Award: EF-1416376

Tropical reef macroalgae are important on coral reefs. They create reef stability, provide a platform for corals to grow, produce calcium carbonate to build reefs over time, and provide food for fish and shellfish. These ecosystem services may become compromised if calcifying algae can no longer be sustained on reefs in the future. As more human-generated CO2 is put into the atmosphere, the oceans take it up. Oceans currently remove approximately 30% of anthropogenic atmospheric CO2. Ocean CO2 uptake leads to the oceans becoming more acidic, frequently referred to as ocean acidification. Under ocean acidification, the seawater H+ increases, leading to a lower ocean pH. Thus, the seawater pH decreases and CO2 concentrations rise under ocean acidification. The CO2 in the atmosphere also increases atmospheric and ocean temperature. As the oceans become more acidic, fleshy macroalgae are predicted to out compete calcifying organisms, such as corals and calcifying macroalgae. Because of the importance of calcifying macroalgae on reefs, this is a major concern. Currently, how reef macroalgae will accommodate ocean acidification conditions is uncertain. For example, will calcifying macroalgae increase photosynthesis with increasing CO2 and offset potential negative effects on calcification? Can calcifying species alter chemistry where they calcify to form new calcium carbonate, or alternatively, does dissolution, the breakdown of calcium carbonate, dominate under low pH? Through a series of experiments using photosynthetic inhibitors, microsensors and radiotracers, photosynthesis and calcification (growth) rates were measured to begin to answer some of these important questions. These experiments were conducted on a wide-range of species to determine potential overarching responses in tropical macroalgae, including species from high-light patch reefs on the Florida Keys Reef Tract and from low-light wall reefs on Little Cayman Islands in the Western Caribbean. Based on a series of experiments, light was found to be very important for tropical macroalgae to maintain high net calcification rates under ocean acidification conditions predicted for 2100 (pH = 7.7) compared to ambient controls (pH = 8.1). This was the case even though the more universal effect across species was greater dissolution of calcium carbonate under ocean acidification in the dark. High light allowed macroalgae to increase the pH at their surface, and likely at calcification sites, above the external seawater. This increase in pH at high light under OA conditions corresponded to the ability to continue to calcify in most species. In some species, this light-triggered calcification was promoted by two processes: photosynthesis and photosynthesis-independent H+ pumps. In high-light species, even though they had greater loss of calcium carbonate (dissolution) in the dark (or nighttime), they were able to sustain daily calcification rates at low pH similar to controls because of high light calcification rates. Low light species, and those with a low capacity to increase photosynthesis or biotically regulate their surface pH, appeared to become overwhelmed at low pH (high H+ and CO2), exhibiting lower calcification rates in the light at low pH. For these species, low light, as well as increased dissolution in the dark, resulted in lower daily calcification rates under ocean acidification conditions. Thus, low light species, and those less able to regulate the pH where they calcify, become more subject to dissolution and an overall lower calcification rate. While the calcification response to ocean acidification was dependent on light and species-specific mechanisms to overcome elevated H+ and CO2 in seawater, calcifiers were more robust to high temperature than fleshy species. At an increase of 3?C above current average summer temperatures, the projection for 2100, most fleshy species growth rates were halved, while most of the calcifying species growth rates were not affected. In fact, a high-light species that was affected by ocean acidification did not reduce calcification under OA conditions at high temperature. Thus, even though some predict an increase of fleshy over calcifying reef macroalgae species in the future under ocean acidification, warmer conditions may provide a window of opportunity for calcifying species to compete with fleshy species during summer months. It will be important to define what strategies dominant macroalgae species from important reefs employ to maintain net positive daily calcification under ocean acidification at projected temperature regimes. These should be investigated across a gradient of light environments from shallow high-light reefs to deeper mesophotic reefs, as light and low-light versus high-light adapted species differ in their response to ocean acidification. As many of these reef systems are already converting to greater algal dominance, the sustainability of calcifying macroalgae is extremely important. On some major reef systems, coral reefs are currently losing more calcium carbonate than they are producing. Thus, our results that show a main effect of ocean acidification on dissolution in tropical macroalgae is disconcerting. The major question going forward is will this organismal macroalgal dissolution be amplified and compound current dissolution on reef ecosystems? Last Modified: 11/22/2019 Submitted by: Marguerite S Koch