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Relationship between living and growing corals and microbial endoliths: parasitism or mutualism

Final Activity Report Summary - CORALS AND ENDOLITHS (Relationship between living and growing corals and microbial endoliths: parasitism or mutualism)

Coral reefs are among the most diverse and dynamic marine ecosystems. They provide natural barriers protecting coasts against storms, food and economic resources for a billion of human beings on the globe. The well-being of coral reefs results from a balance between constructive, primarily because of corals and coralline algae, and destructive forces, consisting mainly of bioerosion. This balance is increasingly threatened by human activities and global warming that mobilise factors such as eutrophication, overfishing, rising atmospheric CO2 concentration (pCO2) and rising seawater surface temperature. It is therefore crucial to better understand functioning of coral reefs under various environmental conditions in order to improve their preservation and to allow for a sustainable development of populations.

Most of the studies concerned with the health and functioning of coral reefs concentrate on coral growth and calcification as well as on factors affecting them such as rising pCO2, due to ocean acidification, and eutrophication, i.e. elevated concentration of nutrients such as nitrate and phosphate. These factors reduce coral calcification. In contrast, research on bioerosion due to the endolithic micro-flora, or microborers, boring macro-organisms and grazers is equally important but has received less attention. Moreover, most of bioerosion studies have focused on the role of macroborers and grazers in coral reef destruction. Roles of endoliths, comprising cyanobacteria, algae and fungi are less understood.

Endoliths are ubiquitous in coral reefs, colonising all kinds of carbonate substrates such as rocks, sediments, live and dead corals, shells, etc. In dead substrates, endoliths penetrate inside substrate dissolving actively calcium carbonate crystals, thus contributing to coral reef destruction. In live substrates such as live corals, endoliths dissolve skeleton but may also contribute to the survival of coral tissues, called polyps, stressed by factors such as rising sea-surface temperature and atmospheric pCO2. In general, endolithic diversity and metabolism, that is production and active dissolution of skeletons, remain poorly studied in both live and dead corals. Additionally, effects of environmental factors such as eutrophication and rising atmospheric pCO2 on their metabolism, is unknown.

As part of an OIF project, I, Dr Tribollet A, investigated endolithic diversity, abundance, distribution and metabolism in corals at different stages of health (i.e. healthy, bleached and dead) which were collected in Kaneohe Bay of Oahu, Hawaii, USA, under controlled conditions in an indoor flume at Hawaii Institute of Marine Biology (HIMB). Endolithic species, abundance and distribution were studied using light microscopy, petrographic thin sections and scanning electronic microscopy. An attempt of determining species of endoliths using molecular tools was made but was unsuccessful. Effects of eutrophication and rising pCO2 on endolithic metabolism were studied by measuring O2 concentration in the flume water, seawater alkalinity, pH and temperature over a period of few weeks.

Although data was still under collection and a few experiments were still underway, the first results showed that skeletons of different live corals from Kaneohe Bay were mainly colonised by the green alga ostreobium and undetermined fungi. Endolithic diversity and abundance was thus less important in live corals than in dead ones. The major result that was obtained concerning endolith metabolism was that dissolution of carbonate due to these micro-organisms increased with rising pCO2 in dead corals (Tribollet et al. submitted to 'Coral Reefs'). It was hypothesised that dissolution of carbonates by endoliths would compensate carbonate production before the year 2100 with dead substrates being way more abundant in coral reefs than live ones, representing more than 80 % of total surface area. If this prediction were confirmed by the next experiments, then major losses of coral reef structure should be expected at the global scale within the next 50 years. Further studies of endolithic metabolism under different environmental conditions combined with rising pCO2 were necessary to better understand future impacts of rising atmospheric pCO2 on coral reefs.