The importance of bacterial functional diversity for mixotrophic flagellate function

Abstract: This project examined the role of functional diversity of bacterial prey organisms on a mixotrophic flagellate’s function. The concept of functional diversity and its role for ecosystem functioning is a subject of ongoing research and controversially discussed. Mixotrophy is the combination of different modes of nutrition in one organism. In the case of this project’s organism, Ochromonas tuberculata, the combination of phototrophy and heterotrophy or phagotrophy. Mixotrophs can have a severe impact on nutrient cycling or carbon fixation in aquatic ecosystems, therefore it is important to investigate what drives their ecological functioning. For this project, two sets of bacterial prey traits were investigated and their effect on the flagellate’s growth rate. The first experiments involved the addition of heat-killed bacterial prey with heterotrophic mode of nutrition (Escherichia coli) and phototrophic mode of nutrition (Synechocystis sp.) on their own and in conjunction to generate a functional diversity. The last experiment investigated heat-killed Gram-negative (Escherichia coli) and Gram-positive bacterial prey (Staphylococcus aureus and Bacillus subtilis) each on their own and in conjunction. The acclimated O. tuberculata cultures in this project were non-axenic and flagellate cell densities were recorded daily over five days after the addition of the prey in each experiment. The results showed that prey with a phototrophic mode of nutrition resulted in a significantly higher growth rate over the course of the experiments than heterotrophic prey or a mix of both under the experimental conditions. For the second set of bacterial functions tested, the results showed no clear effect of the different bacterial functions on the flagellate’s growth rate. This was attributed to the heat-killing method used. The heat-killing method is assumed to have harmed the bacteria structurally and therefore negate the effect of the differences in cell wall properties. Therefore, this question could not be fully investigated. Overall, the experimental setup was not ideal to tackle the research question, nevertheless the results can provide ideas for possible improvements of the experimental setup.