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.

A nanoscopic insight into marine bacterial interactions

I was intrigued after Colin’s lecture (Microbes in ocean process – carbon and energy cycling, 16^th October 2013) touched upon the notion that marine bacteria may have inter-species connections and therefore decided to review this paper, aware that it was published several years ago.

It is understood that around 50% of global ocean carbon cycling is mediated by free-living bacteria, but interactions with primary producers such as Cyanobacteria are poorly understood.  The authors of this paper used atomic force microscopy (AFM) for high resolution imaging of bacteria from whole sea samples, in an attempt to gain a better insight into the interactions between ‘free-living’ bacteria and the organic matter continuum (POM through to DOM as outlined in the lecture).  They found that an average of between 30 - 35% of ‘free-living’ bacteria were in fact conjoined (both heterotrophic and autotrophic Cyanobacteria) and furthermore, up to 55% of the bacteria observed were connected by extensive pili, colloidal gels or networks of up to 20 cells (Fig. 1).  Within the gel matrices, coccolithophore and diatom remnants were found (Fig. 1), adding another dimension to oceanic carbon cycling and how carbon reaches the ocean floor.  The gel may collect particles aiding the coalition of marine snow and influencing sinking rates.

Figure 1.  Examples of the AFM images included in the paper.  The extensive pili network is clearly identified in the first image (a) and the second image (c) shows conjoint cells and a coccolithophore surrounded by a gel matrix.  The colour spectrum denotes elevation.

In the three geographical areas sampled, the authors found bacterial networks and gels for around 36% of the time in the temperate coastal and open ocean samples but interestingly, they did not detect any in the Antarctic samples.  Whilst this was not commented on, it may be that the low temperatures at the Antarctic (around -0.8°C at sampling) do not allow the colloidal matrices to develop.  This could be readily tested in controlled manipulative experiments and if correct, may offer further insight into possible differences in ocean processes at high versus low latitudes.

Whilst this study demonstrates the occurrence of conjoined cells, networks and gel matrices, it does not however, explain any underpinning causes.  The authors surmise that the conjoint bacterial cells may be due to symbioses, parasitic, antagonistic or accidental interactions.  They also suggest that the intimate associations in the bacteria/Cyanobacteria couplings may be adaptive and have biogeochemical implications and whilst this may be the case, might it also be possible that the microbes observed are displaying behavioural plasticity and exploiting the positive patchiness of the environment sampled? Additionally, may it also be possible that the extensive pili and gel matrices are a communication network, potentially connecting cells over a relatively large area? If this conjecture is correct, it may have a tremendous impact on work associated with understanding quorum sensing amongst bacterial communities.

This study offered a novel insight into how the ocean is put together, previously undetected at this scale.  As technology advances, it will be interesting to test hypotheses that are currently difficult to address due to the nanoscopic scales involved.

Malfatti, F. & Azam, F. (2009). Atomic force microscopy reveals microscale networks and possible symbioses among pelagic marine bacteria. Aquatic microbial ecology, 58(1), 1-14.