Understanding the abundance, diversity, and distribution of organisms in the environment is a central goal of ecology with broad implications including ecosystem function and conservation. Ecologists have described many examples of how species interactions, such as competition and predation, or environmental conditions, such as temperature and salinity, influencing community composition. However, the strength and relative importance of these forces and how they interact to determine community composition remains poorly understood. This research project will experimentally elucidate how forces that structure communities act together to determine the abundance and composition of microbial assemblages in lakes. Aquatic microbial communities provide an ideal system for studying forces that influence community composition, because these communities exhibit seasonal patterns of community change, are influenced by a number of biotic and environmental factors, and are relatively easy to manipulate. Observational studies and experimental manipulations indicate that phytoplankton have a substantial influence on temporal patterns of bacterial community change. Environmental conditions including temperature and light have the potential to affect bacteria directly as well as indirectly through modifying bacterial interactions with phytoplankton. This study will explore how bacterial community structure is influenced by species interactions with phytoplankton under different temperature and light conditions by use of manipulative, mesocosm experiments. Bacterial response to all combinations of phytoplankton, temperature, and light will be used to assess the relative influence of biotic and environmental factors on bacterial community composition and how phytoplankton influence changes across varying light and temperature contexts.
Results from this research will enhance the ability of scientists to forecast responses of freshwater microbial communities to environmental change. This project supports the doctoral research of a graduate student and will provide research experiences for several undergraduate students. The principal investigators will participate in outreach efforts aimed at introducing local K-12 students to ecology and environmental science. Hands-on activities will be conducted for students at the Campus Middle School for Girls, and the Don Moyer Boys and Girls Club, and presentations will be made at the Illinois Envirothon, a statewide environmental competition for high school students.
Species interactions (e.g., competition, predation, mutualism) and environmental conditions (e.g., temperature, light, nutrient availability) have been demonstrated to affect the abundance, diversity, and distribution of organisms in the environment. It has also been observed that the effect of species interactions can change under different environmental conditions. For example, competition may become more important in a community as nutrient availability decreases. Our ability to predict how community composition will shift in response to changes in the environment depends on examining species interactions under different environmental conditions. Microbial community composition can affect nutrient cycling and the flux of greenhouse gases in lakes. As a result, it is important to understand factors that cause lake microbial community composition to change. Phytoplankton community composition changes seasonally and interactions with phytoplankton have been demonstrated to cause corresponding changes in bacterial community composition. Environmental conditions, including light and temperature, can change how phytoplankton and bacteria interact while also affecting each community directly. Light availability and temperature decrease with depth and change seasonally; these changes are especially pronounced in darkly stained humic lakes. Thus, light and temperature conditions are potentially critical to understanding the effect of phytoplankton on spatial and temporal changes in bacterial community composition. Our objective was to determine how the combined influences of phytoplankton, light, and temperature affect bacterial community composition. To accomplish this, we carried out an experiment in which bacterial communities from two humic lakes were combined with phytoplankton from one of the two lakes (their "home" phytoplankton, and "away" phytoplankton from the other lake) or no phytoplankton as a control and incubated at two light levels and five temperatures ranging between 10°C and 25°C (Fig. 1, Fig. 2). The effect of light on bacterial community composition was most prominent when bacteria were incubated without phytoplankton present. Direct light effects are potentially due to light stimulating the growth of phototrophic bacteria that can obtain energy from light. The effects of phytoplankton and temperature on bacterial community composition were interdependent. At low temperatures bacterial communities from all phytoplankton treatments were similar to their initial composition. Bacterial communities became increasingly different over time from their initial composition as temperature increased when bacteria were incubated with phytoplankton. Change in bacterial community composition explained by temperature was most pronounced when bacteria were incubated with "away" phytoplankton. When bacterial communities were incubated without phytoplankton, the effect of temperature on bacterial community composition was not significant, or was significant, but less influential, than when phytoplankton were present. These results suggest that bacterial communities in the lakes studied are resource-limited in the absence of phytoplankton, and that the effects of temperature change on bacterial community composition are primarily mediated by phytoplankton. The enhanced effect of "away" phytoplankton on bacteria is likely due to the availability of a different pool of phytoplankton resources than bacteria had become acclimated to in their "home" lake. We additionally characterized the relative abundance of bacterial groups in each treatment combination and found differences in responses among closely related bacteria. Intellectual Merit: These results enhance our understanding of how the environment modifies species interactions and the mechanisms that create and maintain diversity, key concepts for advancing the science of community ecology. Insights gained bring us closer to being able to explain and predict spatial and temporal patterns in community structure, and thus forecast community responses to changes in the environment, including those resulting from global climate change. These findings also improve our understanding of the biology and ecology of freshwater bacteria. Broader Impacts: This project contributed to the training of one Ph.D. student and two undergraduate students (Fig. 3). During the funding period, we developed educational materials to introduce local students to ecology and environmental science and presented this material with hands-on activities to 8th graders at the Campus Middle School for Girls, and to 2nd through 6th graders participating in a "Girls Do Science" summer camp. PI Sara Paver additionally served as the presenter at the aquatics station for the 2011 and 2012 Illinois State Envirothon Competitions.
