Assessing ocean acidification impacts on the reef building properties of crustose coralline algae

Abstract

Crustose coralline algae (CCA), and in particular Porolithon onkodes, play an important reef-building role in modern tropical coral reefs. CCA form thick crusts of Mg-calcite and grow over corals and loose substrate to bind these together. This binding and cementing process is fundamental to the development of structural reefs that are capable of withstanding the high-energy waves in the shallow to inter-tidal areas of the reef. As anthropogenic CO2 emissions continue to increase, the oceans absorb part of this extra CO2 and become more acidic, a process known as Ocean Acidification (OA). There are concerns that OA will have a negative affect on the reef-building capacity of coral reef organisms, in particular on CCA. This is because Mg-calcite is meta-stable and more susceptible to dissolution than aragonite, the mineral used by corals to build skeletons.

The goal of this thesis work was to firstly understand the physical and mechanical properties that enable the CCA to cement the reef and withstand damage from high-energy waves, bioerosion and chemical dissolution. Secondly, to anticipate how OA may interfere with these reef-building properties. These goals were pursued by setting clear aims with associated specific objectives designed to elucidate information relevant to these questions.

Methods were developed for X-ray diffraction to identify the mineral composition of CCA. Nanoindentation was investigated as a tool for determining the mechanical properties of CCA and the measurement of fracture toughness was found to return physically meaningful information relevant to structural reef development.

Study of CCA calcification showed that cell wall Mg-calcite exhibited radial crystal morphology in agreement with published studies on temperate species. However, high-resolution imaging showed the radial crystals were made of banded stacked sub-micron grains within an organic framework. Dolomite was found not only as cell lining by submicron rhombs, but also as the primary calcification of hypothallial cell walls. Dolomite is shown to be resistant to bacterial erosion. A model is developed whereby it is proposed that dolomite formation is dependent on polysaccharide accumulation.

Using nanoindentation, P. onkodes are found to be extraordinarily tough, on par with the measured fracture toughness for metamorphic minerals quartz and corundum. The fracture toughness is enabled by the presence of dolomite cell lining. Contrary to the literature, bacterial erosion is found to be a constructive, not destructive, process.

A survey of P. onkodes from Heron Island fore reef and reef flat showed that dolomite was present in all the fore reef crusts but none of the reef flat crusts. The reef flat crusts did not have fracture resistance except where remineralised. The presence of dolomite cell lining was shown to decrease skeletal dissolution rates by an order of magnitude.

OA experiments showed that skeletal dissolution rates increased with elevated pCO2, but dolomite continued to confer resistance to dissolution. pCO2 levels did not affect the skeletal Mg content or dolomite formation in living CCA. Of concern, and in agreement with the literature, bacterial erosion is accelerated under a combination of elevated pCO2 and temperatures, suggesting this may be the main threat to CCA reef-building in the future. The experimental findings were corroborated by results of a field survey along a natural pCO2 gradient.

In summary, dolomite was found to be an essential component of modern reef development via its contribution to enabling CCA P. onkodes thick crust development and persistence. Reef building by CCA P. onkodes is likely to continue as pCO2 rises up until a tipping point is reached whereby bacterial erosion switches from constructive to destructive.