Response of Benthic Tropical Marine Macroalgae to Ocean Acidification

Macroalgae can modify coral reef ecosystem structure and function through a variety of mechanisms, including mediation of biogeochemistry through photosynthesis and the associated production of dissolved organic carbon (DOC). Although DOC plays a critical role in sponge and microbial loops and coastal carbon cycling, high concentrations can be detrimental to corals and have negative consequences for reef assemblages. Ocean and coastal acidification have the potential to fuel photosynthesis and may alter macroalgal DOC production, but this has not been widely studied across taxa and regions. We exposed fleshy (Dictyota spp.) and calcifying (Halimeda tuna) macroalgae to ambient and low seawater pH for 25 days in an outdoor experimental system in the Florida Keys. We quantified algal growth, calcification, photophysiology, and DOC production across pH treatments. We observed no significant differences in the growth or photophysiology of either species between treatments, except for lower chlorophyll b concentrations in Dictyota spp. in response to low pH. The tolerance of Dictyota and Halimeda to near-future seawater carbonate chemistry and stability of photophysiology, suggests that acidification alone is unlikely to change biogeochemical processes associated with algal photosynthesis in these species. Additional research is needed to fully understand how fleshy and calcifying macroalgae that exist across a range of environmental conditions regulate photosynthesis (via carbon uptake strategies) and how this influences their DOC production. Once we understand the physiological responses of macroalgae to current and future ocean change, then we can more accurately model and infer the indirect impacts of macroalgae on future coral health and reef ecosystem processes.