Rising atmospheric CO2 concentrations are changing the chemistry of the oceans. Specifically, seawater pH is in decline – a process known as ocean acidification (OA). Coral reefs are particularly susceptible to OA. The carbonate structures comprising reefs are built by corals, but OA threatens to weaken and erode calcareous reefs. Another key element in reef dissolution, bioerosion (i.e. the process of sponge growth into calcareous structures causing reef degradation) may increase under future ocean conditions, particularly as a result of warming. The major bioeroding organisms on reefs are sponges belonging to the genus Cliona. Future environmental conditions may shift coral reefs to a state of perpetual decline. It is essential that we understand the role sponges play in that process. The most aggressive bioeroding sponges are those that harbor algal symbionts belonging to the same group found in corals (i.e. Symbiodinium). In corals, the algal symbionts provide the nutrition they need to precipitate calcium carbonate and form large reef structures. In sponges, the energy is used to increase rates of bioerosion. In addition to the interaction with Symbiodinium, most of the sponges host a wide range of bacteria, which are crucial to the sponge behavior.
The combination of ocean warming with ocean acidification causes that corals (and other calcifying organisms in temperate regions) are undergoing a significant decline worldwide, however, bioeroding sponges may have an ecological advantage under future conditions. This might have dramatic consequences because reefs support 25% of marine life. The impact would be not only in terms of food resources obtained from the sea but also tourism would be severely affected. The economic impact is valued in $3.4 billion only in the USA. So, the focus on bioeroding sponges is especially important given how ocean acidification may accelerate the rate of boring into calcium carbonate structures by these type of sponges.
The scientific aim of this project is to study how symbionts interact with each other and with their host in terms of the sponge performance under a climate change context. We characterize the symbiotic community associated to clionaid sponges from tropical habitats and determine the effect of future environmental conditions on sponge bioerosion from multiple perspectives, estimating bioeroding activity and assessing expression of potential genes related to this process.
The main conclusions of the project are (i) clionaid sponges show a species-specific core microbiome with dominant OTUs partly shared among species but with species-specific enrichments; (ii) Symbiodinium populations of Cliona varians was diminished due to an exposure to high temperatures; (iii) temperature also shrank alpha and beta diversities of sponge microbiome; (iv) high bioerosion was observed at high temperature and mid pH conditions. While symbiotic dinoflagellates are reduced at high temperatures, the bioeroding activity is increased. So, bioerosion might be favoured by other mechanisms that need to be further studied. We expect that differential gene expression will shed light on how bioeroding sponges perform under a climate change context.
