Responses of calcifying algae (<I>Halimeda</I> spp.) to ocean acidification: implications for herbivores

Abstract

ABSTRACT: Ocean acidification (OA) can alter the development and physiology of many marine organisms. In addition to calcified invertebrates, studies documenting the responses of calcareous algae are critical because of their prominent role in habitat structure and carbonate production within coastal environments. While many studies report physiological responses, few have examined how OA might ultimately alter interactions with generalist herbivores via shifts in algal chemistry. This study describes a series of experiments that examine the influence of OA on the growth and herbivore defensive compounds of calcareous green algae (Halimeda spp.). One experiment was conducted in an open, outdoor seawater system with H. opuntia, while the other was conducted in an indoor, closed system with H. incrassata and H. simulans. Both experiments were conducted over similar ranges in pCO2 (300 to 2400 µatm) and monitored shifts in calcification and herbivore defenses (calcium carbonate CaCO3] and terpenoid metabolite content). Feedings assays with common sea urchins (Lytechinus variegatus and Diadema antillarum) were further conducted to test the degree to which shifts in algal chemistry influence herbivore feeding preferences. Our results were variable among Halimeda spp., highlighting that OA-induced shifts in chemical composition are species-specific. OA reduced the CaCO3 content (% dry wt) of H. incrassata yet had no effect on H. opuntia or H. simulans. Terpenoid metabolite concentrations were unaltered by pCO2 for all species. Assays with sea urchins revealed that feeding significantly increased on diets of lower CaCO3 and secondary metabolite content. Our work suggests that certain algal species may be relatively more susceptible to OA-induced shifts in chemical composition, and those shifts have the potential to weaken the efficacy of herbivore defenses. Future research on how OA influences marine plant herbivore interactions will improve our broader understanding of how OA stands to alter community and ecosystem properties.