Scleractinia & Symbiodinium: a tragic love story

The exchange of small molecules, or metabolites, between organisms and their environment gives rise to the complexity of life on Earth1. One of the examples of such complexity is ourselves, our body and the full array of microorganisms (microbiota) that live on and in us, humans. 

Just like us, stony corals (Scleractinia) also live in a symbiosis with associated microorganisms. These include bacteria, archea, fungi, viruses, and single-cellular algae called Symbiodinium.

Under normal conditions, these algal cells provide 90 to 95% of the energy required for stony corals to live happily1. The algae provide lipids, carbohydrates, amino acids, and O2 to Scleractinia. In return, the algae get to recycle nitrogen and other inorganic compounds from the coral, that they can use to fuel their own cell metabolism.

Climate change causes a rise in water temperatures; and, this form of environmental heat stress, disrupts the symbiotic relationship between corals and its algae, which results in coral bleaching. 

Researchers have now discovered that heat stress destabilizes this basic nutrient exchange between corals and their algae; and, this turn of events happens long before the bleaching process becomes obvious. 

In fact, Rädecker and his colleagues2 at the Red Sea Research Centre, discovered that heat stress causes the coral to need more energy, which swings the coral-algae symbiosis from a nitrogen- to a carbon-limited state. This actually ends up reducing the movement and recycling of carbon, and creates an animosity between these life partners. 

The coral starts disliking the algae, because the algae it’s not fully doing its work – recycling carbon. As such, the stony coral starts thinking that sheltering such lazy algae is not beneficial anymore. On top of that, the poor coral needs to catabolically degrade more aminoacids, to compensate for the increased energy demands triggered by the warmer waters. Such demanding “cooking tasks” from the coral side, create fury, and actually increase the release of ammonium; which in turn, promote the algae to grow. So, not only does the algae not clean the building properly, but calls its algal friends and they move in. 

Under stress, the coral kicks the algae out. But since this marriage is essential to its life support, the Scleractinia cannot feed itself properly anymore. The stony coral goes hungry, and slowly dies out….

Poor coral…

Underwater love

References:

1          Williams, A. et al. Metabolomic shifts associated with heat stress in coral holobionts. Science Advances 7, eabd4210, doi:doi:10.1126/sciadv.abd4210 (2021).

2          Rädecker, N. et al. Heat stress destabilizes symbiotic nutrient cycling in corals. Proceedings of the National Academy of Sciences 118, e2022653118, doi:10.1073/pnas.2022653118 (2021).

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