Understanding Marine Microbes and Symbioses

There are more than a billion micro-organisms in every litre of seawater and it is now known that microbes dominate the abundance, diversity and metabolic activity of the ocean. They are the unseen majority. Therefore, microbial communities play an immensely important role in sustaining the marine environment.

Current estimates of marine bacterial diversity range from a few thousand species to as many as two million. The limitations of traditional cultivation technology (generally only 0.1 to 1 per cent of marine microbes can be grown in pure culture using conventional approaches) mean that marine microbes have remained largely unexplored.

Importantly, the effect of a change in marine microbial diversity, abundance or biomass both in the water column and on the seafloor is unknown.

However, with the advent of molecular biological techniques, we are now discovering an exceptional diversity of marine micro-organisms and elucidating novel microbial functions including unique biotransformations and new bioactive metabolites, and determining the pivotal role that microbes play in ensuring that marine ecosystems are sustainable.

A symbiosis is a very close association between organisms of different species. In symbioses, at least one member benefits from the relationship, while others in the partnership can also benefit (mutualism), be unaffected (commensalism) or be disadvantaged (parasitism). Marine microbes form a variety of important symbiotic relationships with marine invertebrates such as sponges, anemones, corals, jellyfish, molluscs, starfishes, sea urchins, sea cucumbers and worms.

Symbiotic functions for marine microbes include:

• providing nutrition (through direct incorporation of dissolved organic matter in the seawater or through photosynthesis):
• communicating chemically between bacterial cells, a process known as “quorum sensing”;
• aiding reproductive processes;
• mediating chemical defence;
• facilitating thermal tolerance;
• contributing to coral structural rigidity;
• metabolising waste compounds; and
• producing natural chemicals known as secondary metabolites.

There are also many symbioses where the type of interaction between the host (coral) and their symbionts (microbes) remains unknown. With such a broad range of functions, environmental conditions that affect the distribution, abundance and/or diversity of symbiotic marine microbes could significantly affect host fitness and survival.

Global climate change will have a direct effect on the Great Barrier Reef. The primary influence will be a 1-3^oC increase in global sea surface temperature, along with predicted sea level rises. Other associated effects include increased acidity and run-off from the land.

Climate change will have a significant impact on marine microbes, potentially altering microbial diversity, function and community dynamics. Although microbes constitute by far the largest diversity and biomass of all marine organisms, they are often ignored during discussions about climate change.

This is despite the fact that microbial life on our planet has played a central role in either accentuating or mitigating the effects of climate change. Global elemental cycles of carbon and nitrogen are completely dependent on microbes, and changes to temperature, nutrient availability and acidity will have major impacts on microbial processes central to the climate debate.

The multidisciplinary Marine Microbes and Symbioses team is exploring the diversity and function of marine tropical microbial systems. It will first approach this enormous task by examining the cornerstone of coral reefs: the association between the animals and their symbionts.

Two model systems will be explored simultaneously: corals and sponges. Both exist in coral reef ecosystems due to their microbial associations and both are clearly under threat from coral bleaching and disease.

The team will examine:

• the biodiversity of symbiont groups within their hosts;
• the vulnerability and adaptive capacity of symbiosis in response to environmental change;
• the role of symbionts in regulating or causing disease; and
• the molecular mechanisms that underpin symbiosis and adaptation.

Group leader: Professor Linda Blackall CV in PDF file.

March 3, 2008