Thursday, 12 December 2013

The plasticity of nitrogen metabolism in chemoautotrophic symbionts

I found this paper while I was looking for more research on the vent tubeworms that are dependent on microbial symbionts in order to survive. This paper focuses on the chemoautotrophic symbionts of the vent tubeworm Ridgeia piscesae and aims to determine whether the nitrogen metabolism of these bacteria show some sort of plasticity depending on the environmental conditions since the tube worms display a variation in the phenotype in different conditions as well.

Nitrogen is important for the growth and maintenance of organisms and an exogenous nitrogen source is required; however, dissolved organic nitrogen is rare around hydrothermal vent habitats and inorganic nitrogen is rather abundant (e.g. NO3- or NH4+). Nitrogen metabolism of the tubeworm symbioses is not well studied in comparison to carbon and energy metabolism, especially to what extent they utilise of exogenous ammonium (NH4+) is unclear. As found in Riftia tubeworms (e.g. Robidart et al., discussed in the lecture), most symbionts have roles in dissimilatory/assimilatory nitrate reduction (DNR/ANR) or in dissmilatory nitrate reduction to NH4+ (DNRA; gene glnA) as a source for reduced nitrogen for biosynthesis. However, it is unclear if there are variations in nitrogen uptake and utilisation by the same species of tube worm living in different habitats where ammonium and nitrate concentrations fluctuate dramatically.

R. piscesae are ideal model organisms since they occur in a range of habitat with NH4+ concentrations varying between 10 and 950 ┬ÁM, and for this study, the authors collected two phenotypes of R. piscesae, long-skinny (LS) and short-fat (SF) which exhibit very distinct variations in size, tube shape and colour, and plume shape from two different locations. The LS phenotype occurs around low temperature vent fluids with lower NH4+, and the SF phenotype at rather high temperature chimneys with higher NH4+. However, both phenotypes identified to provide a habitat for identical symbionts as confirmed by 16S rRNA analyses. Gene expression of key enzymes involved in symbionts DNR and ANR, as well as host and symbiont ammonium assimilation from the two sites were examined.

Results showed that all Ridgeia hosts take up NO3- since it is the most readily available inorganic nitrogen source, and they use it via dissimilatory nitrate reduction pathways. It was also found that the LS phenotype directly uses NH4+ as major nitrogen source for the symbiosis, whereas the SF phenotype uses a certain DNR pathway for the synthesis of NH4+. A difference in genes expression between the two phenotypes has also been observed, for instance glnA expression was much higher in the LS phenotype and it can be assumed that this reflects an increase in enzyme production to enable the acquisition of NH4+ in colder conditions with less NH4+ around.

This study was the first of its kind to highlight the flexibility of the symbionts for nitrogen metabolism in changing availability of nitrogen sources and this could mean that the symbionts have great impact on the host’s fitness and survival which enables the symbioses to live in physiologically changing environments.

Liao, et al. (2013). Characterising the plasticity of nitrogen metabolism by the host and symbionts of the hydrothermal vent chemoautotrophic symbioses Ridgeia piscesae. Molecular Ecology.

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