Should I burrow?

Marine sediments harbor an abundance of microbial communities and the credit for this, as well as facilitating their biogeochemical processes, goes to bioturbating fauna such as crustaceans, polychaetes, and bivalves that dominate infaunal assemblages (Woodin, 1974) and act as ‘ecosystem engineers’ (Taylor and Cunliffe, 2015). They also alter the geochemical properties of sediments and impact microbial populations. But what about the microbial community in the (neighbouring) sediment that is undisturbed?

Papers mentioned in Taylor and Cunliffe (2015) state that bacterial abundance is greater in burrows than in the surrounding unaffected sediment and the diversity between them is distinct. This is because they provide distinct microhabitats and are geochemically different in composition.

Hediste (Nereis) diversicolor, the model organism in this paper, has semi-permanent burrows that impact sediment geochemical properties by aiding oxygenation of water and mixing of the sediment. Therefore, this study compares the microbial (bacteria and eukarya) diversity between bioturbated sediment (which stimulates oil degradation) and un-bioturbated sediment.

DNA and RNA were extracted from the un-bioturbated sediment (without H. diversicolor) and burrow samples. To assess microbial activity and differences in community, RNA samples from both types of sediments were used for 16S rRNA and 18S rRNA Q-RT-PCR and were amplified. 16S rRNA genes from DNA samples from the two sediments were also amplified by PCR. An Ion Torrent was used for sequencing and the data was analysed using the an open-source bioinformatics software package. Using RISA, they assessed the effect of H. diversicolor on bacterial communities in oil-contaminated sediment. Using a combination of T-RFLP and clone libraries, they assessed the changes in structure of bacterial communities in polluted sediments.

The total eukaryotic and bacterial communities were more active in burrows (18S and 16S rRNA transcripts were significantly higher, respectively). When 18S (for eukarya) and 16S (for bacteria) rRNA transcript OTUs were analysed, the burrow population formed distinct clusters. In H. diversicolor burrows, the orders Alteromonadales, Methylococcales, Oceanospirillales and Thiotrichales were more abundant. According to 18S rRNA sequences, fungi (Basidiomycota) dominated both types of sediment. Through interactions with oil-degrading bacteria, fungi are known to directly degrade hydrocarbons. 

This paper also focused on relative abundance of obligate hydrocarbonoclastic bacteria (OHCB). From 16S rRNA sequences with DNA and RNA gene library analysis, Cycloclasticus were found in high abundance in burrows and were also present in unbioturbated sediment. Alcanivorax, Marinobacter and Oleibacter were also prevalent in the burrows and were either absent or present in low amounts in the unbioturbated sediment. Alcanivorax, Marinobacter and Cycloclasticus have been shown to possess oil degrading properties (Taylor and Cunliffe, 2015).

OHCB has shown to respond to changes, such as oil influx (Taylor and Cunliffe, 2015). Assessment of the impact of the Deepwater Horizon oil spill in the Gulf of Mexico using 18S rRNA gene pyrosequencing showed that fungi dominate post-spill communities too. In H. diversicolor burrows, Cycloclasticus, Alcanivorax and Deltaproteobacteria (major components after the Prestige oil spill) were less abundant and this is due to oxygenation of the burrows causing a switch in metabolism, resulting in more rapid hydrocarbon degradation (Taylor and Cunliffe, 2015). The potential for bacterial communities to remediate polluted sediments is reduced in the presence of bacterivorous meiofauna, which decrease mineralization rates and alter bacterial community structure (Näslund et al., 2010).

This paper provides a good understanding of the relationship between burrowing organisms and microbial abundance and diversity. It also highlights the importance of these microbial communities in natural remediation through the process of degradation of polluted (oil contaminated) sediments. This in turn, gives us an insight into the anthropogenic impacts on the sediment and how organisms that inhabit these sediment cope as a result. To gain a deeper perspective of this, the feature of the burrow (such as lining) and the host invertebrate’s characteristics is important to establish. Although this paper has touched upon some biological limiting factors and Woodin (1974) has demonstrated the importance of biological interactions to the determination of species abundance in sediment, it would be interesting to see the effect of physical limiting factors. Another concept to bear in mind would be the stability-time hypothesis to further explain patterns of diversity of the hosts as well as the microbes. 

Bibliography

Paper Reviewed -

Taylor and Cunliffe. (2015). Polychaete burrows harbour distinct microbial communities in oil-contaminated coastal sediments. Environmental Microbiology Reports, 7(4), 606–613.

Additional references - 

Woodin, S. A. (1974). Polychaete Abundance Patterns in a Marine Soft-Sediment Environment: The Importance of Biological Interactions. Ecological Monographs, 44(2), 171-187.

Johan Näslund, F. J. (2010, May 13). Meiofauna reduces bacterial mineralization of naphthalene in marine sediment. Retrieved from www.nature.com: http://www.nature.com/ismej/journal/v4/n11/full/ismej201063a.html

Charles C. Steward, S. C. (1996, March 28). Microbial biomass and community structures in the burrows of bromophenol producing and non-producing marine worms and surrounding sediments. MARINE ECOLOGY PROGRESS SERIES, 133, 149-165.