While reading several microbial diversity studies I came across some where certain environmental variables were being correlated with microbial abundances. One of those variables was change in porewater concentration with depth. However, from my understanding, whether it's sulfate reduction or iron reduction, the activity is based on the second derivative of the porewater concentration. So I'm not sure how porewater concentration and microbial abundances can be correlated without taking this into account. Or have I misunderstood something fundamental here?
If "abundances" mean "cell numbers", a correlation cannot be done in my opinion. You might have high numbers of certain species, but the are "dormant" in the sediment. Rather, porewater concentrations are related to "activities": location or layer of activity (first derivatives), consumption/production rates (second derivatives).
As always it comes back to what question you are addressing and what you actually want to find out about your system. Helmut is right that the presence of an organism (detected by its sequence) does not mean it is active so that would immediately suggest that any correlation is unlikely to informative about the ecology of the system. Activity will be related primarily to flux rather than standing concentrations for many compounds. To this you must also consider some substrates are in excess (like sulfate used by sulfate reducers in many marine and brackish systems) and others can be reduced to concentrations that are too low for some organisms to utilise (acetate for methanogens in high sulfate systems). Finally, and this may well be the hardest issue to resolve, porewater concentrations are only a bulk measure of available nutrients/compounds and the microbes themselves may experience very much higher and lower concentrations on a micro-scale in situ.
However, a correlation may help you develop an idea of factors are important to a community, which you can then test in further experiments, and so may be helpful in that way.
Here's an example of such a study.
Article From the Cover: PNAS Plus: Correlating microbial community p...
This looks like (on a very brief look) a good example of the point I ended on in my previous answer. They conclude not that they have found the physiologies of the groups they have correlated with porewater compounds but that they can now have some real data that allows them to hypothesise about the physiologies of these groups. As these hypotheses may be testable experimentally that is a pretty good outcome form a piece of work! Don't expect correlation to answer questions, but to help you pose better ones!
I guess my next question is when making these correlations how are the geochemical factors (stated above) accounted for in the models?
Hi Carolina,
If you found a correlation it would be a fluke, as there is'nt really such a thing as a porewater profile. 2D methods show that in fact are heterogenous porewater distributions that shift throuigh time. At best a profile would be a average, but as solutes react with each other and solid phase during porewater extraction, what measure is the product of these chemical reactions. The 2D methods can be used to generate "real" averaged depth profiles, but often there is almost as much conc variability laterally as vertically.
I've attached some papers on 2D porewater Fe(II) distributions, the L&O methods paper (Fig. 9) shows a simple example of averaged porewater profiles of Fe(II) and sulfide from 2D codistributions and what you would likely measure using a conventional porewater extraction method (really simple only considers precipitation of Fe(II) by sulfide and not the myriad other possible reactions).
Cheers,
Dave
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Climate Science
- Climate Change