Microbial partnerships


Zooxanthellae are photosynthetic, single celled algae that exist in a symbiotic relationship with coral Photo: Cathy Liptrot

Coral reefs and many other marine organisms depend on partnerships with microbes for their survival.

For instance, corals are home to photosynthetic single-celled algae called "zooxanthellae". As well as providing food for the coral, these algae are responsible for the distinctive colours of the reef.

In return for food, the coral gives the zooxanthellae shelter and some essential nutrients, making a symbiotic, or mutually beneficial, relationship.

Several studies have shown that the types of zooxanthellae in the partnership affect the coral’s physiology, with some helping the corals to grow quickly and others improving corals’ tolerance for extreme temperatures.

This relationship is also very delicate, and coral bleaching occurs when the colourful zooxanthellae are lost due to extended periods of high temperature and strong light.

For a long time it was believed that coral bleaching was simply a mismatch between the needs of the two symbiotic partners, but we now know it involves the coral, multiple species of zooxanthellae plus associated fungi, bacteria, viruses and algae for whom the coral is host. The coral and its associated microorganisms are known collectively as the coral "holobiont".

We are studying the biochemical and molecular interactions within the holobiont that lead to coral bleaching and disease, so that we can predict what happens to the coral–algal symbiosis under changes in external conditions.

However, recent experiments and field studies at AIMS suggest that symbiotic relationships can also help with adaptation to climate change.

This applies also to other marine invertebrates, such as sponges, sea anemones, jellyfish, molluscs, echinoderms (which include starfish and sea cucumbers) and nematodes, with symbiotic microbes.

As with corals, the microbes in these relationships may provide food, but they potentially also help with thermal tolerance, reproduction, structural rigidity, waste reduction and, through chemicals produced as secondary metabolites, defence and immunity.

We’ve observed rapid changes in the symbiotic microbes of organisms like corals and sponges under changing climate conditions, so now we’re researching how important they are to the host organism’s ability to acclimatise. Plus, because the microbes can be passed to subsequent generations, they may contribute to long-term adaptation.

Understanding how microorganisms contribute to their hosts’ ability to adapt is essential if we are to reliably predict the consequences of global change.

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