Tracing the food chain from magnetotactic bacteria to protozoa under laboratory and environmental conditions

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Fossilized magnetosomes produced by magnetotactic bacteria (MTB) are a crucial proxy in paleomagnetism studies, that are used to reconstruct the timing of past geological events in Earth history. However, the environmental and ecological processes controlling the abundance of MTB in natural environments, and their magnetosomes, are poorly understood. One key aspect of MTB ecology that remains poorly characterized, is how their abundance in natural environments is controlled by microbial predators: predatory protozoa (protists). We will test hypotheses surrounding whether protist grazing plays a role in controlling MTB abundance in nature and the effects of protist grazing on the preservation of magneotosomes. I propose to address this challenge by applying a novel experimental setup that will, for the first time, directly identify the full diversity of protists that consume MTB under environmental conditions and reveal rates of MTB grazing and magnetosome dissolution under controlled laboratory conditions. My lab has developed a new experimental setup that allows for identification of protist grazing on isotopically labeled prey cells, that we will apply to natural environmental settings to reveal the diversity of protists that prey on MTB in both marine and freshwater environments. Using a controlled laboratory setup with an oxic-anoxic gradient, we will perform protist grazing experiments with mutant and wild-type MTB to identify which MTB phenotypes play a role in helping the MTB escape predation by certain protists. This will be done in collaboration with Prof. Dr. Dirk Schüler (University of Bayreuth) who maintains a large culture collection of mutant MTB strains with varying phenotypes, that we will apply in our protist grazing experiments. This will reveal the MTB phenotypes that are critical for their survival in the presence of key protist predators, and how they avoid predation under certain environmental conditions. We will also assess rates of magnetosome dissolution by the protists, that can dissolve the magnetosomes after the MTB have been ingested. For those magnetosomes that are ingested, but not completely dissolved, we will assess the changes in magnetosome morphology and magnetic properties. Altered magnetic properties of ingested magnetosomes will be studied in collaboration with Prof. Dr. Stuart Gilder (LMU Munich), by means of first order reversal curve (FORC) analysis of ingested magnetosomes at the end of the grazing experiments. This will reveal how protist grazing effects the magnetic properties of the magnetosomes, an important but poorly understood factor in the magnetosome fossilization process. Because the magnetosomes produced by MTB are a crucial proxy used in paleomagnetism studies of past geological events, the results of this project will aid the interpretation of magnetofossils in the geological record.