The repellent substances are actually supposed to protect ship hulls from growth with algae and the like. However, study results suggest that gentle antifouling coatings could also be used in another way: on surfaces intended for coral settlement, they could give cnidarian larvae a decisive developmental advantage over competing algae, researchers report.
The hull of ships is a habitat that marine organisms such as algae, barnacles or mussels like to colonize quickly. The proliferating growth can quickly become a significant burden: the so-called biofouling increases the drag and the weight of ships, causing billions in damage worldwide. To contain the problem, ship hulls are coated with antifouling. These are usually paints that contain biocidal active ingredients that are intended to prevent the problem organisms from fouling. It seems surprising that the researchers led by Lisa Röpke from the Leibniz Center for Tropical Marine Research (ZMT) in Bremen came up with the idea that this concept of all things could have potential for coral protection. But as they explain, corals also have a kind of biofouling problem.
competition with algae
Algae in particular compete with the tiny larvae of the cnidarians for suitable settlement sites. Competing for space and light, algae have a particular advantage where coastal areas are polluted by sewage, as they thrive in such conditions. This represents a further problem for coral reefs, which are already heavily burdened by climate change and other anthropogenic harmful effects. The competitive pressure is particularly critical in the case of targeted settlement projects. In order for reefs to recover and become more resilient, hard corals must be able to reproduce and their larvae to spread and settle.
As part of their study, Röpke and her colleagues have now investigated the extent to which special antifouling coatings could be used to curb the growth of algae, which often affects the coral larvae when they settle. "We wanted to find a coating that would reduce the growth of algae on a suitable settlement site without affecting the larvae," says the scientist. The scientists coated carrier plates, such as those used to expose small coral heads in seawater aquariums and in reef restoration projects, with three test antifouling coatings specially developed for the study. They were designed in such a way that it seemed possible to suppress algae while at the same time protecting the larvae.
One of the coatings contained the biocide DCOIT (dichlorooctylisothiazolinone), another consisted of cerium dioxide nanoparticles. The researchers explain that these can curb bacterial growth on substrates, which in turn has a major influence on whether algae subsequently settle in one place. The third coat was a silicone coating which, thanks to its smooth and water-repellent surface, prevents fouling organisms from settling.
Potential for coral conservation projects
After the researchers had stored the coated carriers together with lime, green and brown algae in aquariums for five weeks, it became clear: "Both the silicone and the DCOIT coating were particularly effective at reducing the growth of algae on the carriers," says Röpke . The scientists then added coral larvae of the genus Acropora to the test carriers and examined 24 hours later how many of the tiny larvae had settled on the carriers. It turned out: "It was particularly pleasing to see that the coatings did not have a negative effect on the colonization behavior of the coral larvae," reports Röpke.
The bottom line is that there is fundamental potential: "Antifouling coatings can reduce fouling intensity in a relevant way, while at the same time maintaining a robust level of coral colonization," the scientists write. "It is possible that this method could be used to design areas in reefs that keep algae growth in check and thus give the corals a head start in their development. That would be an important help for reefs, especially since the majority of hard corals usually only spawns once a year,” says Röpke.
However, further research is now necessary, emphasizes the scientist. “At the moment there are no reliable and at the same time harmless antifouling coatings on the market. Effective coatings that are only harmful to fouling organisms could be a sustainable alternative to toxic paints," says Röpke. In the case of the antifouling paints in her study, the effects on larvae of other coral species and also on other reef organisms must first be examined, the researcher points out.
Source: Leibniz Center for Tropical Marine Research, specialist article: Scientific Reports, doi: 10.1038/s41598-022-19997-6