microlayer of a coral differ substantially [13].…”