Diversity and dynamics of bacterial communities in coastal sands

Interactions between land, ocean and atmosphere allow for highly dynamic coastal ecosystems, with sands acting as filters and mediums in which a variety of materials accumulate. These interactions and accumulations make sands diverse habitats for microorganisms with many niches available and an ever-changing environment. It is thought that the dynamic nature of coastal sands results in high selection pressure from the environment, allowing only the best-adapted microorganisms to permanently establish themselves.

The authors of this paper (Gobet et al, 2012) set out to discover more about the dynamics of bacterial populations of coastal sands with particular emphasis on temporal fluctuations. Utilising pyrosequencing techniques of ribosomal genes, they examined the effects of a variety of environmental factors on the studied bacterial populations. Their study compared the similarity of bacterial community composition of communities from three coastal compartments, sand, pore water and the adjacent water column, sampled from the island of Sylt, Wadden Sea, Germany.

The key findings of their study were that there were a low proportion of shared bacterial populations (less than 0.2% between the pore water and the sand grains and also low between the sand and the water column, based on community composition and evenness) at the time of sampling. The sampled water column consisted mostly of Bacteroidetes and Alpha-/Gamma- Proteobacteria, with Verrucomicroba and Actinobacteria also representing a significant presence and the sand’s community consisted of mostly Bacteroidetes, Gammaproteobacteria, Deltaproteobacteria and Planctomycetes species both in accordance with previous research. The pore water had a similar composition to the sand with some temporal variation that was accounted for by exchange of land and marine microbial communities as a consequence of a constantly changing environment. A positive relationship between sediment depth and bacterial species richness, with the exception of Cyanobacteria and Bacteroidetes, was found at most of their sampling sites, an observation that was in agreement with prior research (Urakawa et al., 2000 and Böer et al., 2009b), and due to habitat stability.

Other findings include a high turnover of bacterial diversity in regards to sediment depth and time at OTU (phylum) level, indicating a highly dynamic community, with some seasonal fluctuations observed in abundant groups such as Gammaproteobacteria and Planctomycetes possibly due the changing spring weather (the sampling was carried out in March 2006, a period of turbulent conditions in the Wadden Sea) and 50% of bacteria present at all times in all three depths of sand were Gammaproteobacteria and Deltaproteobacteria, representing 23% and 10% of total sand cell counts. 40% of the total OTU cell counts, the rare biosphere, only appeared once per sample, and represented a large fraction of the community that may be subject to substantial change. They concluded that a large turnover of the bacterial community composition could be due to a variety of factors including migration, complexity of environment, rare species becoming dominant in suitable conditions (i.e. the seed bank hypothesis) and extracellular DNA having an effect on their results. Community composition was found to be more varied than microbial functions such as biomass and extracellular enzyme activities, indicating that resident species are responsible for these functions and the rare biosphere has little effect.

Although comprehensive in their approaches, the main issue I had with the study was their limited sampling time, which was around a year. This meant that the data set acquired from their sampling (they had included some previous long term data) only represents the community structure of Sylt’s coastal sands for that year, not taking into account variations over longer time periods, especially when it comes to the highly variable nature of Northern European seasons. Also their molecular techniques didn’t differentiate DNA originating from living populations and extracellular DNA, the presence of which may have skewed their results, but they argue any effect from this DNA was probably negligible.

The authors identified areas for further study in to bacterial populations including research into a discrepancy between the temporal dynamics of communities in sediments and water columns, which had been shown to be seasonally cyclic and further research into the response of the rare biosphere to the environment.

Reference        

Gobet A, Böer SI, Huse SM, Beusekom JE, Quince C, Sogin ML, Boetius A and Ramette A (2012) Diversity and dynamics of rare and of resident bacterial populations in coastal sands The ISME Journal, 6, 542–553.