Mercury Adaptation among Bacteria from a Deep-Sea Hydrothermal Vent
Costantino Vetriani, Yein S. Chew, Susan M. Miller, Jane Yagi, Jonna Coombs, Richard A. Lutz, and Tamar Barkay
Appl. Environ. Microbiol. 2005. 71: 220-226.
The Big Picture:
Mercury contamination from industrial sources is a huge issue, as are, in fact, all forms of heavy metal contamination of surface waters. Lead, cadmium, mercury, copper... they are all pretty toxic to people drinking the water, to fish, etc, and they tend to be biologically magnified as they are concentrated up the trophic levels. That is, they persist in an animal, and are passed to the thing that eats that animal. Man, at the top of many food chains, gets a whopping dose.
So, one idea that is popular is bioremediation, with plants or bacteria taking the heavy metals out of the water or soil and 'sequestering' them in some easily harvested or bio-inavailable form. If harvested, then the waste can be contained. If bio-inavailable, then the material becomes inert and no longer an immediate threat.
Anyway, the hunt for plants and bacteria that do things with heavy metals has been long. Along the way, we have learned much about the processing and cycling of metals in biological systems. This paper picks up there, with the realization that there are some cool things happening at the deep sea hydrothermal vents.
These vents are like mini-volcanoes on the ocean floor, where hot water comes out of the crust into the otherwise frigid, lightless, and pretty lifeless ocean floor. Bacteria do all sorts of cool things with the chemicals down there to get energy, and they pass that energy to little animals, eventually to bigger and bigger animals. For all sorts of information about symbiosis down there, look to the works of Cavanaugh.
Anyway, one interesting thing is that the hot water sucks cinnabar out of the rocks. Cinnabar is a red mercuric sulfide compound. The hot water sucks out the mercury and then swirls it into the ocean. There is a distinct gradient of mercury in this fashion.
For the non-microbial evolutionary biologist, this seems mildly interesting. However, thinking about enrichment cultures... when you want a bacteria to learn to digest a toxic chemical, you need to start at low concentrations (so it can survive) and steadily increase the concentration to keep challenging it, until it can handle the high concentrations necessary. In nature, by having a steady gradient of mercury in a spatially structured environment, also aligned with a chemo-nutrient gradient, there will be competition for the other chemicals leading the bacteria up the mecury gradient. Ideal conditions for the evolution of mercury processing/decontamination.
Well, that's pretty much what these researchers found. They found that merA was present in a whole bunch of bacteria, even when their isolations didn't select for mercury resistance. They found that mercury resistance was related to the temperature at which the bacteria grew... which makes sense given the gradient of temperature going up towards the vent. They did nice physiological tests and demonstrated the bacteria making the mercury less bioavailable. Then they sequenced a bunch of merA genes and tested the MR protein for optimum temperature. They found that the high temperature bacteria had high temperature MR.
They also found that one of their merA was in another cluster in their tree. This is consistent with putting the root of the merA at the deep sea vents. However, this small tree that they constructed with other known examples of merA was not terribly convincing in and of itself.
Personally, I take issue with both the size of the tree and the method of building it... neighbor joining alone, with some moderate bootstraps (85 at a key node), and not too many merA sequences outside of their experimental samples. I'd prefer to have seen a larger tree with maximum likelihood backing up their conclusions. However, this would require going and isolating other mercury resistant samples, sequencing them, and adding them to the tree. It is pretty far afield for an otherwise sturdy paper.
One place this might be useful is in an industrial setting, where a high temperature bioreactor could be used in a mercury containing waste stream. It might be possible to feed the bacteria and keep the stream at a temperature that discourages outside contamination. Mercury could be pulled out in a particular stage of waste decontamination.
I'd like to see them isolate some more strains and get a bigger merA tree before they really speculate about the evolution. At the same time, this was a fascinating paper on the ecology of the system, and poses some interesting issues. For one, a person might be able to selectively isolate bacteria from different regions of the plume by using crossed mercury/temperature/pressure gradients. This could be a real boon to vent ecologists.
Jolly fun read!
Costantino Vetriani, Yein S. Chew, Susan M. Miller, Jane Yagi, Jonna Coombs, Richard A. Lutz, and Tamar Barkay
Appl. Environ. Microbiol. 2005. 71: 220-226.
The Big Picture:
Mercury contamination from industrial sources is a huge issue, as are, in fact, all forms of heavy metal contamination of surface waters. Lead, cadmium, mercury, copper... they are all pretty toxic to people drinking the water, to fish, etc, and they tend to be biologically magnified as they are concentrated up the trophic levels. That is, they persist in an animal, and are passed to the thing that eats that animal. Man, at the top of many food chains, gets a whopping dose.
So, one idea that is popular is bioremediation, with plants or bacteria taking the heavy metals out of the water or soil and 'sequestering' them in some easily harvested or bio-inavailable form. If harvested, then the waste can be contained. If bio-inavailable, then the material becomes inert and no longer an immediate threat.
Anyway, the hunt for plants and bacteria that do things with heavy metals has been long. Along the way, we have learned much about the processing and cycling of metals in biological systems. This paper picks up there, with the realization that there are some cool things happening at the deep sea hydrothermal vents.
These vents are like mini-volcanoes on the ocean floor, where hot water comes out of the crust into the otherwise frigid, lightless, and pretty lifeless ocean floor. Bacteria do all sorts of cool things with the chemicals down there to get energy, and they pass that energy to little animals, eventually to bigger and bigger animals. For all sorts of information about symbiosis down there, look to the works of Cavanaugh.
Anyway, one interesting thing is that the hot water sucks cinnabar out of the rocks. Cinnabar is a red mercuric sulfide compound. The hot water sucks out the mercury and then swirls it into the ocean. There is a distinct gradient of mercury in this fashion.
For the non-microbial evolutionary biologist, this seems mildly interesting. However, thinking about enrichment cultures... when you want a bacteria to learn to digest a toxic chemical, you need to start at low concentrations (so it can survive) and steadily increase the concentration to keep challenging it, until it can handle the high concentrations necessary. In nature, by having a steady gradient of mercury in a spatially structured environment, also aligned with a chemo-nutrient gradient, there will be competition for the other chemicals leading the bacteria up the mecury gradient. Ideal conditions for the evolution of mercury processing/decontamination.
Well, that's pretty much what these researchers found. They found that merA was present in a whole bunch of bacteria, even when their isolations didn't select for mercury resistance. They found that mercury resistance was related to the temperature at which the bacteria grew... which makes sense given the gradient of temperature going up towards the vent. They did nice physiological tests and demonstrated the bacteria making the mercury less bioavailable. Then they sequenced a bunch of merA genes and tested the MR protein for optimum temperature. They found that the high temperature bacteria had high temperature MR.
They also found that one of their merA was in another cluster in their tree. This is consistent with putting the root of the merA at the deep sea vents. However, this small tree that they constructed with other known examples of merA was not terribly convincing in and of itself.
Personally, I take issue with both the size of the tree and the method of building it... neighbor joining alone, with some moderate bootstraps (85 at a key node), and not too many merA sequences outside of their experimental samples. I'd prefer to have seen a larger tree with maximum likelihood backing up their conclusions. However, this would require going and isolating other mercury resistant samples, sequencing them, and adding them to the tree. It is pretty far afield for an otherwise sturdy paper.
One place this might be useful is in an industrial setting, where a high temperature bioreactor could be used in a mercury containing waste stream. It might be possible to feed the bacteria and keep the stream at a temperature that discourages outside contamination. Mercury could be pulled out in a particular stage of waste decontamination.
I'd like to see them isolate some more strains and get a bigger merA tree before they really speculate about the evolution. At the same time, this was a fascinating paper on the ecology of the system, and poses some interesting issues. For one, a person might be able to selectively isolate bacteria from different regions of the plume by using crossed mercury/temperature/pressure gradients. This could be a real boon to vent ecologists.
Jolly fun read!
posted by BenK at 12:31 PM
0 Comments:
Post a Comment
<< Home