The Never Ending Battle of Synergism

In this study, Chan et al. investigated the effects of hypoxia and the ubiquitously dispersed flame-retardant BDE47 on the bacterial communities in marine surface sediments. It has been shown that the number of hypoxic areas has doubled each decade since 1960, and the largest increase in hypoxic areas in this decade is predicted to occur in Asia. Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in plastics, textiles, resins, building materials and electronic equipment since the 1970s, and now become ubiquitous in the global environment especially around Asia. It has not yet been investigated the impact of PBDEs on microbial communities in marine sediments, especially the synergistic effect with hypoxia on more than one functional group of bacteria.

Surface sediments used for this study were collected from Tung Lung Chau, a pristine site in Hong Kong. Taking samples from surface sediments may have been a limiting factor in this study as deeper sampling would have been better for the recovery of rare species and a higher level of confidence for any observed differences. 

Laboratory experiments were carried out to test the hypothesis that hypoxia and PBDEs would alter the bacterial community composition in marine surface sediments. Microcosms with different combinations of PBDEs and dissolved oxygen levels were set up, and the changes in the bacterial community composition on marine surface sediments over 28 days were characterised using terminal-restriction fragment length polymorphism (T-RFLP) and 16S rRNA gene clone library construction.

Both hypoxia and BDE47 alone significantly altered the bacterial community and reduced the species and genetic diversity. UniFrac analysis revealed that BDE47 selected certain bacterial species and resulted in major community shifts, whereas hypoxia changed the relative abundances of taxa, suggesting slower but nonetheless significant community shifts. These two stressors targeted mostly different taxa, but they both favoured Bacteroidetes and suppressed Gamma-proteobacteria. Most importantly to notice is that the impacts of BDE47 on bacterial communities were different under hypoxic and normoxic conditions.  PBDEs are ubiquitous whilst hypoxia affects very large areas in the marine environments, which highlights the need to consider risk assessments and bioremediation strategies for BDE47 in a broader context of interaction with hypoxia.

The results here are in contrast with a previous study on planktonic microbial communities that showed that hypoxic zones had a higher microbial diversity in comparison to normoxic and anoxic zones (Zaikova et al., 2010). The enrichment of nutrients in the water column might explain the observed increase in pelagic microbial diversity reported. However this contrast highlights how important it is to study multiple habitats in the ocean before assuming all habitats will react the same.

This paper came up with an interesting and important hypothesis, however there was not a lot of interpretation on the importance and reasoning behind the changes in composition. In order to further the understanding on the functional changes of the bacterial communities in the marine surface sediments in response to hypoxia and BDE47, future study on the functional gene profiles using functional genes microarray or metatranscriptomics would be essential.

Reference: Chan, Y., Li, A., Gopalakrishnan, S., Shin, P. K., Wu, R. S., Pointing, S. B., & Chiu, J. M. (2014). Interactive effects of hypoxia and polybrominated diphenyl ethers (PBDEs) on microbial community assembly in surface marine sediments. Marine pollution bulletin.

Can be found on: http://www.sciencedirect.com/science/article/pii/S0025326X1400280X

Additional references: Zaikova, E., Walsh, D. A., Stilwell, C. P., Mohn, W. W., Tortell, P. D., & Hallam, S. J. (2010). Microbial community dynamics in a seasonally anoxic fjord: Saanich Inlet, British Columbia. Environmental microbiology, 12(1), 172-191.