Scientists Study Antifouling Coatings, Coral Growth
A marine biologist from the Leibniz Center for Tropical Marine Research (ZMT) in Germany recently studied how antifouling coatings can curb algal growth while still protecting tropical coral reefs.
The study results have since been published in the journal Scientific Reports.
About the Research
According to the release, the study stems from tropical coral reefs coming under increasing pressure from ocean warming and acidification, marine pollution, dynamite fishing and other human activities in coastal areas. Stony coral reproduction and the dispersal and settlement of their larvae must succeed for these reefs to recover and become more resilient.
However, these larvae are exposed to several dangers, such as predators, stirred-up marine sediment preventing their settlement or algae disputing suitable settlement sites. Algae reportedly have a particular advantage in polluted coastal areas compared to coral larvae.
But, researchers say algae—similar to mussels or barnacles—can cause a problem for maritime shipping by attaching themselves to surfaces such as ship hulls. These surfaces can be treated with antifouling coatings, but they usually contain biocidal agents designed to prevent fouling.
“Our goal was to find a coating that would reduce algal growth on a suitable settlement site but not affect the larvae,” said ZMT marine biologist Lisa Röpke.
Röpke coated various carrier plates, such as those used to maintain small coral colonies in marine aquariums and in reef restoration projects, with three antifouling coatings for the study. These new coatings were reportedly developed and tested for the first time in the study, and are potentially more environmentally friendly than previous agents.
The research center reported that one of the coatings contained biocide Dichlorooctylisothiazolinone (DCOIT), while another consisted of cerium dioxide nanoparticles. Both can curb bacterial growth on substrates, another major impact on whether algae settle in a place.
For the third coating, a silicone coating that prevents fouling organisms from settling due to its smooth and water-repellent surface was used.
The coated plates were placed in aquariums with coralline, green and brown algae for five weeks. Afterwards, the plates were transferred to jars containing the coral larvae of the genus Acropora. Röpke examined how many larvae had settled on the carriers after 24 hours.
“We found that both the silicone and DCOIT coatings effectively reduced algal growth on the carriers. What was particularly exciting, however, was that the coatings did not negatively affect the settlement behaviour of the coral larvae,” Röpke reported.
“It is possible that this method could be used to design surfaces in reefs that keep algal growth in check and thus give the corals a head start in their development. For reefs, this would be an important help, especially since a large proportion of stony corals usually spawn only once a year.”
ZMT reports that the researchers hope that the use of coated surfaces for the settlement of coral larvae, in addition to other methods, could still preserve one or the other reef area as a refuge from which dying reef areas could be repopulated.
“At the moment, there are no reliable antifouling paints on the market which are sustainable and non-toxic. Effective coatings only harmful to fouling organisms could be a sustainable alternative to toxic antifouling paints,” Röpke said.
However, she added that the effect of the new antifouling coatings in her study on larvae of other corals and on other reef organisms still needs to be investigated.
Environmentally Friendly Antifouling Research
Late last year, a study conducted by Egypt’s National Institute of Oceanography and Fisheries utilized algae to create environmentally friendly, antifouling marine paints.
For the study, researchers utilized extracts from four different Egyptian marine macroalgae: Ulva fasciata, Cymodocea nodosa, Padina pavonia and Colpomenia sinusa.
The water soluble polysaccharides (WSP), proteins and lipids were combined with paint into sixteen compositions, aiming to act as a biocide to create environmentally safe, antifouling marine paints. Each type of these algal extracts was mixed solely by 2% (w/w) for WSP and protein and 1% (w/w) for lipid with the prepared paint formulation.
These paints were applied to unprimed steel panels, hung on a steel frame alongside a control and submerged in the Eastern Harbour of Alexandria, Egypt. Researchers collected sea water samples to analyze during assessment, as well as visually inspected and photographed the panels.
After 171 days of immersion, results showed:
The best results were with panels coated with the formulations containing WSP. Researchers also report that the measured hydrographical parameters were within the normal range indicating that the paint compositions are environmentally safe.
In March, a research team from the Shenzhen Institute of Advanced Technology of Chinese Academy of Sciences and Xiamen University announced the results of its study of a natural-based coating, which reportedly exhibited effective antifouling and lower toxicity.
To find an effective method to control fouling, the research team looked to a natural product with antifouling properties, camptothecin (CPT), as an environmentally friendly antifoulant. Rather than completing lab tests, researchers utilized a more real-world method in a sea trial.
Tested panels were made from six different materials, typically used for constructing underwater sensor housing. These included three metals and three plastics.
In July 2019, the panels were partially coated with the CPT-based paint and hung under a floating raft in Xiamen Bay, China, for nine months. The panels were submerged at a depth of one meter (3.3 feet).
After the nine months of submersion, the university reports that the CPT-based paint reduced biofouling by 73.33%-96.41% compared to the control unpainted areas (100% coverage). The team noted that the antifouling of plastic was better than of the metal materials.
Additionally, researchers deployed three underwater sensors under a moored surface buoy platform in Daya Bay, Shenzhen, China, in June 2020. The sensors reportedly remained clean after four months of deployment in the marine environment.
“Our results suggest that the CPT-based paint could be used as a potential solution to control the biofouling of sensor housings for long-term in situ applications in marine environments,”said professor FENG Danqing, one of the corresponding authors of this study.
“It is worth noting that the CPT-based paint also has great potential for other artificial submerged structures in the marine environment, such as ship hulls, oil platforms and aquaculture facilities,”added professor LI Jianping, who was the co-corresponding author of this paper.