Communities of bacteria in the soil and in aquatic environments work together to assist each other. For scientists, it is often difficult to study these organisms because their cultivation relies on the inclusion of the other species being in their environment as well. Attempts to culture individual species on, say, a petri dish usually fails.
This can be mitigated to some extent by taking entire soil samples and studying effects on the bacteria therein that way, but soil is also a difficult medium to work with and to measure the effects of perturbation on. In that respect, water samples are a much simpler and oft-chosen choice.
Based on that, researchers at the Georgia Institute of Technology chose to use water samples for their research into rare species within microbial communities. What they found was that these specific organisms are far more important to the health of the rest of the community than expected.
For their experiment, they gathered water samples from Lake Lanier and put them into separate and contained 20 liter “mesocosms”, which are controlled experimental environments set outdoors. The purpose is to replicate the original environment as much as possible, so that any actions done on the samples reflect how the bacterial communities in the wild would react.
Then, the scientists added various organic contaminants to the mesocosms, such as 4-nitrophenol (a pH indicator and intermediary for a variety of industrial products), the herbicide 2,4-D, and also caffeine. What they found after spreading around the introduced substances was that genetic changes were slowly spread throughout the community between different species of bacteria via gene plasmids. The genes being spread usually offered some sort of resistance to the contaminant, especially 2,4-D.
Providing Community Assistance
The surprising part was the original source of these plasmids that the rest of the bacteria uptook. It was the rare bacteria. Now, to understand this group better, there has to be an explanation of what rare means. These bacteria make up a very small amount of the individuals within a community, usually a tenth of one percent.
But there is a huge amount of species diversity within this group, with hundreds of different species with a low population that ultimately make up 20-30% of the overall number of species in the entire community. It’s just that there is a very small number of them individually.
This experiment revealed that this small population of diverse bacteria appear to act as a “genetic reservoir”, which means they keep rare copies of genes like contaminant resistance or stress tolerant genes. In general, if the majority of the bacteria community used these genes all of the time when they weren’t needed, it would just be a drain on their overall fitness.
Therefore, it appears that these rare low-numbered bacteria species retain these genes for future use and then spread them to other bacteria species when needed by using plasmid packets. This allows the overall community to respond to environmental conditions far faster than if they had to wait for natural selection to randomly provide the resistance genes.
Even Bacteria Work Together
Overall, this research shows the interaction and connectedness of bacterial communities and how they work together to survive environmental conditions and also how they rely on the small population numbers of many rare species to provide the more numerous species with genetic material when necessary.
This also shows just why it is so difficult to cultivate any of these bacteria on their own and why scientists have to find special ways of studying these types of bacteria.
Photo CCs: US Army Rangers parachute into Lake Lanier 140508-A-PP104-171 from Wikimedia Commons