One of the basic tenets of aquatic ecology is that primary production by algae, through the process of photosynthesis, supports all aquatic food webs and is responsible for approximately half of the primary productivity on earth. In reality, many algal species are mixotrophic; that is, they are capable of supplementing photosynthetic production with heterotrophic consumption of bacteria, other algae, or even dissolved and particulate material transported into lakes from terrestrial sources. The proposed dissertation research will use natural stable isotopic tracers to quantify the amounts of algal production due to photosynthesis versus mixotrophy. Estimates of mixotrophy from laboratory and field experiments will be combined with predictive models of mixotroph abundance across the continental United States to evaluate the importance of mixotrophy in freshwater food webs.
Mixotrophy represents a potentially important natural process that has been known for some time, but is poorly understood. Results from this study will allow more accurate descriptions and models of aquatic food webs. This knowledge will provide better predictions of changes to freshwater ecosystems - such as the occurrence of toxic algal blooms - that are increasingly exposed to human-induced modifications. The investigators will recruit undergraduates from under-served groups to participate in the research through ongoing SUNY-ESF programs. In addition, the research will be carried out at SUNY-ESF's Cranberry Lake Biological Station, which is an active teaching campus throughout the summer. Students enrolled in courses at the station will participate in the study.
While algae, the foundation of aquatic food webs, are typically thought of as primary producers, the reality is that many species also are predators. These "mixotrophs" are both photosynthetic and also capable of feeding on other microscopic organisms. This unique form of omnivory is utilized by these species to overcome environmental factors such as low light and/or nutrient availability, which can limit strictly photosynthetic growth. Given that mixotrophic algae may function simultaneously as both producers and consumers in aquatic systems, it has been difficult for researchers to quantify their impacts on nutrient and energy cycles and how their presence corresponds to the environmental conditions. We systematically examined mixotrophy and its impact on aquatic food webs First, we utilized a new technique that we have developed using stable isotope tracers to measure a mixotroph’s carbon gained from photosynthesis in conjunction with controlled growth experiments. Some elements have multiple stable isotopic versions, varying only in the number of neutrons their atoms contain (for example Carbon-12 and Carbon-13). The ratio of these isotopes in an organism can be used to quantify the contribution of multiple food sources in an organism’s diet (i.e., you are what you eat). Our research used a modified version of this technique to differentiate between carbon obtained by the mixotrophic algae consuming a single food source (bacteria) and carbon from photosynthesis by these algae. NSF funding allowed us to fully develop this technique, in conjunction with growth rate studies, to assess how mixotrophy is utilized by a variety of algal species. Each species was grown under a range of light levels, from extremely high (similar to that seen in a shallow, clear lake) to total darkness. For each light level tested, half of the algal cultures were supplemented with a bacterial food and half had no food, so algae could only obtain energy from photosynthesis. Samples were collected throughout to measure each algal species’ growth and bactivory rates. At the end of each trial samples also were collected to measure stable carbon isotope values. We found that for six of the seven species tested, the addition of bacterial food allowed mixotrophic algae to grow under some conditions where it was not able to persist with no food. Some mixotrophic algae were able to grow in complete darkness by consuming bacteria. Isotopic values also differed substantially between algal species, indicating that some mixotrophs are able to assimilate high percentages of bacterial carbon, while others integrated very little carbon from bacteria, even when predation rates were very high. This work demonstrates that mixotrophy provides several alternative advantages, with some species mainly obtaining carbon and energy from bacterial prey, while others primarily utilize bacteria as an alternative source of growth-limiting nutrients. In addition to facilitating the development of our novel in-lab isotope techniques, NSF funding also allowed us to conduct field research on mixotrophy. First, we assessed the abundance of mixotrophs in a 40-lake sampling effort in the northeastern U.S., focusing on lakes of differing nutrient and staining statuses. Algae, bacteria, dissolved organic carbon (DOC), and nutrient samples were collected, as well as measures of each lake’s physical parameters (e.g., maximum depth, temperature, light penetration). We chose lakes across a range of DOC content, as the associated staining compounds are thought to reduce light penetration and decrease bioavailability of nutrients. Also, we used funds to conduct in situ enclosure experiments in Cranberry Lake, NY to assess how the relative abundance of natural mixotrophic algal populations was affected by nutrient, light and bacterial additions. This aspect of our work was crucial, as it allowed us to test whether factors that support photosynthetic growth (i.e., light and/or nutrient additions) as well as the availability of food (bacteria additions) would result in observable changes in the abundance of mixotrophs under natural conditions. Cranberry Lake is a highly stained system, with low light in the water column, so light addition was necessary to accomplish our goals. This grant allowed us to design, develop, and test a new system of inexpensive underwater lights. We subsequently produced a peer-reviewed method paper describing the construction of these devices, making them available to a wide-variety of researchers. Funding from the National Science Foundation was critical in allowing for the completion of co-PI Gillette’s dissertation research and allowed for greatly expanded research findings in all areas pursued. It also allowed for the hiring of two post-graduation bachelor-degree students who assisted with field and lab research, both of whom subsequently went on to pursue graduate degrees. Additionally, while experiments were conducted at Cranberry Lake, numerous undergraduate students assisted with both the enclosure experiments and the construction of the lighting devices, providing them with valuable experience in both plankton ecology and field research techniques.
