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Acclimatisation through changes in symbiosis

Acclimatisation in corals can occur due to 'symbiont shuffling' Photo: AIMS

AIMS is investigating whether a process called symbiont shuffling will allow coral to acclimatise to climate change.

Acclimatisation is the response of organisms to environmental change through non-genetic processes. It is different to adaptation, which involves inheritance of a genetic change.

Growth rates of corals, and their upper temperature tolerance limits, depend partly on single-celled algae—called zooxanthellae—which dominate the coral tissues. Early in life, most corals take up several kinds of zooxanthellae. Later, one type usually dominates the tissues while the other types persist at very low densities.

However, some of the low-density zooxanthellae—in particular the ones that have a higher bleaching tolerance threshold—are still healthy and may be able to increase their densities after the coral has bleached. This shift in the algal symbiont community is called ‘symbiont shuffling’.

Symbiont shuffling can change a coral’s physiological tolerances including its upper temperature tolerance limit.

But not all corals shuffle symbionts under stress. Our current research is following the dynamics of different symbiont types before and during stress to identify the mechanisms that prevent and allow shuffling to occur.

Our recent experimental and field studies have also highlighted how some coral reef species, including corals and sponges, can rapidly alter their bacterial symbionts in response to climate change conditions. We are assessing whether these symbiotic shifts influence the ability of the host to acclimatise to new environmental conditions.

AIMS is also investigating how non-genetic inheritance in corals may help them cope with climate change.

Non-genetic inheritance, or transgenerational acclimatisation, is the transmission of acquired traits from one generation to the next.

A growing body of evidence from non-coral organisms suggests that environmentally induced changes to DNA may allow much faster evolution than genetic adaptation. These ‘epigenetic’ changes result from changes to non-gene parts of the DNA sequence that affect the way genes express themselves.

Favourable symbiotic shifts that enhance growth or infer environmental tolerance to the host may be passed to subsequent generations, enabling long-term acclimatisation of these organisms under climate change.