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An exceptional "dual feeding" strategy underlies a Mediterranean coral's resilience to rising sea temperatures, according to a in Nature.

The stony coral Oculina patagonica is known to feed itself with or without algae. Now, the first direct comparison of algae-hosting cells in O. patagonica and other coral species reveals how this flexibility works at the genetic and cellular level.

First discovered in the Gulf of Genoa in 1966, O. patagonica was thought to be an invasive species from the Atlantic, until recent studies showed it is native to the Mediterranean, quietly persisting in small numbers for millions of years until changing conditions helped it expand.

The coral has spread rapidly since. It has established populations along most of the shallow coasts of the Mediterranean Sea, where water temperatures range from around or below 10ºC in winter to over 30ºC in summer.

"When first documented in Levantine waters, it was thought that O. patagonica would not survive because summer temperatures were too high, but against all predictions, it established itself and populations are growing," explains Dr. Shani Levy, first author of the study who carried out the work at the Centre for Genomic Regulation (CRG) in Barcelona.

Oculina is a type of stony coral, marine animals that build hard skeletons. Stony corals host photosynthetic algae in their cells and rely on them for energy in a mutually beneficial symbiotic relationship. The algae can provide up to 90% of a coral's energy requirements, fueling the formation of massive calcium carbonate skeletons that provide the structure and foundation for coral reefs.

However, Oculina is different. Its skeleton is small and its partnership with algae is optional. When the Mediterranean's sea temperatures climb past 29°C, it expels algae from its host cells, losing its brown-orange color. This bleaching event would be fatal for other corals, but Oculina survives long enough to regain algae in its host cells during the cooler autumn waters. It can even survive without any algae at all, with populations found in caves or at depths of 30 to 40 meters, where little light penetrates.

"Oculina's ability to live without a photosynthetic partner allows it to settle in deeper waters where less light is abundant, or tolerate turbid waters, where light is blocked by increased sedimentation caused by navigating ships. That's a huge advantage in the human-altered Mediterranean Sea and one of the reasons we chose to study this species," explains Dr. Xavier Grau-Bové, co-author of the study and postdoctoral researcher at the Centre for Genomic Regulation (CRG) in Barcelona.

The authors of the study explored the stony coral's resilience at the genetic, cellular and evolutionary level. The research group specializes in combining genome sequencing with single-cell sequencing to uncover how life's cellular building blocks evolved in species, especially in non-model or understudied organisms.

They sequenced Oculina patagonica's genome and analyzed tens of thousands of individual cells to look at which genes are used when the marine animal contains symbiotic algae vs. when it doesn't. The researchers also built similar cell atlases for two tropical stony corals that depend entirely on algae to allow for direct comparison between species.

They discovered that when algae are present, Oculina's cells rely strongly on lipids, or fats, obtained from the symbionts. These lipids are important building blocks and can be stored for later use, giving the coral a more stable energy reserve than sugars alone.

When algae are absent, Oculina shifts its cell programs, boosting the activity of immune-like cells that could be clearing away dying symbiotic cells. It also expands gland and digestive cells, allowing the coral to capture and digest particles directly from the water. This mode of feeding is known as heterotrophy.

"Oculina is resilient because it does not strictly depend on photosynthetic products from the algae," explains ICREA Research Professor Arnau Sebe-Pedrós, senior author of the study and researcher at the Centre for Genomic Regulation (CRG).

"It can obtain photosynthetic products when the algae are present, and that's probably optimal, but Oculina can also survive feeding only heterotrophically, eating small organic particles and plankton captured and digested in the gut," he adds.

When the researchers compared cell atlases, they found the same cells and gene pathways Oculina uses to feed heterotrophically in other corals that are algae-dependent, but they are switched. The discovery means the ability to feed without algae is not unique to Oculina, but an ancient ability that is present across corals, likely conserved from a common ancestor.

"Oculina's life strategy seems to be one of resilience through diversification. It didn't need to invent a completely new lifestyle from scratch but rather dust off some old tools in its toolbox," says Dr. Grau-Bové.

The study is a window into how marine species are adapting to climate change in the Mediterranean. As a semi-enclosed sea, Mediterranean waters experience sharper swings in temperature, salinity and nutrient inputs than the open ocean.

"It acts like a natural stress test," says Dr. Levy. "Corals and other organisms living here are already coping with more extreme fluctuations, so the Mediterranean gives us a kind of preview of how marine life might fare under accelerated climate change."

While the study suggests corals like O. patagonica may have a better chance of surviving in warming oceans, the researchers caution that Oculina itself is not a major reef-builder, so it's unlikely to mitigate the loss of coral reefs, ecosystems which cover less than 1% of the ocean floor yet support a quarter of all marine species.

"The best way to help any marine ecosystem, reefs included, to withstand this warming world will always be to prevent the warming in the first place," concludes Dr. Grau-Bové.