Conservationists and the general public alike are becoming increasingly aware of coral bleaching and the negative impact it has on the marine ecosystem. This natural phenomenon has been studied and in the past decade, several discoveries have shed light on the factors that trigger it and possible measures we can take to reduce or prevent coral bleaching episodes. Recently, a team of researchers found out that exposure to direct sunlight increases the intensity of bleaching episodes and also causes higher recovery times. 

Similar findings are helping scientists/ marine biologists across the world mitigate and reduce the severity of this occurrence. So, what is coral bleaching and what triggers it? . Due to increasing ocean temperatures (due to global warming), the waters around reef ecosystems are rising rapidly. Corals are incredibly sensitive to changes in their surroundings and a sudden spike in temperature causes coral bleaching. Frequent fluctuations in temperature disrupts the symbiotic relationship between the coral host and its dinoflagellate symbionts. These symbionts, commonly referred to as zooxanthellae, help nourish the coral with food they prepare through photosynthesis. This is an important source of nutrition for most reef-building species of coral. Increase in ocean temperature causes the coral to shed their algae covering, which causes malnourishment and eventually leads to  bleaching. 

Ongoing research on coral nutrition has given us a better understanding of the diet requirements of these fascinating marine creatures. But, latest study results in the field show that isolating certain microbiomes could greatly benefit corals susceptible to heat stress. The push to improve the heat tolerance of corals has led scientists to test the benefits of certain bacteria and other microbes on coral health. Since corals already share a close relationship with a variety of dinoflagellates, scientists were able to derive from the dietary benefits of probiotics in humans and implement them into a coral’s diet in a bid to fortify its heat tolerance properties.

A group of scientists led by GEOMAR Helmholtz Centre for Ocean Research Kiel are spearheading this research project and the findings are published in journal Microbiome. The team looked into the benefits of probiotics in human health and immunity and tried to isolate microorganisms that could aid corals with heat tolerance abilities.

Study Methods and Findings 

Corals rely on a range of microorganisms present in oceans to sustain themselves. Most coral species rely on microbes, such as dinoflagellates, other protists, fungi, bacteria, archaea, and viruses. The research team was aware of certain functions these dinoflagellates performed to help the corals like keeping up a constant supply of carbon and several essential amino acids. But this time, the microbes were studied for their ability to help corals develop better heat resistance.

“The idea is that probiotic bacteria with beneficial functions could help a coral to better withstand heat stress,” explains Dr. Anna Roik from GEOMAR, lead author of the study, which was funded as part of a Future Ocean Network project at Kiel University. “In the current study, we tested the approach of a ‘microbiome transplantation,’ inspired by microbiome-based applications we know for example from clinical treatments,” Roik continues.

Using two primary reef-building species of coral found in the Andaman Sea off The coast of Thailand, the team were able to study the impact of these microbes on wild specimens. Using a process known as microbiome transplantation, the research team conducted experiments on Pocillopora and Porites species and recorded the changes in heat tolerance levels in corals. The team inoculated the test coral environment with heat-tolerant “donor” coral tissue. They chose heat resistant samples, studied the microbiome levels and types on it’s tissue and isolated them and introduced them into corals that did not display natural heat tolerance. 

“We then used material from the coral tissue of the donor corals to inoculate conspecific, heat-sensitive recipients and then documented their bleaching responses and microbiome changes using a genetic analysis method called 16S rRNA gene metabarcoding,” said Dr. Anna Roik from GEOMAR, lead author of the study in the report

The team found that recipient corals from both test species showed lower effects of bleaching and recovered faster from a stressful episode. The team subjected the test corals to temperatures of 34C, which is higher than the maximum heat threshold of corals in the region. By using the microbiome makeup from heat-tolerant coral species, the team were able to negate a large chunk of bleaching effects. 

“The results show that the inoculated corals were able to resist the heat stress response for a short time,” explains Prof. Dr. Ute Hentschel Humeida, head of the Marine Symbioses Research Unit at GEOMAR and co-author of the study. “In addition, the microbiome data suggest that the ‘inoculated’ corals may favor the uptake of putative bacterial symbionts,” Dr. Anna Roik continues. “However, further experimental studies are required to unravel the exact mechanism of action, as well as long-term field-based studies to test the durability of the effect,” says the marine biologist, looking ahead.

Future Applications

Another similar study performed on corals in the Red Sea showed similar results. This study, published in february 2021, showed that corals underwent a dynamic genetic change when exposed to healthy strains of bacteria found in heat resistant corals. Corals in this region have to endure vast temperature changes and are a perfect specimen for the study. This team used a single coral species and inoculated it with a BMC consortium consisting of six bacteria strains. 

The team observed that the new coral’s microbiome underwent “dynamic genetic and metabolic alterations that boosted its chances of survival under heat stress.”

These results, observed in a lab setting, can be implemented into wild corals with the help of planning and devising clever delivery mechanisms. The researchers of the second study are looking to experiment with scaling up the operation into the wild and seeding reefs using robots guided by AI and cameras. The chosen strain of probiotics could be injected into sediments or directly to vulnerable corals. Also, different delivery systems could be tested out as probiotics are available in liquid, pill, capsule or chewable forms for humans. Similarly, scientists are looking at slow-release ‘pills’ before a predicted bleaching event, helping the corals survive a tough period.