Nitrogenase Activity and Temperature

While being here in Iceland, Jackie and I both have a great opportunity to develop a research project.  Over the past couple of weeks I have noticed that particular species of nitrogen fixers are growing in select stream temperatures.  For example, “Rock” Nostoc -  Nostoc c.f. pruniforme (Kützing) Hariot, is only found in colder stream temperatures, while “Pink” Nostoc, - Nostoc spongiaeforme Agardh ex Born Flah, is found in warmer streams.  It was interesting to see such a distinct species preference to temperature.  I started to wonder what would happen to these species if the stream temperature that they are acclimated to were to change.

"Rock" Nostoc - Nostoc c.f. pruniforme

            It is well documented that global temperatures are increasing (NASA 2013). All organisms, in general, have several physiological processes that are regulated by temperature-dependent enzymes including cellular respiration, photosynthesis, and for a special group, nitrogen fixation.  Enzymes have a threshold for both cold and hot temperatures. As Jackie mentioned in the previous post, The Number, a select group of organisms have the ability to obtain and use nitrogen gas from the atmosphere, which is unavailable to non-nitrogen-fixers. These organisms are called cyanobacteria and they have a specific enzyme, called nitrogenase, which allows them to do this.  Nitrogenase functions to break the triple bond in nitrogen gas to yield ammonium, which is then used by the cyanobacteria to build biomolecules essential for growth.  Fixing nitrogen is an energetically expensive process and, therefore, not advantageous in nitrogen-rich environments.  However, the streams we are working in are not nitrogen rich, and many species of cyanobacteria are definitely present, with some clear shifts in species composition across the temperature gradient.  Given these observations, I began to wonder, how does temperature affect nitrogenase activity in different aquatic cyanobacteria species?  Is the rate strictly driven by temperature, or have these species adapted to certain stream temperatures in ways that lead to differing relationships between temperature and nitrogen fixation rates among the different species? Can cyanobacteria found inhabiting cold streams rapidly increase nitrogen fixation rates in warmer streams and vice versa?

            In order to investigate this question, we will be doing a reciprocal transplant experiment. We will collect dominant cyanobacteria (mostly Nostoc pruniforme) found in cold streams (~10˚C) and transplant them into both colder (~5˚C), warmer (15˚C), and hot (25˚C) streams. We will do the same for Pink Nostoc - Nostoc spongiaeforme, whose resident mean stream temperature is about 15˚C, as well as other dominant cyanobacteria species.  By the end of the transplants, cyanobacteria from each of their resident streams will be relocated to other streams spanning this temperature gradient, with 5-6 replicates for each species. I know I mentioned previously, that temperatures are increasing, so why put samples into colder temperatures?  Assuming nitrogenase reacts to temperature like other enzymes, its activity rate should decrease in colder temperatures and increase in warmer temperatures, up to some threshold.  Placing the dominant cyanobacteria species across a wide temperature gradient and measuring their nitrogen fixation rates will help us to better interpret and understand the relationship between temperature and enzymatic activity and where and under what conditions we should find each species.

          

"Pink" Nostoc - Nostoc spongiaeforme

  The goal of this experiment will be to see how these different species respond to different temperatures, and their ability to acclimate to a new environment. It is important to know the temperature threshold these species can withstand and how nitrogen fixation rates are likely to change in a warming world. If their temperature threshold is limited, it might be possible that in coming years, community composition will shift due to rising temperatures which could lead to species being out competed or lost entirely. Nitrogen-fixers play an important role in ecosystems where nitrogen is limited because they provide a source of nitrogen for other organisms, which feeds back on the ecosystem as a whole by influencing photosynthetic rates, the production of invertebrates and fish, and how other elements like carbon and phosphorus cycle as well.