Eat Your Probiotics

According to the most recent IPCC report, marine heatwaves have doubled in frequency in the last four decades. This is a direct result of the planet-warming greenhouse gases humans have released into our atmosphere; the ocean has absorbed more than 90% of the excess heat.

For coral reefs, marine heatwaves can be particularly problematic. Under normal conditions, corals host symbiotic algae (zooxanthellae) inside their tissues – which perform photosynthesis. The algae use the energy gained by photosynthesis to feed the corals (in return for a safe home), and their pigments give corals their bright colors. When temperatures rise, however, corals become heat-stressed and eject their algal friends – leading to a loss of color known as coral bleaching (Figure 1). Because the photosynthetic algae provide more than 90% of the coral’s nutritional demands, prolonged heatwaves and bleaching eventually lead to coral death.

Additional Players – the Holobiont

Many hundreds of bacterial species also live in the coral, and they help their host by fighting pathogens or providing other micronutrients. This group of bacteria is referred to as the coral “holobiont,” and the three units – the coral, the photosynthetic algae, and the holobiont – work together to create a healthy coral reef. However, studies have shown that marine heatwaves also negatively impact the coral holobiont; the community of bacteria present in corals becomes much less diverse at higher temperatures, as some species are kicked out under stress.

Previous research showed that coral bleaching could be slightly mitigated if corals were treated during a heatwave with a beneficial cocktail of such bacteria. This is effectively a dose of probiotics – just like humans take for our gut microbiomes when we consume foods like yogurt. While the observation that “probiotics” could prevent bleaching in corals was promising, an international team of scientists wanted to find out whether additions of probiotics could also prevent widespread coral death.

Coral on the Hotseat

To accomplish their goal, the team collected coral samples of the species Mussismilia hispida from the Coroa Vermelha reefs off the coast of Bahia, Brazil, and then established them in a series of aquaria. In a heat stress experiment, the water temperature in the aquaria was gradually turned up from 26ºC to 30ºC (78.8ºF to 86ºF), where it remained for ten days before returning to 26ºC for a recovery period of nineteen days. Half the corals were occasionally squirted with a beneficial cocktail of six microorganisms native to M. hispedia (a probiotic treatment), while the rest were squirted with saline solution as a negative control for comparison.

Following the experiment, it was determined that while 100% of the corals with the probiotic treatment survived the heat stress experiment, only 60% of the control (saline solution) corals survived. Furthermore, the team found by measuring the corals’ photosynthetic efficiency that while the photosynthetic ability of corals in the control group was far lower at the end of the experiment than before the heatwave, the photosynthetic ability of corals in the probiotic group rebounded during the recovery period – and had returned to normal by the end of the experiment.

Photosynthetic efficiency – (F_m – F₀)/F_m – was calculated as the maximum amount of light energy absorbed by the coral during photosynthesis, minus the background fluorescence, divided by the maximum. The higher the number of functional photosynthetic organs, the higher the maximum light that can be absorbed, and the higher the photosynthetic efficiency.

A Not-So-Silver Bullet

We continue to learn that the ocean and its creatures are not isolated pieces of a puzzle. All components of the ocean are interconnected, including corals and bacteria invisible to the naked eye. While we figure out how to combat climate change, and prevent the worst of its effects, the results of this research show that we may be able to buy corals a little bit more time by using tricks that nature has already devised (Figure 2). There are challenges of course; applying probiotics to entire reefs would be a logistical nightmare. Furthermore, each coral species may need a suite of different microorganisms – just as a human’s gut flora and ideal diet may differ from their pet dog’s. But while we figure out how to save our oceans, we will continue to need promising restoration treatments like the one described in this study. Probiotics might be an important tool in our toolbox, particularly for coral reefs.