Are Coral Reefs Doomed?
Probiotics to the rescue!
Would you like to swim in the midst of scintillating fish in coral reefs? Would your kids? The future is bleak for diving fans, with the ever-increasing bleaching of these gorgeous ecosystems.
What is coral bleaching?
Corals are animals that have a symbiotic relationship with algae, Zooxanthellae, that coats the corals’ surface, giving them their color, and most of their food. Exposure to stress like increased water temperature and pollution causes the algae to leave the coral’s tissue. The coral thus turns white, weak, and anemic, having lost its main source of nutrients. White corals look like skeletons, but they are not yet dead, although a prolonged loss of algae will cause them to die. This is the process called bleaching.
How common is coral bleaching?
Stressful events probably have always happened, but due to human activities and global warming, such events are becoming much more frequent, and do not let corals recover from the precedent event when the next stress hits, leading to massive coral extinction.
Earth has lost 50% of its underwater paradises over the last 30 years and is expected to lose 90% by 2050.
As the planet warms, over 90% of the heat is captured by oceans, only making matters worse. And without coral reefs, marine biodiversity drops steeply, with major impacts on the food chain. According to National Geographic, an estimated 4000 fish species and 25% of marine life rely on coral reefs at some point in their existence.
Is there still hope? What can be done?
Despite the dark situation, there’s hope to save the corals. Coming from bacteria.
Raquel Peixoto, a marine science professor at Saudi Arabia’s King Abdullah University of Science and Technology, has been working for years on the development of probiotics to support corals in this crisis. She has described previously mechanisms of action, challenges, opportunities and pinpointed the prospect consortia of potentially helpful microorganisms.
Indeed, other organisms beyond algae associate to the coral’s holobiont, contributing to its biology — in particular, to stress tolerance and adaptability, more specifically through nitrogen fixation, sulfur cycling, scavenging reactive oxygen species (ROS), antibiotic production...
This observation led researchers to test the hypothesis that coral’s beneficial microorganisms could strengthen the animal’s resilience, in a new study tracking the survival of corals Mussismilia hispida exposed to bleaching conditions in the presence and absence of a probiotic inoculum.
The scientists simulated a heatwave by exposing corals in aquariums to heat, with the highest point reaching 86°CF/30°C for 10 days, and they sprayed aquariums with either a control saline solution or Beneficial Microorganisms for Corals (BMCs), namely with a consortium coming from the natural reef’s ecosystem. The BMCs selected were an association of Bacillus lehensis, Bacillus oshimensis, Brachybacterium conglomeratum, Planococcus rifietoensis, and Salinivibrio sp.
These bacteria were selected based on the absence of antagonist activity against other selected BMCs, and no previous record of being harmful to humans or other marine life. Beneficial traits included nitrogen fixation, denitrification, dimethylsulfoniopropionate degradation, ROS scavenging potential, and antagonistic activity against two coral pathogens, Vibrio coralliilyticus, and Vibrio alginolyticus.
What happened in this study?
After the challenge, the stressed control corals expressed a 40% mortality, while the corals exposed to the beneficial bacteria consortium all survived. They were much quicker to recover their photosynthetic efficiency.
Stressed corals seem to have a quicker uptake of bacteria from their environment. When they are weakened by the heat and absence of their algal symbionts, it appears crucial to bring on board good bacteria, which will occupy the niche and create a barrier effect, protective against the colonization by pathogens — quite similar to what happens in humans. But there’s much more to it.
Transcriptomics, the science that reveals how genetic expression changes in response to their environment, showed reprogramming in over 2000 groups of genes with pronounced differences between the control and probiotic-treated group. It was possible to identify that the short-term genetic responses involved immunity, inflammation, apoptosis (the process of programmed cell death), and cellular reconstruction, among others.
This splendidly confirmed the usefulness of the probiotic approach to sustaining coral reefs.
Interestingly, the BMCs appeared to influence the overall composition of the coral’s microbial community, although at T3 the probiotics delivered were no longer discernable.
Is it possible to scale up in the reality of oceans?
While it is easy to spray bacteria in a tank, it is much more difficult to do so in the vastness of the oceans. According to Raja Dhir, co-founder of Seed Health, a probiotic company partnering with the research project, even depositing the bacteria directly on the surface of the corals gives them very little probability of attaching.
One option explored by Raja Dhir and Raquel Peixoto is to deliver the bacteria through a sort of patch coated with a gel-like material that mimics the coral mucous layer and could help bacterial transfer. The team also evaluates the feasibility of using robots to cover more coral reefs more quickly, in this race against time.
“We take the stance that if it’s human-induced behavior which is causing the decline of these ecosystems, then we have a moral imperative to intervene in the most fragile of those ecosystems if we know we can do so safely and efficaciously.” Raja Dhir
The coral problem is not solved yet, and our responsibility and first action should of course remain to mitigate climate change, but we can also start to address the downstream effects of our human activities.