Can osmotrophy in Gonyostomum semen explain why lake browning drives an expansion of the alga in parts of Europe?

Gonyostomum semen experiences a remarkable expansion in parts of Europe, partially driven by rising levels of dissolved organic matter (DOM), also called lake browning. One way to explain the stimulating effect of browning is to assume that G. semen combines photosynthesis with osmotrophy, the ability to utilize DOM as source of nutrients or energy. The present work tested this idea on a nearly unialgal G. semen bloom. Continuous vertical profiling found that each night, G. semen moved into the oxic-anoxic interface or the anoxic hypolimnion, i.e., into places where redox processes can give access to DOM via dissolution of ferric oxide-organic matter complexes. In the hypolimnion, conditions were right for a build-up of DOM, which, however, never happened. Optical analyses of hypolimnetic DOM indicated that the build-up was prevented by the removal of terrestrially derived DOM, which was correlated with G. semen. During the daytime, G. semen migrated into the epilimnion. Here, net oxygen production was near zero, indicating that G. semen combined photosynthesis with heterotrophic nutrition. Optical analyses of epilimnetic DOM hinted at the utilization of terrestrially derived DOM. Taken together, the above findings provide circumstantial evidence for G. semen combining photosynthesis with osmotrophy in nature. The proposed utilization of terrestrially derived DOM suggests that lake browning may favor G. semen by improving the access to DOM that the alga can utilize. Browning may also benefit G. semen by promoting summer anoxia that causes a reductive release of DOM from complexes with ferric oxides, improving the alga’s access to DOM.