How do marine symbioses effect nitrogen fixation and transformations?

A review of: Fiore, C. L. et al. (2010). Nitrogen fixation and nitrogen transformations in marine symbioses. Trends in Microbiology 18 (10): 455-463

This is a clear and extensive review of the symbiotic relationships and their related influence on nitrogen fixation and nitrogen transformations in the marine environment. The note aims to evaluate current understanding of these symbioses and identify areas of interest for further research.

A description of the processes and the various microbes involved in nitrogen fixation, nitrification, denitrification and ANAMMOX is given. This provides a useful summary of these important global processes in the marine environment and helps the reader to reach a deeper understanding of the numerous symbiotic relationships which have been observed between nitrogen-fixing prokaryotes and oceanic invertebrates. These include sponges, corals and sea urchins as well as various protists including dinoflagellates, tintinnids, radiolarians, diatoms and tunicates. Further associations have also been reported in sponges and corals involving nitrification, denitrification and ANAMMOX. The paper should be consulted if a more detailed description of the role of nitrogen fixation and other nitrogen transformations in the following symbiotic systems is desired.
· Shipworms
· Diatoms
· Corals
· Sponges

These relationships between a eukaryotic host and their nitrogen-fixing, nitrogen- transforming symbiont can be characterised by; the coevolution of host and symbiont, the benefits and costs to each organism and the degree of integration. These interactions are commonly associated with the transfer of metabolic products. Many symbioses are essential to each organism particularly in environments where each partner would be unable to survive either aposymbiotically or as a free-living entity. This can be crucial in oligotrophic conditions where nitrogen-fixing associations could increase nitrogen fixation and better utilise inorganic nitrogen resulting in an ecological advantage.

The influence of these symbioses is yet to be reliably calculated. However various interpretations of the possible effects are proposed, specifically surrounding the global biogeochemical cycling of essential nutrients. Current nitrogen budgets have been determined without the consideration of nitrogen fixation and further nitrogen transformations which are facilitated by symbioses between marine prokaryotes and a diverse range of hosts. The wider implications and significance of these associations are currently unknown. Recent research advocates that sponges hold a vital role in nitrogen cycling in coral reef systems that has been previously overlooked. On a global scale, the symbionts of dinoflagellates and diatoms have also gone unnoticed as potentially important contributors to global nitrogen cycling. In light of this, revised nitrogen budget calculations should be inclusive of these factors in order to resolve the gaps in current estimations.

The effect upon these symbioses because of the existing threat of anthropogenic environmental change is unknown. Environmental pressure from eutrophication is suggested to inhibit nitrogen fixation as well as the symbiotic relationship frequently observed between nitrogen-fixing bacteria and various hosts. This is due to an abundance of particulate and inorganic nitrogen at affected sites. Ocean acidification from increased atmospheric CO2 concentration is identified as a hazard to calcifying organisms which are hosts of nitrogen-fixing symbionts (e.g. sea urchins, corals and shipworms). This is reported to have limited effect upon denitrification rates; however an increase in CO2 concentration is suggested to reduce nitrification. These parameters all contribute to a potential decline in species diversity of hosts sheltering these microbial symbionts. This suggests a reduction in symbiont diversity will occur, possibly reducing evolutionary adaptation to changing conditions.

Three major avenues of additional investigation are highlighted;
· The physiological characterization of known symbioses.
· The discovery of new symbioses involving nitrogen fixation and other nitrogen transformations.
· Monitoring the effect of the evolutionary and ecological development of these symbioses in relation to their hosts and ecosystems.

Filling these gaps in our understanding is essential in determining the impact of these symbioses on host ecology and formulating their role in oceanic biogeochemistry.