It is a well known fact that many bacteria like to live in groups. Moreover, just as human societies strive to facilitate group living, (through grocery stores that provide food for otherwise unsustainably crowded cities, and municipal waste centers to ensure that our concentrated living spaces are kept clean enough to remain livable), bacterial populations employ a variety of strategies that enhance collective life.
The bacterial analog to human group-living strategies is called quorum sensing. Quorum sensing, or QS, is really an umbrella term used to describe a range of bacterial behaviors that enhance group life. QS behaviors are considered common to all bacteria and have likely been tracking bacterial evolution since the first true cells of Precambrian earth crawled out of their prebiotic soup. Here I’ll focus on a particular group of bacteria that employ some very ecologically important QS strategies.
Bacteria are broadly divided into several distinct phyla that occupy a range of earth habitats and employ an enormous variety of survival strategies. Proteobacteria are one such major group that is particularly interesting to me because of their posited dominance in the zone of plant-nutrient uptake known as the rhizosphere. The significance of proteobacteria in rhizosphere microbial populations has really only been examined in temperate forests, and hopefully some of my own work will bring a tropical perspective to our understanding of rhizosphere community composition.
It turns out that proteobacteria release a specific signal molecule known as N-acyl-homoserine lactone, or (AHL) in order to alert other proteobacteria of their existence. Thus proteobacteria are able to “quorum sense” their environment and receive information of the relative density of their fellow quorum members. Moreover, the release of AHLs operates via a positive feedback mechanism- higher concentrations of AHLs attract more bacteria, resulting in even higher concentrations of AHLs.
Bacteria don’t employ quorum sensing simply because they enjoy each other’s company. In fact, there are almost certainly trade offs associated with group life, including higher levels of predation, lower oxygen availability and buildups of toxic waste products. However, the benefits that can be obtained by living in groups are also significant. Soil bacteria produce extracellular enzymes that release soluble, digestible molecules into their environment. A greater concentration of bacteria results in a greater concentration of free enzymes and thus a more nutrient-rich environment. In the rhizosphere, high concentrations of bacteria can also exert positive feedbacks on rhizosphere priming, the mechanism by which plants release sugars into the soil to nourish the bacterial community.
Though bacterial QS behavior has been well studied for decades, most research has been in the field of disease ecology and few studies have experimentally demonstrated QS to be an important phenomenon in soils. One recent study tackled this problem through a detailed examination of bacterial populations in rhizosphere soil, using controlled pot experiments. The authors found that AHL, the QS signal specific to proteobacteria, was 10 times more concentrated in rhizosphere soils compared to bulk soil. Similarly, bacterial densities in the rhizosphere were about 10 times higher. Furthermore, the researchers discovered a tight correlation between QS and enzyme activity. In particular, enzymes involved in acquiring nitrogen were closely linked to QS expression. Nitrogen is considered the major plant-limiting nutrient in temperate ecosystems, and it is believed that one of the primary motivations for a plant to “prime” the soil around its roots is to access soluble nitrogen from microbial decomposers.
I knew almost nothing about quorum sensing before reading this paper. As always, I found myself overwhelmed by the nuanced mechanisms that bacteria employ in order to explore and shape their environment, coupled with the interactions of plants that can manipulate this complex community for their own nutrition. Quorum sensing could well be an important control in rhizosphere nitrogen availability, and will have to be studied in more detail to fully assess its role in controlling ecosystem productivity.
DeAngelis, K.M., Lindow, S.E. & Firestone, M.K. Bacterial quorum sensing and nitrogen cycling in rhizosphere soil. FEMS Microbiology Ecology 66, 197-207 (2008).