The Trichodesmium consortium: the marine N-cycle at the microscale?

About half of all of the carbon fixed in photosynthesis on Earth takes place in the oceans. Most of this occurs in those regions which are far from land and which are usually quite poorly provided with the plant nutrients required to fuel growth. In the most desert-like regions found in the tropical and subtropical oceans, some bacteria use sunlight for photosynthesis in just same way as the plants on land and are also able to exploit the abundant nitrogen present in the overlying atmosphere. This nitrogen-fixing process not only supports the growth of these bacteria under these hostile, nutrient-poor conditions but also adds this additional source of nitrogen to the oceans that then acts a fertilizer for other marine life.

The nitrogen-fixers are usually found in ocean regions where there is a deficit in the availability of other forms of nitrogen and so are at a particular advantage in these areas. The nitrogen deficit is due to other types of bacteria that remove nitrogen from the ocean in a process known as denitrification. Usually, these denitrifiers, as they are called, are only found to be active in deeper waters where the reduced availability of oxygen allows them to thrive under conditions where other life is usually quite sparse. We have found, though, that the denitrifiers and the nitrogen fixers seem to be able to live together nearer to the surface and, what is more, that they are both active in the cycling of nitrogen in the oceans.

The research to be carried out in this proposal is focussed on firstly putting names to these denitrifiers, so that we can recognise where they are found in the oceans, and secondly, on trying to find out what these bacteria might be doing within this newly discovered association. This is important because the oceans help to drawdown carbon dioxide from the atmosphere and the rates at which they do so depend on how fertile they are. If the denitrifiers are removing large quantities of nitrogen from the surface waters then this will affect the overall productivity of the oceans. We rely on the microscopic plants found in the ocean to lock away some of the extra carbon dioxide that is released to the atmosphere by the burning of fossil fuels. If their vital nitrogen supply is reduced by denitrification then this unfixed carbon dioxide will be left in the atmosphere to contribute to future global warming.

Another important facet of this research is that the powerful greenhouse gas nitrous oxide can be released in the denitrification process as well as being consumed. Not only does this gas warm the planet when it is vented to the atmosphere but it is also responsible for breaking down some of the ozone layer that filters out hazardous ultraviolet rays from the sun. We need to know also, therefore, what the likely scale of the impact of nitrous oxide cycling within this consortium of bacteria is and how it might be affected by future climate change.

By improving our understanding of the scale of nitrogen inputs and losses that might occur in the newly discovered bacterial association we should be in a better informed position to assess how significant the partners in the consortium are in the larger scale global nitrogen cycle that influences the fertility of the land as well as the oceans.

It is envisaged that the main beneficiaries of the research outside of the immediate scientific community are likely to be those regulatory authorities concerned with the management of the European coastal margins and the provision of safe bathing and recreational waters in these regions, and conservation agencies concerned with the biodiversity of these regions.

The coastal margins are under threat from environmental change and in particular from future warming and deoxygenation. Trichodesmium blooms have caused serious loss of amenity in Australian coastal waters in recent years by closing beaches due to the production of surface scums. Their association with epibiotic bacteria and other organisms has also been implicated in the production of potent toxins including those causing the neurological symptoms of paralytic shellfish poisoning. Scums washed onto beaches pose a further but underappreciated threat to health because they can be borne further inland on winds when desiccated following stranding.

Trichodesmium is already present in the warmer temperate waters of the southwestern approaches to the UK and is routinely found in summer months in western coastal margins influenced by currents flowing toward the UK from the Gulf Stream. This is not a new phenomenon but one that is likely to become more frequent as the marginal seas around the UK become warmer. Analogous to the fate of surface cyanobacterial scums in freshwater environments, the collapse of Trichodesmium blooms injects organic material into the sediments where it stimulates the respiratory activity of the microbial community and deoxygenation. If Trichodesmium blooms become more frequent in our marginal seas it is likely that these deoxygenation events will begin to impact on the benthic biodiversity of UK waters

The research planned is concerned most directly with the role of the epibiotic flora in nitrogen cycling in Trichodesmium but can be expected to have wider applications to those agencies charged with maintaining the health of UK coastal waters since it will inform on aspects of the biology of these associations. These associations are not just implicated in the production of harmful toxins but are also the inocula for microbial processes that take place within the water column and sediments as blooms decay.