Dragon’s Eye: the truth behind the legends

There are couple of legends about Rogoznica Lake (Dragon’s Eye) that include Greek gods, brothers and even aliens. However, from the scientific point of view, Dragon’s Eye is a marine lake located on the eastern Adriatic Coast (Croatia) with rare and specific features. It represents an extreme marine environment with a water column stratified into oxic, hypoxic and permanently euxinic (anoxic and sulfidic) bottom layer.

Occasionally, depending on meteorological conditions, these layers overturn causing the spread of anoxia throughout the entire water column. The only organism able to survive these conditions are bacteria and archaea. Microbes that are able to live and thrive in anoxic aquatic environments are very interesting for evolutionary implications as well as for predicting future phenomena. Contemporary examples of these environments are observed in meromictic lakes, and as such, they can serve as valuable proxies for investigations of redox transition zones. We investigated microbial communities that successfully cope to these extreme conditions using high-throughput sequencing of 16 rRNA gene. Results revealed that bacteria and archaea in the lake are tightly coupled with their environment.

Despite the extreme conditions they are also found to be very diverse, however, most of the diversity is hidden within a large proportion of unclassified bacteria, confirming that these environments are still unexplored. Long-term geochemical data show warming, sulfide and ammonium accumulation in the bottom layer of the lake, as indicators of global climate change. These changes can have adverse effects on microbial communities and biochemical processes in the lake.