It is widely accepted that abiotic stress causes
unfavourable shift in the coral holobiont. Metagenomics are a relatively cheap
technique, which enables extensive community analyses. Changes in bacterial
taxa associated with stress enable predictions to be made regarding “healthy”
and “disease-related” assemblages. However, the functional role of bacteria
within the holobiont is less well understood, and although bacterial taxa can
be quantified, the outcome of the new community remains unclear.
Vega-Turber et al (2009) sequentially sequenced the
microbiota of corals over a 64-hour period, during exposure to four abiotic
stressors: temperature, dissolve organic carbon (DOC), nutrients; nitrogen and
phosphorus, and pH. The sequences were annotated for known functional genes and
classified into a hierarchical system with three levels.
All abiotic stressors resulted in an increase, at least
two-fold, of genes encoding virulence, fatty acid catabolism/ anabolism, and
RNA and protein metabolism. Yet at the lower hierarchical level, virulence
varied between stressor. During temperature stress the function of virulence
genes were related to invasion and intracellular resistance, whereas the
virulence function in nutrient treatment was related to toxin and antibiotic
production. This might be explained by the difference in bacterial taxa between
stress treatments. Temperature stress resulted in a significant decrease in
Cyanobacteria, Firmicutes, and Symbiondinium, but had a large increase in
fungi. Whereas, in pH stress, the community decreased in Fusobacteria, and
Cyanobacteria increased.
The complexities associated with determining the function of
bacteria within the holobiont is evident. Changes in abiotic conditions can
result in changes in genetic expression, highlighting the bacterial plasticity,
which I find fascinating. The implications of synergistic effects of abiotic
stress could be severe, and is likely to be why severely degraded reefs
struggle to recover.
This study was highly interesting, and as this aspect of
bacteria is so foreign to the large majority of people, more studies like this
will contribute hugely to the understanding of the holobiont. I think it would
be interesting to do a similar study, but on a healthy coral, as everyone is
fascinated with how genetic expression changes according to stress, but have
not considered how it changes temporally.
It may have been more appropriate to use lower temperatures
however, as here only used an increase of 5°C, which is
not comparable to changes in natural ecosystems.
Thurber, R. V.,
Willner‐Hall, D., Rodriguez‐Mueller, B., Desnues, C., Edwards, R. A.,
Angly, F., ... & Rohwer, F. (2009). Metagenomic analysis of stressed
coral holobionts. Environmental Microbiology, 11(8), 2148-2163.
Hay Kat, interesting post! I really like the functional approach as I think it tells you a lot more about what organisms are doing which is really the key point in any ecosystem, both within animals and in the external environment. Could you unpack what you meant by 'encoding RNA'? Presumably these were a specific type of RNAs as all genes encode for mRNA, did the authors say what kind of function were these RNAs fulfilling?
ReplyDeleteMaybe that bit reads awkwardly, so more genes were active in RNA metabolism (which is the breakdown, synthesis and modifications to the genes). This is showing that modifications are occurring during stress - but this is assigned at the widest level. Every gene they could regonise, was assigned a category at three levels - the widest to the most specific. Which allowed broad comparisons, which could then be narrowed in on, to see the function within that category - if u get what i mean?!
ReplyDeleteSo they initially used RNA metabolism as a broad indicator that something was happening and then 'zoomed in' to look at changes in specific functional genes or have I understood that wrong? Out of interest how were they measuring the RNA metabolism?
ReplyDeleteSorry - I totally hadn't looked at this!
DeleteYes you are right, they used three levels of classification and could zoom in as you say. This allowed them to compare the differences in expressed genes to different abiotic stressors - as seen in the virulence mentioned above - all "virulence" but very different forms toxin production Vs invasion!
I don't think its a measure of RNA metabolism as such, but genes (that they have categorised as having a role in RNA metabolism) increased/ were up regulated, during stress!! Hope this helps!
I dont know the extact genes they assigned as having RNA metabolic function
Hi Katrina, i find this paper interesting as well! It is fascinating to see that abiotic stressors can induce change in the bacterial community (or maybe the proportion of the microbial community) instead of wiping out the whole population due to changes in environmental conditions. It does show some light in the survival of the corals which are in danger due to climate change. In a way that bacterial community might be a part that prevent the wiping out of corals population.
ReplyDeleteYes I agree, it is facinating how they can change their genetics in response to abiotic conditions. I do have hope that bacteria will enable the survival of coral populations, based on the probiotic hypothesis proposed by Rashef et al, 2006. He proposed that the genetic diversity of the bacterial community will be able to over come all stressors.
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