Shedding light on the role of light in lake ecosystems – the role of photoheterotrophy

The aim of this project is to evaluate the importance of a little studied function of microorganisms that is of importance for our view of the role of lakes in the global carbon cycle. More specifically, the aim is to evaluate the causes and consequences of photoheterotrophy in lake ecosystems.

Photoheterotrophic organisms can use light as an energy source without performing traditional photosynthesis by fixing carbon dioxide. The consequence of photoheterotrophy is that less carbon dioxide is released by respiration in the presence of light.

Research indicate that the role of photoheterotrophs is largely underestimated, and that the degree to which bacteria release carbon dioxide in lake ecosystems is overestimated.

We hypothesize that a high abundance and activity of photoheterotrophs is affecting the carbon dioxide balance in lakes.

Further, it is hypothesised that respiration measurments made in darkness is overestimating the production of carbon dioxide when photoheterotrophs are present. There are two different types of photoheterotrophs known. One is using bacteriochlorophyll as a light harvesting pigment while the other use a form of rhodopsin.

We hypothesise that these two different types of photoheterotrophs are, at least partly, promoted by different environmental conditions, where the availability of iron is one key factor.

The hypotheses will be tested by field observations and experiments, by for example metagenome sequencing, in a 4-year PhD student project.