Chemical and Biological Controls on Coral Nucleation

Abstract

Coral reefs are vibrant and important ecosystems in the oceans, but reefs today are under threat from multiple sources. One such threat is ocean acidification due to anthropogenic climate change, which is reducing both seawater pH and the thermodynamic driving force for CaCO3-based biomineralization (oversaturation or ?). It is known that coral skeletal growth will decrease in an acidifying ocean, but the detailed mechanisms driving this response are still poorly understood. In addition, there is a long-standing debate in the field regarding the relative impact of skeletal organic matrix proteins on calcification and nucleation, with some claiming that these proteins can mitigate the effects of ocean acidification. Nucleation is the first step of the skeleton growing process, and as such is the step that is thought to determine the pace, pattern, and strength of the coral skeleton, and by extension, the development of the very framework that holds reefs together. We made the first quantitative measurements of inorganic aragonite nucleation and its sensitivity to ?, mapping the energy landscape of nucleation kinetics, and finding that it is more sensitive to oversaturation than bulk mineral growth rates in the environmentally relevant range of ? values. Furthermore, we combined inorganic mineral growth rates from the literature with the nucleation rates measured in this work in a numerical model to make predictions about how these two processes work together to affect the overall skeleton. In order to determine the extent of biological control over nucleation, we measured the quantitative effects of a peptide from the acidic domain of a matrix protein from Stylophora pistillata, as well as matrix protein analogues and polypeptides with different functional groups, on nucleation rates and other growth properties. We found that although the matrix protein and analogues had an effect on nucleation, it is unlikely that this protein can counteract the effects of ocean acidification. Overall, this work provides a comprehensive look at inorganic aragonite nucleation and the balance of chemical and biological controls on inorganic rates, as well as how these inorganic studies pertain to corals. These experiments quantify the limits of corals’ ability to respond to the environmental pressure of ocean acidification on skeletal growth.