CoralChem - The Mechanics of Coral Calcification Revealed by a Novel Electrochemical Tool Kit

Tropical coral reefs are a very important marine ecosystem contributing $30billion in ecosystem services each year worldwide. They are diversity hotspots, offer coastal protection and sustain important economic activities like fisheries and tourism. Despite being some of the largest bioconstructions on the planet, the 3D framework which underpins their ecosystem function is constructed by stony corals in a space about 1/10th of the width of a human hair sandwiched between the living coral and the existing calcium carbonate skeleton. Coral reefs face a multitude of anthropogenic threats on a range of scales from local pollution and over fishing to global warming and ocean acidification. In order to best predict the fate of these important ecosystems in the face of continued anthropogenic change and to ensure the most effective local mitigation efforts are carried out to conserve them, we need to better understand how they build their skeletons. Through laboratory studies where corals are subjected to simulations of the future (e.g. water temperature is increased, or pH lowered, or both) we have good evidence that coral calcification will be much reduced in the warm, acidic oceans of our future. However, exactly what our future holds is uncertain and the environmental change they will face is, and will be, multifaceted, multifactorial and synergistic involving simultaneous changes in water temperature, acidity, nutrient levels, sea level, light levels and food supply, amongst others. So much so, extrapolating the simple laboratory experiments where one variable is changed at time to predict what coral reefs will be like in the future is fraught with uncertainty. Instead a mechanistic understanding is required such that the processes involved in skeletal construction, and their environmental sensitivities, can be encoded in a numerical model to more accurately predict the fate of this important ecosystem.

What is needed to achieve this are new ways to sense the processes occurring in the tiny space sandwiched between the coral animal and existing skeleton. Although small probes can be inserted into this space they frequently break which makes such research difficult, frustrating and expensive, and it is hard to work out exactly where you are within the coral animal - especially since the coral is a living organism inhabiting a moving fluid (seawater). As a result, such measurements are far from routine and are reported in only a handful of publications. Consequently, our current understanding is insufficient to predict how corals will respond to anthropogenic stressors.

In this proposal we will make a robust, low cost, solid-state microelectrode that senses pH more reliably than those currently commercially available and one that will resist breakage more easily. By controlling this electrode with a novel self-positioning system we will know exactly how close the skeleton is from the electrode tip and we will be able to maintain its position allowing us to make prolonged and reliable measurements of the evolution of pH in the calcifying fluid for the first time. This advance draws on developments in the field of scanning electrochemical microscopy and uses the sensing tip like a radar to determine its distance from the skeleton (and other layers in the coral cellular structure). This proposal is an important first step towards being able to reliably measure the carbonate system in the calcifying space of corals so as to better identify the mechanics of calcification. Performing measurements within the calcifying coral cavity will present challenges but the probe we propose has good potential for commercialisation for a wide range of applications beyond the scope of this project, especially in field locations (e.g. on a research ship, in a field station, or marine biology lab) that are common in the environmental sciences but present a significant challenge to existing technology.

The environmental sensitivities of the processes involved in coral calcification govern the response of the skeletal phenotype to environmental change. While we know what processes likely occur, we currently lack an understanding of their relative importance and how they are impacted by environmental changes. Consequently, predictions of the future fate of stony corals, and the reefs they support, are uncertain. The mechanistic understanding of coral calcification that is required is lacking in no small part due to the difficulty of observing the micron-sized seawater filled calcifying space that exists between the coral animal and its skeleton.

An important process in calcification is the modification of the pH within the calcifying space. Sensing calcification fluid pH is therefore required to unpick the processes involved in coral calcification. However, commercially available micro-pH electrodes are expensive and fragile often breaking against the coral skeleton, leading to the existing studies being scarce, unreliable and limited in scope. Here we will make a low-cost, robust and reliable, solid-state dual pH/O2 micro-electrode. By drawing on developments in Scanning Electrochemical Microscopy we will combine this microelectrode with a self-positioning system and together these advances will allow for the reliable and prolonged measurement of pH, accurately positioned within the calcifying space, irrespective of the animal movements. Our preliminary applications with coral will demonstrate the utility of this novel sensor and its promise in other areas of the environmental sciences where reliability and expense of commercially available micro-pH sensors is currently a limiting factor. The approach will be the subject of future proposals to build a multi-barrel probe with solid-contact ion selective electrodes to simultaneously monitor pH, calcium and carbonate ion concentration.