Chemical grazing defence of bacterial populations in the ocean
Final Report Abstract
Within the microbial loop, a tightly coupled food web, predation of protist onto bacterial and archaeal prey is a key process for nutrient cycling, microbial diversity, community structure, and evolution. Grazing defence mechanisms, such as changes in cell size or chemical interactions may play a key role. Following the hypothesis that, similar to viral predation, host specificity of protozoan grazing mediated by genetically encoded defence mechanisms significantly affects the genotypic structure of microbial populations, we aimed to investigate chemical grazing defence mechanisms in closely related strains of different Vibrio populations. The first aim was to find candidates for further investigations. Therefore, we conducted a predation matrix of 240 different strains with four protist predators to find potential candidates for further investigations. We used Illumina MiSeq sequencing of the hsp60 genes of the different strains to determine their predation susceptibility. Unfortunately, the sequence quality was low and the applied linear model could not be fitted to all samples. Consequently, we decided follow a different research question, which arose from a master thesis project conducted by Estelle Clerc. The aim of this master thesis was to investigate chemical grazing resistance mechanisms of biofilm forming Vibrio breoganii. In a predation assay of 42 strains of V. breoganii and 8 control strains, we discovered that predation susceptibility follows a gradient, suggesting that predation resistance is caused rather by a gradual process than a grazing resistance mechanisms. The results showed that grazing susceptibility is highly correlated with initial cell numbers of the biofilms. This suggests that growth rate, which can determine cell numbers in biofilms, is the underlying mechanism defining predation susceptibility. Growth rate is one of the basic features of all bacterial cells. Consequently, we decided to follow this line of research. Since biofilms are complex environments, where growth rates are difficult to assess we used liquid cultures of V. cyclitrophicus for the following experiments. Following the hypothesis, that growth rate is positively correlated with cell size and also with predation rate, we aimed to investigate grazing preferences on large versus small cells. We used equal amounts of cells of fixed large and small one V. cyclitrophicus strain for incubation experiments with two different predators. FC and SMR analyses indicated a higher predation rate for larger cells. We adjusted the experimental conditions and repeated the experiment with three strains. The results of these experiments are still pending. In addition to the analyses of the results of the experiment to determine size dependent predation rates, two more experiments were planned and will be conducted in the future. Firstly, we aim to investigate if growth rate determines predation rate in a competition experiment with a slow and a fast growing strain challenged with different predators. Secondly, we aim to investigate the evolutionary effect of predation on the growth rate of a fast growing strain in a long term incubation experiment using a turbidostat. These experiments would allow us to draw conclusions about the correlation of growth rate and predation rate dynamics of closely related strains of Vibrio and potentially bacterial strains in general. In order to investigate the importance of cell size changes in the environment, I monitored the bacterial community and cell size during a dinoflagellate bloom in the enclosed environment of a salt pond. Analysis of the sequences and samples for SMR measurements are still pending. In addition, I was able to join an expedition to Antarctica to investigate predatorprey interactions in a glacial influenced marine environment. Unfortunately, first experiments showed that the tracer experiments did not achieve enough labelling of the samples.
Publications
- 16th International Symposium on Microbial Ecology, Montreal, Canada: Chemical defence mechanisms in Vibrio strains
S. Thiele, J. Elsherbini, A. K. M. Kauffman, M. D. Johnson, T. J. Mincer, M. F. Polz
- Annual Meeting of the Association for General and Applied Microbiology (VAAM), Jena, Germany: Chemical warfare – Chemical defence mechanisms in Vibrio strains
S. Thiele, M. D. Johnson, A. K. M. Kauffman, J. Elsherbini, M. F. Polz
- Annual Meeting of the American Society for Microbiology (ASM), New Orleans, USA: Grazing Rates of Amoebae Depend on Prey Growth Rates in Biofilms of Closely related Bacteria
S. Thiele, E. E. Clerc, M. F. Polz
