Hurricane Irene wipes her salty shoes on the Ocean's welcome mat

It has only been in the last 20 years that the idea of microbes forming colonies on surfaces in shallow, intertidal and deeper waters, has come to light. But now we are faced with exciting research showing the complex nature of microbial mats and how their diversity and interactions with the microhabitat around them are formed. These delicate systems are placed under many pressures everyday such as light, nutrient and oxygen availability. Preisner et al looked into microbial mats and how taxa within these diverse systems can provide stable biological reservoirs that are influenced by environmental stressors such as salinity brought on by natural disasters, in this instance, Hurrican Irene which hit the coast at the end of August 2011. Microbial mats can arguably have some of the highest diversity which, these in turn, influences foundations of ecological processes so any additional stressors should be recognised and taken into account when understanding these delicate microbial ecological interactions. Much of the greatest diversity is within low abundance taxa, which this study takes into account. While much of the biomass of microbial mats is common within a few dominant taxa, both the dominant and ‘rare’ taxas show different activity levels within environmental pressures.

The paper studied a microbial mat on San Salvador Island in the Bahamas on August 1^st-2^nd in 2011 (Pre Hurricane Irene) and again on August 1^st – 2^nd in 2012 (Post Hurricane Irene). This microbial mat consisted of well-defined layers, which as we understand from general knowledge of microbial mats, can have steep gradients of oxygen, light and sulfide availability. These gradients were taken into account to an extent in this paper, however it is not the primary focus.

Post disturbance the reduction in salinity showed that general community composition was not affected significantly diurnally however general archaea decreased from 57% to 33% and Bacteria increased from 43% to 67%. Most taxa showed low activity across conditions but activity levels in general did increase after disturbance. This highlights how these communities are sensitive to environmental change. Pre-disturbance showed to favour Halobacteria as they are shown to thrive in lower salinities, Halobacteria held 48% of the community biomass predisturnbance however these results were favouring towards one OTU halobacteriaceae. Halobacteriaceae are defined by their halophilic nature and are abundant in hypersaline conditions. Post disturbance showed abundance and activity decrease in Halobacteria from 48% to 31%, however abundance and activity increased in Proteobacteria, Cyanobacteria, Thermoplasmata, Thaumarchaeota and Crenarchaeota.

Cyanobacteria are known to be tolerant of a range of salinities however results did show a decrease in its nitrogen fixating abilities. Cyanobacteria are important for the formation of microbial mats, filamentous Cyanobacteria such as Phormidium are dominant members in the mats and although not mentioned in this paper, it is important to remember the interaction of this microorganism with microbial mats so when disturbance occurs we can understand any impacts it will have on healthy mats.

The sample site chosen is often disturbed by natural disasters like hurricanes, so is therefore used to environmental pressures such as wide ranges of salinity, high irradiance and high temperatures. This study site does seem ideal for studying larger communities of microbial diversity in relation to salinity, however it does suggest that these communities are already well adapted to cope with pressures, therefore it would be interesting to see the diversity and activity rates in communities in other, less pressurised areas.

Paper reviewed: Preisner, E. C., Fichot, E. B., & Norman, R. S. (2016). Microbial Mat Functional and Compositional Sensitivity to Environmental Disturbance. bioRxiv, 063370. http://www.biorxiv.org/content/biorxiv/early/2016/07/12/063370.full.pdf