Applied and Environmental Microbiology, August 2002, p. 3859-3866, Vol. 68, No. 8
Association of Microbial Community Composition and Activity with Lead, Chromium, and Hydrocarbon Contamination
W. Shi, J. Becker, M. Bischoff, R. F. Turco, and A. E. Konopka

So... I wrote this up and blogger ate it. *sigh*

Here goes, again.

The Issue... many sites are contaminated with pollutants such as heavy metals and oil, solvents, etc.
Heavy metals here are represented by Chromium and Lead... Chromium is used in cleaning agents and paints,
lead in paints, solder, plumbing, etc. Oil is represented by "Total Petroleum Hydrocarbons" or TPH.
The question of how bacteria can "mineralize" (i.e. eat) the TPH to clean up a site has been asked many times.
People have tried to develop bacteria, use existing wild bacteria, etc, to eat a variety of solvents like benzene
or toluene, as well as crude oil.
Heavy metals, on the other hand, are often best immobilized by plants, which are then harvested and decontaminated.
They tend to inhibit bacterial action, or so conventional wisdom goes.

The question is: how to deal with a multiply contaminated site? Very few superfund sites have a single chemical to worry them.

The authors take a real site where the bacteria have been living in contaminated soil for a number of decades. There are various parts of the soil containing various amounts of Cr, Pb, and TPH.

Three questions:
1. do the bacteria populations vary depending on the pollution levels?
2. do the bacteria have different metabolic states depending on the pollution levels?
3. will the bacteria respond to introduction of "good food" in contaminate sites?

The answers:
1. Two variables determine 92% of the variation in populations between sites. The main variable is TPH. Where TPH is high, fungi are present and bacteria are relatively scarce.
2. The bacteria do get suppressed by heavy metals. However, they have developed a degree of tolerance not found in strains from uncontaminated sources.
Because there is a sigmoidal curve in the response of amino acid uptake and sugar utilization to heavy metal concentrations, the authors suspect that different parts of the population have different tolerances to the metals. I disagree. It looks to me like sub-population structure would create a step-like pattern with several sigmoid curves superimposed. Instead, we see a single smooth sigmoid, which differs only slightly from sample to sample. Thus, there is one messy threshold, which is standard for toxicity analysis of a single population per sample. This avoids their entire discussion about microhabitat and bioavailability.
A very interesting result is that by adding Lead Nitrate to some samples, they kill the bacteria, while in others they stimulate them. Why should the lead stimulate the bacteria? Well, they eliminate the idea that the nitrate is being stimulatory by adding potassium nitrate. One thing is that this effect only occurs in addition of the chemical to whole soil samples, not to extracted cells. Possibly, the chemical is reacting with something in the soil. Another possibility is that they are not measuring increased biomass, but increased sugar mineralization. Possibly the bacteria in that sample have a way to use sugar to pump out lead ions, and they are churning through the energy to survive the lead or repair damage, etc. An intriguing result none-the-less.
3. Adding alfalfa to the soils provided some promising results. In the most contaminated soils, they started slowly, but ended up getting surprising amounts of carbon mineralization. It looks like the alfalfa was successful in convincing the bacteria to eat the TPH... which is exactly the desired outcome.

So, a good paper, kudos to the authors!