A recent body of research has begun to study how diversity within microbial communities, such as bacteria and fungi, can affect ecosystem processes. Since microbes are easily overlooked by students there are a few initial points that are worthy of emphasis to students. 1) Microbes are incredibly diverse with estimates ranging from thousands to millions of microbial “species”* per gram of soil! 2) Microbes, especially bacteria, contain most of the nitrogen and phosphorous, and approximately half of the carbon stored in living organisms. 3) Microbes are responsible for the bulk of decomposition and perform essential steps in the carbon, nitrogen, sulfur and phosphorous cycles. 4) Our limited knowledge of microbes, especially in the environment, is due to methodological hurdles.
This graph is from McGrady-Steed et al. 1997 in the journal Nature. In this paper the researchers asked the question, what is the effect of diversity on ecosystem predictability? (Emphasize to students that one of the goals of science and ecology is to build predictive models off of empirical research and data). The researchers defined predictability as low or reduced variability in an ecosystem process. Specifically in this set of experiments the scientists manipulated the diversity of aquatic microbial communities and then measured rates of ecosystem respiration. The x-axis in the above graph has realized species richness (# of different microbial species) as the independent variable. The y-axis in this case is actually a measure of variability. Notice that the y-axis is the standard deviation of carbon dioxide flux.** The negative correlation indicates that as microbial species richness increases the variability in carbon dioxide production decreases. What is important to emphasize is that as variation decreases, predictability increases — or put another way — as diversity increases the predictability of an ecosystem process increases.
*There is currently no agreed upon definition of a microbial species. Ask students why the commonly taught Biological Species Concept might not work as a unit of taxonomy for microbes.
**It may be worthwhile to draw a simple graph on the board and show students how error bars briefly work. For example. one bar with a small error bar and a second with a large error bar. Indicate to the students what this means about the populations that were measured. (For a brief summary: http://en.wikipedia.org/wiki/Error_bar).
1. What might be some of the challenges to studying microbes (especially in the environment)?
2. What are some of the recent technological and methodological advancements that have enhanced the study of microbial communities (e.g. Polymerase Chain Reaction, sequencing)?
3. Why is being able to predict ecosystem processes important? How can robust predictive models help us manage events of disturbance, extreme weather, and climate change?
4. How is human and public health dependent on microbial communities?
5. Thought question: if microbial communities contain large amounts of carbon how could they be involved in climate change processes (in both positive and negative ways)? Below: helpful graphic of carbon cycle. Emphasize to students that much of the respiration (production of CO2 indicated by blue up-arrows) is due to microbial processes. Notice that rates of photosynthesis and respiration are nearly equal.