Dark carbon fixation is a common process in stratified boreal lakes. Chemolithoautotrophs (ChLithO) in boreal lakes

CO2 fixation (i.e primary production) is a key function of all ecosystems as it is the source of carbon and energy for the whole food web. It also plays an important role in mitigating climate change as CO2 is the most important greenhouse gas. Even if photosynthesis is regarded as the most important carbon fixation pathway, in a light limited ecosystem prokaryotes able to fix carbon in the absence of light (chemolithoautotrophs) can also be a significant source of energy. Boreal lakes, notoriously colored and stratified in regards of oxygen and nutrients, present ideal conditions for this so called dark carbon fixation by the chemolithoautotrophs. However, the prevalence of dark carbon fixation in boreal lakes remains unknown. Here, we measured dark carbon fixation in Swedish lakes from the boreal and boreo-nemoral zones, during summer stratification. We detected dark carbon fixation in 16 out of 18 studied lakes , and conclude that dark fixation is a widespread phenomenon in boreal lakes. Our data further suggests that chemolithoautotrophic activity is mostly driven by iron-oxidizing bacteria. The chemolithoautotrophic guild is diverse and seems to be composed of ubiquitous bacteria as well as of taxa specific to certain lakes, such as Ferrovaceae, which seems to be favored by lakes with low pH. These results are particularly exciting as they suggest that dark carbon fixation could partly compensate for the poor photosynthetic capacity in lakes with dark watercolor.