Project: Canary on the coral reef: ecophysiology of tropical crustose coralline algae across latitudinal gradients

In this project, the investigator will study the effects of increasing temperatures and acidification on the physiology of tropical crustose coralline algae (CCA) - mineralogy and net calcification - with the objective of identifying which species can serve as bioindicators of, or are acclimated to, reef impacts related to global climate change. To achieve this outcome, the investigator will conduct field research at established sites with on-going ocean acidification research across a broad geographic range. The broadening participation activities involve education and outreach to Pacific islanders at the remote field sites where students will engage in hands on research activities.

Rising sea surface temperatures (SST) and ocean acidification (OA) threatens the ability of calcified organisms to build carbonate reefs. Current understanding of the effects of OA on coral reefs originates from static laboratory studies largely focused on growth of corals. But tropical crustose coralline algae (CCA) have been proclaimed as the "canary on the coral reef" because some species dissolve in these OA "shock" experiments. The exact physiological mechanism is unknown, but may be related to the magnesium content of the skeletal tissue, which renders the calcite more soluble in acidified conditions. The mole fraction of Mg in CCA in situ is subject to temperature and salinity and is positively correlated with warmer SST. This relationship has been exploited for climate reconstruction studies, but has not yet been considered in the context of OA. If CCA are not phenotypically plastic for Mg fractionation, then warming may exacerbate the response of these species to OA. The overall objective of this research is to examine the ecological importance of Mg content in recently deposited skeletal tissue of common, pantropical CCA across a wide latitudinal range that encompasses natural spatio-temporal gradients in carbonate saturation and SST. The project includes three specific objectives: 1) Describe the biogeographic and fine-scale patterns of Mg content in the carbonate skeleton of several species of common tropical CCA. 2) Explore the relationships of temperature and seawater carbonate chemistry with Mg content in several species of CCA. 3) Test whether Mg content alters the biological response (e.g. species-specific net calcification rates and community composition) of CCA to global warming and/or ocean acidification. Both powdered X-ray diffraction analysis and laser ablation inductively coupled plasma mass spectrometry will be used to quantify Mg2+ content relative to Ca2+ in the calcite lattice of CCA skeletal tissue; growth increment patterns will pinpoint the timing of deposition. High frequency time-series seawater pH or pCO2, salinity, and SST data, complimented by discrete sampling for the remaining carbonate parameters (from established OA monitoring programs), will resolve daily, tidal, and seasonal cycles and be related with coincident biological data (calcification and production rates and Mg content) in situ.