The changing earth and Cyanobacteria - Oxygen revolution in the past, what comes now?

Cyanobacteria is the oldest known fossil on earth. They are highly distributed in any type of environment and play a crucial role within the nitrogen cycle by fixing nitrogen from the atmosphere through photosynthesis. In the marine environment, cyanobacteria distribution seems to be concentrated around the equator with a few phylogenetic groups scattered further out towards colder waters. This global biogeographical distribution, limited by a physiologically feasible temperature range at which cyanobacteria can perform nitrogen fixation, has now been experimentally tested to help our understanding of the nitrogen cycle. 

Brauer et al. (2013) used the unicellular diazotrophic cyanobacterium, Cyanothece, as a model organism in their very straight forward experiment which examined the reasons for limitations of fixing nitrogen in high and low temperatures. Growth rates, nitrogen fixation rates and respiration rates of batch cultures were monitored in the lab which provided evidence that in cold waters nitrogenase activity is limited to an increasing respiratory cost per molecule of fixing nitrogen. Low temperatures also delay the onset of nitrogenase activity which consequently allow the cyanobacteria even less time during the day to fix nitrogen. 

Being a simple lab experiment, this study`s mechanistic explanation of temperature dependence of nitrogen fixing organisms is part of a bigger picture. Brauer et al. mentioned that their study not only explains global biogeographical distribution of Cyanothece, but it might also apply to many other nitrogen fixing unicellular cyanobacteria. Information about which role temperature plays in  nitrogen fixation is particularly important as global warming is predicting an increase in water temperatures, especially in higher latitudes. 

I wonder how many other organisms and processes in the ocean can influence this straight forward chemical reaction. I think this study is a constructive approach to increase our understanding of the nitrogen cycle as well as to help predict future changes. For a better understanding it would be interesting to compare cyanobacteria abundance in samples collected in different temperature ranges over the last decades. Devices such as the Continuous Plankton Recorder (CPR) sample zooplankton and phytoplankton and give information about geographical distribution, seasonal cycles and year-to-year changes in abundance over a large spatial area. Would it not be useful to haves something similar to the CPR for smaller organisms including unicellular cyanobacteria? Details about composition of picophytoplankton samples could then provide us with information about other limiting factors for nitrogen fixing organisms.

Cyanobacteria were playing the most important part in the oxygen revolution during the Archaean and Proterozoic Era, changing the atmosphere to a habitat more suitable for life. Present and future global warming, causing temperature increases in our oceans, seems to make conditions for cyanobacteria even more suitable. Lets hope that they still have the same intentions as 3.5 billion years ago.

Brauer V. S., Stomp M., Rosso C., van Beusekom S. AM., Emmerich B., Stal L. J. and Huisman J. (2013) Low temperature delays timing and enhances the cost of nitrogen fixation in the unicellular cyanobacterium Cyanothece. ISME Journal, 7, 2105-2115.

http://www.nature.com/ismej/journal/v7/n11/full/ismej2013103a.html