How important is nitrogen?

A review of: Capone, D. G. (2008) The marine nitrogen cycle. Microbe 3: 186-192.


This paper is a neat and informative review of the marine nitrogen cycle and its potential impact on biogeochemical cycling. It gives a helpful description of the history and development of this research area spanning from the initial inception of the idea of nitrogen fixation in the early 1800s, through to the current discoveries of the present day. Alfred Redfield is highlighted for his shrewd observation which identified the role of marine plankton in maintaining the ratio of phosphorus and nitrogen in the deep ocean – the Redfield ratio. He also outlined the debate over which nutrient is the primary limiting factor in our oceans, which is still a highly contested area of discussion today.

Various studies have implicated Trichodesmium as a key global contributor of oceanic nitrogen input. However, parallel geochemical investigations were advocating a larger nitrogen input than could be accounted for by Trichodesmium on its own. This implies that either alternative agents weren’t noticed, the input of Trichodesmium was underestimated, or both. Nevertheless, Trichodesmium is undoubtedly an example of the major role that diazotrophs hold in nitrogen input in the sea. Researchers are aware of many diazotrophs in the sea but how much nitrogen input they have is unknown. Recent discoveries of diazotrophs less than 10µm in size have been observed among plankton of tropical and sub-tropical waters. The assessment of abundance of other diazotrophs, like heterosystous cyanobacteria of the genus Richelia, is being investigated. It is believed that, even with low numbers, microbes of this size division with nitrogenase activity could produce a significant quantity of nitrogen across large volumes of the ocean. Unfortunately, methodological problems of culturing oceanic bacteria are making it difficult to determine these affects.

The nutrients that limit the productivity of nitrogen fixers seem to be subject to the different ocean regions and determined by the relative availability of phosphorous against iron. Experimental evidence of this can be observed in the Atlantic Ocean. Diazotrophs dwelling in the surface waters in the northern expanses of the Atlantic Ocean show an increased prevalence of phosphorus stress than iron stress. This is due to the input of iron in to the sea via atmospheric dust transported from the desert regions of North Africa. This raises concentrations of dissolved iron to high nanomolar levels, which subsequently drives the drawdown of phosphorous until it becomes limiting. Recent investigations have also advocated that dissolved concentrations of CO2 affect oceanic nitrogen fixation rates. By doubling the partial pressure of CO2 in culture to emulate predicted oceanic conditions that could occur over the next 100 years, it was reported that an increase in nitrogenase activity and growth rate in Trichodesmium can be observed.

Feedback processes are advocated to preserve levels of global fixed nitrogen within reasonably stringent boundaries. This can be concluded from geochemical models, field investigations and paleoecological evidence. It is suggested that global processes that remove biological nitrogen are being overestimated or inputs of the same element are being underestimated. The future of this research area lies in the application of new sampling and measurement technologies as well as the refinement of current modelling methods. I think that climate change scientists should incorporate marine nitrogen fixation in to their work, in order to combat anthropogenic pressures such as anticipated increases in oceanic temperature, CO2 concentration and the run off nitrogen based fertilisers produced to support our growing population.