Project: Ocean acidification, temperature and light effects on carbon-use mechanisms, calcification, and growth of tropical macroalgae: Drivers of winners and losers

Ocean sequestration of atmospheric CO2 enhances the availability of CO2 in seawater and lowers ocean pH, commonly referred to as 'ocean acidification'. Marine macroalgae are likely to respond to these two changes in ocean chemistry (elevated CO2 and acidification) in ways that have the potential to reduce the sustainability of coral reefs and other coastal ecosystems with potential economic consequences. There are two major forms of marine macroalgae on reefs: fleshy species that are characterized by a rapid growth potential which allows them to become 'nuisance species' and out-compete corals, and calcified species that are slower growing, but help cement the reef and promote coral larval settlement. Currently, there is very little information on fleshy macroalgal photosynthesis and growth responses to increased ocean CO2. Further, there is an inadequate understanding of ocean acidification effects on macroalgal calcification. The proposed research will examine the fundamental pathways of inorganic carbon uptake for photosynthesis across a range of pH and CO2 levels simulating ocean acidification into the future (2100). How photosynthesis, growth and calcification are modulated by light and temperature under ocean acidification will also be examined. These data will be used to identify ocean acidification effects on ecologically important macroalgae and consequential impacts to coral reef ecosystems.

Macroalgae responses to ocean acidification (OA) are likely to be distinctive compared to phytoplankton and microalgae due to their low surface area to volume ratios, high external boundary layer resistance to CO2 and higher irradiance requirements. Understanding specific mechanistic responses of tropical macroalgal photosynthesis and calcification to elevated pCO2, temperature and irradiance is critical to develop predictions of OA effects on macroalgal dominated communities of the tropics, including those that grow near their thermal limits. The research objectives are to (1) provide new insights into the biochemistry and physiology of photosynthesis and calcification that drive growth responses to OA and warming in ecologically important tropical macroalgal species, (2) elucidate photosynthetic C-use mechanisms in tropical species to understand OA influences on HCO3- use, (3) determine if photosynthesis-calcification processes become uncoupled by OA, (4) clarify the role of irradiance in photosynthetic and calcification responses to OA, and (5) examine the synergistic effects of OA, temperature and light on the thermal optima of photosynthesis and growth in species living close to their thermal limits. To meet these objectives, a series of short-term physiological experiments will be conducted to ascertain HCO3-use mechanisms, Ci uptake kinetics, potential to employ carbon concentrating mechanisms (CCMs), and species-specific linkages between photosynthetic C-use mechanisms and calcification in ten dominant fleshy and calcareous tropical macroalgae species. These biochemical and physiological data will subsequently be used to interpret longer-term (20 d) growth (organic and inorganic calcification and crystal formation) responses in aquaria studies conducted over three different seasons, and photosynthesis response surface experiments to gradients of pH, irradiance and temperature.