The rise of sponges in anthropocene reef ecosystems

Coral reefs around the world have undergone profound changes in recent decades. Human activities have contributed to the massive loss of corals in the tropical oceans.

The degradation of reef-generating corals is expected to worsen under current climate trajectories, but our work shows that most reef sponges are sufficiently resistant to tolerate the predicted weather conditions for 2100.

In our latest research, we examine how future reefs with more sponges could work compared to current coral-dominated ecosystems.

Sponges on coral reefs

On the Great Barrier Reef, the amount of live corals has declined over the last 30 years. Recurring bleaching episodes have profound implications for the ecology of reef systems and the resources that reefs can provide to humans.

Marine sponges are found in the world's oceans. They are among the oldest known multicellular organisms and first appeared in the fossil record about 580 million years ago.

During this long evolutionary history, sponges have experienced various environmental conditions and have shown remarkable persistence to survive after the mass extinction of the late Triassic, there are 200 million of them. ; years. Although sponges are found in shallow and deep waters, from tropics to poles, they are particularly important for coral reefs. Filter filters are an essential link between the seabed and the overlying seawater body.

Sponges pump large amounts of water and kill bacteria, plankton and dissolved food. They also have symbiotic partnerships with various communities of microorganisms that can provide nutrients and secondary metabolites that enhance their defense against predators and infections.

Tolerance to sponges and super larvae

The latest report of the Intergovernmental Panel on Climate Change suggests very different results for coral reefs with an increase in seawater temperature of 1.5 ° C or more. 2.0 ° C. Even if we manage to maintain the warming of the oceans at 1.5 ° C, the corals will nevertheless be seriously affected.

However, we have shown that many species of sponges are more tolerant than corals to the impacts of climate change. We think that sponges could be the future "winners" of coral reefs.

Our work explored the tolerance of four Great Barrier Reef sponge species to ocean warming and ocean acidification levels predicted for 2100. No species were affected by the moderate climate change scenarios in which we increased the temperature by 1.5 ° C. However, the environmental conditions foreseen in the most extreme scenarios (temperature increase of 4 ° C) had significant negative effects for some species.

Although higher temperatures may be detrimental to the health and survival of some sponge species, ocean acidification appears to have negligible effects. Research conducted in natural carbon dioxide vents also confirms the overall pH tolerance of many species of sponges.

Our experimental work has shown that responses to the combined effects of ocean warming and acidification vary among sponge types. While acidification exacerbated the warming effect of sponge species feeding on plankton, it mitigated the warming effect of species with photosynthetic symbiots.

Sponges react differently throughout their lives. Larvae of the abundant sponge Rhopaloeides odorabile have a thermal threshold 4 ° C higher than that of their parents. Survival and settlement of common reef sponge larvae Carteriospongia foliascens are not affected by the worst climate change projections.

These results suggest that sponges have an inherent ability to tolerate climate change, but that this tolerance is not maintained in adult populations.

Resilience of sponges

In our most recent research, we have explored the potential mechanisms that underlie the tolerance of sponges to warming and acidification. We measured the lipid and fatty acid composition of sponge species with different environmental sensitivities. We found that sponges containing a higher proportion of storage lipids and some long-chain polyunsaturated fatty acids were more resistant to warming.

These lipids and specific fatty acids probably preserve the function of the cell membrane and other cellular processes in the face of heat stress. Further study of how sponges alter the lipid content of their membranes in response to rising temperatures has revealed a potential mechanism by which ocean acidification could increase resistance to heat stress by increasing sterol-stabilizing production. membrane. Our research shows that lipids and fatty acids play an important role in the response of sponges and can contribute to their survival in the oceans to come.

How a sponge reef could work

Sponge-dominated reefs are likely to function very differently from existing coral-dominated systems. Reefs where sponges are already the most abundant taxa have been reported in Indonesia and the central Pacific. Some researchers also consider that many Caribbean reefs are mainly dominated by sponges.

Recent research has modeled the functioning of reef ecosystems with an increased abundance of sponges. He highlighted the need to better understand how changes in the dominant group of reef organisms could alter marine food webs. While sponge-dominated reefs are unlikely to provide humans with the same resources as coral reefs, they provide habitat and food for some reef species. They are also responsible for nutrient recycling and contribute to the structural complexity that is expected to have positive effects on reef biodiversity.

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Could coral reefs become sponge reefs in the future?