Coastal Microbial Mats… The Truth Behind the Ocean’s Nitrogen Fixation.

Coastal Microbial Mats… the truth behind the Ocean’s nitrogen fixation.

Microbial mats (MM’s) are important to the foundation of the oceans, they carry out a variety of important processes, an example of this is nutrient cycling. A large part of this is down to the nitrogen fixation which is fuelled by microbial groups of cyanobacteria that are within layers of MM’s. These cyanobacterium groups are known as active diazotrophic communities. Despite how important the cyanobacterium are, there is not a lot of known information on these microbes, this study was undertaken to shed light on this.

 The research took place at Elkhorn Slough estuary, Monterey Bay, CA, USA in October of 2009. The approach was to establish what was fuelling the nitrogen fixation within microbial mats and to gain a better understanding of what caused nitrogen fixation and how.   

It had long been assumed that all cyanobacterium played a heavy role in the full N² fixation from microbial mats. This hypothesis was proven via cultivation base studies (Paerl et al., 1991; Bebout et al., 1993). This however didn’t allow a clear understanding of how these diazotrophic communities worked within complex ecosystems, this has now been explored.

To explore this topic in its entirety a wide range of analysis and methodology was used, enabling a vast amount of results. This included biogeochemical, molecular and high-resolution secondary ion ma In-situ water the nutrient and keep as close to the outside conditions as possible.

Biogeochemical (ARA’s):

Two separate 10mm width by 10mm length mat cores and horizontally layered into 3 triplicate layers. The 3 triplicate mat cores were tested every 3 hours, 3 control mat cores where also taken used as a negative control.  The 3 test cores where separated and incubated within ethylene, this was later measured with gas chromatography to assess the depth distribution of nitrogenase activity.

N-15 incubation – the triplicate mat core samples were placed into 14ml serum jars topped with a stopper to test Gas exchanged for N-15. These were incubated in dark for 10h before samples being split in half for analysis. Half of sectioned cores were analysed by IMRS, the other half of sectioned cores were kept for later Nano-Sims analysis.

N-15 incubation of culture of cyanobacteria was also undertaken using the same techniques mentioned above however these where incubated at a temperature of 22˚C and where tested on a 8/16h day to night light cycle.

Molecular:

DNA and RNA were co-extracted from the upper 2mm of mat cores this was achieved by combining phenol–chloroform extraction with parts of the RNeasyMini and QIAamp DNA Mini Kit used for this analysis. The upper parts of the mat cores showed to have the most N-fixation, which is why this part of the core was sampled.  RNA reverse transcribed with reverse transcriptase enzyme into a single stranded complementary DNA (cDNA), this was analysed via ss spectrometry (NanoSIMS) techniques.  

Findings showed that Cyanobacteria filaments are able to split into heterocystous (cells that are specialized nitrogen-fixing and usually formed during nitrogen starvation periods) and vegetative cells, this means that they are splitting the nitrogen and photosynthesis into separate cells. This was established when heterocystous cells became apparent. Photosystem II was found to be not present but photosystem I was in high numbers as well as the presence of ATP and luciferin.

Because of this, Cyanobacteria are important during the night time due to the highest rates of nitrogen fixation happening during the night in comparison to the day time where photosynthesis has the higher advantage of sunlight. Results also demonstrate that within the presence of oxygen nitrogenase enzyme cyanobacteria are inhibited. 

This study has allowed for new discoveries to be made within the microbial field and has allowed microbiologists and like-minded ocean scientists to gain better understanding of ocean processes. Thorough analysis techniques, such as using a combination of single and standard cell analysis it has enabled researchers to link cell functions and characteristics to recognize previously unidentified microbial groups that work among complex ecosystems and how these microbes link into key ocean processes.