Are marine invertebrate corals thermally tolerant?

Monika Raman, PSG College of Technology, Coimbatore

Due to increased anthropogenic pressure, coral reefs have suffered significant losses in recent years. Coral bleaching, or the loss of Symbiodiniaceae resulting from ocean warming, is currently one of the leading causes of reef deterioration. Corals lose their source of nourishment when their algae symbionts die, which typically results in widespread coral death. Now it’s necessary to gain a piece of knowledge on coral bleaching and the mechanisms that determine susceptibility or tolerance to thermal stress.

Corals can bleach and die at temperatures as low as 1–2°C above their maximum monthly mean (MMM). Coral bleaching susceptibility is linked to genetic variations in corals and algal symbionts; in bacteria and microalgal symbionts. However, it’s unclear whether similar mechanisms or variables contribute to coral temperature tolerance across species and locations. 

What may be the reason for coral’s thermal tolerance?

The temperature tolerance limits of corals from the Northern Red Sea, for example, are exceedingly high, reaching >5°C beyond their summer maximum, which is deadly for corals elsewhere. Despite sea surface temperatures regularly surpassing 30°C in the summer, the Red Sea as a whole is one of the hottest ocean basins, where corals thrive.

These higher thresholds are due to historical selection, as corals recolonized the Red Sea from the Arabian Sea and travelled through the warmer Southern Red Sea. A collaboration of researchers from Germany, the United States, New Zealand, and Israel have expressed doubts regarding the constancy of this temperature tolerance over the Red Sea’s gradient. 

They were also unsure if the same holobiont assemblages and molecular processes were performing at various geographical scales. “We used multi-temperature acute thermal stress tests to identify thermal tolerance limits of corals to find a solution,” said Voolstra, the lead author of the study.

Coral Bleaching Automated Stress System

Using the Coral Bleaching Automated Stress System, the researchers wanted to see the consistency of thermal tolerance limits across geographical scales (CBASS). They were able to do several comparable short-term thermal stress studies using the Stylophora pistillata colonies. They used colonies from 3 central red seas (CRS – hot region) about 0.3 – 10 km overwater distance and North Red Sea site – Gulf of Aqaba (GoA – Cooler region) about 1.000 km over water distance.

The relative temperature thresholds of GoA and CRS corals were very comparable (7°C higher than MMM). The absolute thermal thresholds of CRS corals, on the other hand, were on average 3°C higher than those of GoA corals.

Do holobiont assemblages play a role in their thermal tolerance?

“We determined gene expression and microbiome response of the coral holobiont to examine the molecular underpinnings,” says Valenzuela, one of the co-authors of the study. The transcriptomic responses to heat stress differed significantly between GoA corals and their symbiotic algae, with vigorous reaction in GoA corals and their symbiotic algae against a surprisingly subdued response in CRS colonies. Furthermore, temperature-induced expression of coral and algal genes was seen in GoA corals, whereas CRS corals exhibited constant high expression. Under heat stress, the bacterial community composition of GoA corals altered substantially, whereas CRS corals had stable assemblages.

Following that, a direct comparison of the response patterns underlying the calculated temperature thresholds revealed variations in gene expression response and holobiont topologies, suggesting that different thermal tolerance mechanisms work at different geographical scales. In contrast to a trend of persistently increasing heat resistance in CRS corals that can no longer attune, they read GoA corals’ behaviour as that of a robust population reaching a tipping point.

Their research illustrates the value of using standardized short-term heat stress experiments to determine thermal thresholds, followed by molecular interrogation, to detect variations in thermal tolerance response patterns and related holobiont characteristics (e.g., alleles, genes, microbes). Such response variations imply unique heat tolerance mechanisms by which the coral communities compete with ocean warming.

Also read: HEI10: How do sex cells receive the right genetic mix?

Reference:

1. Voolstra, C. R., Valenzuela, J. J., Turkarslan, S., Cárdenas, A., Hume, B. C. C., Perna, G., Buitrago‐López, C., Rowe, K., Orellana, M. V., Baliga, N. S., Paranjape, S., Banc‐Prandi, G., Bellworthy, J., Fine, M., Frias‐Torres, S., & Barshis, D. J. (2021). Contrasting heat stress response patterns of coral holobionts across the Red Sea suggest distinct mechanisms of thermal tolerance. Molecular Ecology, mec.16064. https://doi.org/10.1111/mec.16064

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