The energy-diversity relationship of complex bacterial communities in Arctic deep-sea sediments

This paper describes and discusses the links between energy, bacterial activity and bacterial diversity at different taxonomic levels, as well as identifying the bacterial taxa which are most likely affected by changes in energy availability. Their reasons for conducting this research was down to the rarity of studies linking energy availability to the bacterial diversity patterns, and in recent years advances in methods such as high-throughput fingerprinting has made them much more available. The authors believe that it is important to unravel the relationships between environmental conditions, organism diversity and ecosystem functions if we are to understand the effects of global change.

They chose the natural energy gradient of the Arctic continental slope (as it covers a range of phytodetritus fluxes and represents both mesotrophic and oligotrophic deep-sea settings) to place depth transects across; minimising any confounding factors by sampling across different ocean provinces. Their study began in September 1993 during RV Polarstern cruise ARK IX/4, in which they took samples from 17 stations from the outer Laptev Sea shelf into the deep Eurasian basin. Sediment cores were sliced 1cm thick horizontally, and sediment samples from the same stations were used to measure environmental-parameters, potential enzyme activity and DNA extraction.

For community structure analysis, DNA was extracted using ‘UltraClean Soil DNA Isolation Kits’ from 1g of sediment, and stored in Tris-EDTA buffer. 42 samples made up of various sediment horizons (0-1, 1-2, 4-5cm) were analysed using Automated Ribosomal Intergenic Spacer Analysis (ARISA), a technique developed for the rapid estimation of microbial diversity and community composition. A set of 10 samples were also chosen for 454 Massively Parallel Tag Sequencing (MPTS). In order to keep analysis over different taxonomic levels consistent, they used a subset of the 454 MPTS dataset for further analysis, in which only sequences with a complete assignment up to genus level were retained. A high Spearman’s rank correlation between dissimilarity matrices of the reduced and original dataset confirmed that ecological patterns were consistent in both.

The results shown prove a strong relationship between changes in alpha-diversity (sample richness) and beta-diversity (changes in community structure between sites) with changes in pigment concentrations. OTU (defined by ARISA) richness and pigments concentrations showed a strong positive, linear relationship until around 2µgcm^-3 sediment was reached, after this the relationship started to level off. The two molecular techniques ARISA and 454 MPTS, exposed similar ecological patterns; when 454 MPTS was applied to the subset of samples, a similar linear relationship was found, appearing to level off at higher pigment concentrations (3µgcm^-3).

Many more results following statistical analysis of varying methods show similar correlations and with regards to change in richness with increasing energy availability in the form of phytodetritus, suggest an overall positive response of bacterial OTU which was strongest at oligotrophic conditions (defined by low levels of pigment concentrations 2-3µgcm^-3); as well as benthic meiofauna and megafauna. An increased phytodetritus input sustains an increase in bacterial abundance and biomass, in line with the ‘more individuals’ hypothesis of the species energy theory.

Further studies of natural and experimental systems are required in order to decipher the mechanisms which can be held responsible for the establishment and preservation of energy-diversity relationships in bacterial communities- and if these can be extended to a global scale. Beyond energy availability-diversity relationships for complex bacterial communities, this study showed strongly implies that any environmental changes affecting primary productivity and particle export will cause shifts in bacterial community structure and function in the Arctic, which in turn may affect key processes such as the carbon cycle.

A review of:

Bienhold, C., Boetius, A., and Ramette, A., (2011). The energy-diversity relationship of complex bacterial communities in Arctic deep-sea sediments. The ISME Journal. pp. 1-9.