Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

Abstract

Global and local anthropogenic stressors such as climate change, acidification, overfishing, and pollution are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by non-calcifying algae. These changes could reduce the ability of coral reef ecosystems to maintain positive net calcium carbonate accretion. However, relationships between community composition and calcification rates remain unclear. We performed field experiments to quantify the metabolic rates of the two most dominant coral reef substrate types, live coral and dead coral substrate colonized by a mixed algal assemblage, using a novel underwater respirometer. Our results revealed that calcification rates in the daytime were similar for the live coral and dead coral substrate communities. However, in the dark, while live corals continued to calcify at slower rates, the dead coral substrate communities exhibited carbonate dissolution. Daytime net photosynthesis of the dead coral substrate communities was up to five times as much as for live corals, which we hypothesize may have created favorable conditions for the precipitation of carbonate minerals. We conclude that: (1) calcification from dead coral substrate communities can contribute to coral reef community calcification during the day, and (2) dead coral substrate communities can also contribute to carbonate mineral dissolution at night, decreasing ecosystem calcification over a diel cycle. This provides evidence that reefs could shift from slow, long-term accretion of calcium carbonate to a state where large daily cycling of calcium carbonate occurs, but with little or no long-term accumulation of the carbonate minerals needed to sustain the reef against erosional forces.

Characterizing Reef Net Metabolism Via the Diel Co‐Variation of pH and Dissolved Oxygen From High Resolution in Situ Sensors

“…Macroalgae are efficient consumers of CO 2 maintaining high pH in the water column during the day (Smith et al., 2013). However, allelopathic effects on the microbial communities can enhance daytime respiration, which may lower the pH in microenvironments on coral or rubble surfaces and crevices promoting dissolution (Islam et al., 2016; Romanó de Orte et al., 2021). Although we have no direct evidence of such processes taking place at English Caye, allelopathy‐mediated coral dissolution could potentially explain the observed daytime decoupling of pH:DO.…”

“…Although we have no direct evidence of such processes taking place at English Caye, allelopathy‐mediated coral dissolution could potentially explain the observed daytime decoupling of pH:DO. Daytime dissolution is unusual as coral dissolution usually occurs at night when the absence of photosynthesis allows respiration to drive down pH and aragonite saturation state (e.g., Cyronak et al., 2018; Romanó de Orte et al., 2021; Webb et al., 2021). However, the dissolution of dead corals at seawater aragonite saturation states of >1 have been observed, proposed to be mediated by microbial respiration (Romanó de Orte et al., 2021), indicating that daytime dissolution is possible.…”

“…Healthier reefs are generally associated with higher hard coral cover and lower macroalgae presence (Mumby et al., 2007). Following coral mortality, algal assemblages are usually the first colonizers of the bare substrate (Romanó de Orte et al., 2021), and once established, macroalgae can damage adult corals and suppress coral recruitment (Brooker et al., 2016). Phase shifts to the dominance of other life forms, including soft corals and sponges, can also occur following a disturbance to a reef (Norström et al., 2009).…”

“…This algal‐coral symbiosis is also referred to as the “biological engine producing the reef” (Roth, 2014). Dissolution typically happens at night (e.g., Moav‐Barzel et al., 2023; Page et al., 2016; Romanó de Orte et al., 2021) as a consequence of decreasing pH and [CO 3 2− ], which results from CO 2 generated by respiration no longer being consumed by photosynthesis. Net reef dissolution can be an indicator of reef ecosystem health decline, and has been associated with vulnerability to environmental stress (Hoegh‐Guildberg et al., 2007).…”

“…Coral reefs are subject to numerous intensifying environmental stressors, including rising sea surface temperatures and ocean acidification, and anthropogenic activities, such as overfishing and nutrient loading (Bove et al., 2020; Bozec et al., 2016; Pendleton et al., 2016; Romanó de Orte et al., 2021; Silbiger et al., 2014, 2018). This has resulted in a global decline of coral reef health, with an approximate 50% reduction in coral cover from 1957 to 2007 (Eddy et al., 2021).…”