Estimating atmospheric nitrogen fixed by cyanobacteria at the ecosystem scale.

It has long been known that diazotrophic cyanobacteria are capable of fixing atmospheric nitrogen into biomass for their own nutrition, conferring the advantage that they are free from dependence on dissolved organic/inorganic nitrogen (DON and DIN) supplies. It is also known that this process may contribute significantly to the amount of nitrogen available in an environment. This study aimed to quantify the transfer of diazotrophic nitrogen into an estuarine ecosystem during a summer bloom of Nodularia spumigena.

The relative contribution of diazotrophs to total nitrogen input has already been shown to vary greatly between aquatic systems, dependent on the trophic status of the ecosystem, as well as the seasonality, frequency and size of algal blooms. Studies in lake systems have produced estimates ranging from 6-82% of annual nitrogen input, whereas estuaries have been the focus of fewer studies, with estimates range from 3-39% of annual nitrogen input being fixed by diazotrophs. The conventional view is that diazotrophic cyanobacteria are less successful in estuarine systems due to increased grazing pressure from zooplankton and environmental stress.

The experimental method used in this study is promoted as a more cost-effective way of estimating the diazotrophic nitrogen contribution to an environment. The authors explain the expenses and limitations of the mesocosm-scale incubation assays and the ¹⁵N labelled N₂ tracers used in past studies, and endorse their own in-situ techniques as an economically viable alternative. A comparison of stable nitrogen isotopes ¹⁵N and ¹⁴N in pre- and post-bloom samples, together with measurements of plankton abundance using chlorophyll fluorescence, among other techniques, were used to ultimately determine diazotroph contribution.

These techniques generated a conservative estimate of diazotrophic nitrogen flux of 146 tons, accounting for 177% of the total summer nitrogen-load. The authors acknowledge the difficulties of placing this result in the context of total annual nitrogen flux, yet by using data from a study the previous year, they estimated a total annual nitrogen yield of 695 tons, taking into account fluvial, atmospheric and diazotrophic sources. They therefore state that this ‘back of the envelope calculation’ suggests diazotrophic cyanobacteria are responsible for 22% of the annual total nitrogen load.

However, it had been shown in a previous study that N. spumigena are un-grazed in the estuary studied, so the results may offer limited inferences for other estuarine systems where cyanobacteria are more heavily grazed, as well as estuaries that experience less severe algal blooms. These limitations, along with any sources of potential methodological error are acknowledged by the authors.<>

Despite the differences between estuarine systems, the result of 22% contribution to total nitrogen load by diazotrophs sits directly central in the range of previous estimates. This study thus does not challenge the conventional view in its field, its contribution is rather in its suggestion that the experimental method used is the most cost effective technique for further research in the field.

Woodland, R. J., & Cook, P. L. (2013). Using stable isotope ratios to estimate atmospheric nitrogen fixed by cyanobacteria at the ecosystem-scale.Ecological Applications.