With a little help from friends – the importance of symbiotic algae in coral temperature tolerance

Written by Maha Cziesielski

The dire state of coral reefs worldwide has become an international topic of concern. With an estimated one billion people directly depending on coral reefs for their various ecosystem services (WWF), it should come as no surprise that more proactive approaches to conserving them has gained attention. Increasing water temperature and acidification have been deemed the two most detrimental climate change consequences affecting coral health. The elevated water temperatures cause the breakdown of the relationship between the coral and their endosymbiotic algae (lives within the coral) of the genus Symbiodinium; an event known as bleaching.

Tolerance to increasing temperatures can vary between species, both locally and globally. Many species of corals have managed to inhabit reef systems of various temperature regimes, indicating the ability of corals to adapt to different environments. In an attempt to decipher underlying cellular mechanisms of adaptation, researchers studied temperature stress response of the anemone Exaiptasia pallida from three geographically and thermally distinct regions; North Carolina, Hawaii and the Red Sea (Fig.1). E. pallida (Image 1) acts as a model organism for corals; it does what mice have done for human research.

Picture1
Figure 1. – Annual average temperature profiles of sampling locations of the three investigated Aiptasia strains originating from North Carolina (Wilmington), Hawaii (Kāneʻohe Bay) and the Red Sea (Al Lith). The average monthly temperature is denoted by a solid line. Shading around represent maximum and minimum temperature (temperature data taken for closest location to sampling area from www.seatemperature.org). Annual profiles show that Red Sea has the warmest waters, while North Carolina has largest temperature fluctuations (taken from Cziesielski et al., 2018).
Picture2
Image 1. – A picture of the small sea anemone Exaiptasia pallida, used as a model organism for coral research.

Whole transcriptome and proteome analysis, total mRNA, and protein in an organism (Fig. 2, includes definition of terms mentioned), were conducted for each anemone under heat stress for the first time. While the authors reported commonalities in response between different strains of anemones, they also found that those originating from the Red Sea had a stronger response to oxidative stress, which is caused by the increased release of superoxide radicals. Production of these radicals, or reactive oxygen species (ROS), rises during heat stress and can hinder cellular function and damage cell structure – a primary cause of coral bleaching. The ability to efficiently ‘detoxify’ cells may therefore indicate a higher temperature tolerance in the Red Sea anemones.

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Figure 2. – Illustration of the central dogma of molecular biology highlighting the different omic layers. Transcriptome is the complete set of RNA transcripts produced by the genome at any one time. Proteome is the complete set of proteins expressed by an organism. mRNA is a subtype of RNA, which carries a portion of the DNA code to other parts of the cell for processing.

To validate that the detoxification potential of Red Sea anemones was higher than in anemones from colder waters, the authors directly measured the production of ROS during heat stress. Indeed, Red Sea anemones showed significantly lower levels of ROS production – but so did their Symbiodinium. In fact, ROS production in the algae directly reflected in the anemones, suggesting that the improved response accredited to Red Sea anemones could actually come from their symbionts (Fig. 3).

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Figure 3. – ROS produced by Symbiodinium (A) in hospite and (B) in isolate of different anemone strains (CC7: North Carolina, H2: Hawaii, RS: Red Sea). ROS was measured for control (25 °C) and heat stress (32 °C) conditions and normalized against total symbiont counts. Error bars denote ±1 standard errors. *: p < 0.05; n.s.: not significant (taken from Cziesielski et al., 2018). Only in the Red Sea anemones was there no increase in ROS production.

Re-analysis of the data in a manner different to standard procedures revealed that there was no significant difference in the response capacity of Red Sea anemones compared to others; reduced oxidative stress experience is driven by the symbionts temperature tolerance. Thus, anemones from the Red Sea may have adapted to warmer waters through association with more temperature tolerant symbionts. This highlights the importance of the coral-algae relationship and provides hope for corals to adapt in the future. The question remains; can they adapt fast enough?

References:

Cziesielski et al., 2018 – Multi-omics analysis of thermal stress response in a zooxanhellate cnidarian reveals the importance of associating with thermotolerant symbionts. Proceedings of the Royal Society B, 285(1877).

WWF (Accessed 11.06.18): http://wwf.panda.org/our_work/oceans/coasts/coral_reefs/coral_importance/

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