Microorganisms Collected from the Surface of Freshwater Lakes Using a Drone Water Sampling System (DOWSE)

New tools and technology are needed to study microorganisms in freshwater environments. Little is known about spatial distribution and ice nucleation activity (INA) of microorganisms in freshwater lakes. We developed a system to collect water samples from the surface of lakes using a 3D-printed sampling device tethered to a drone (DOWSE, DrOne Water Sampling SystEm). The DOWSE was used to collect surface water samples at different distances from the shore (1, 25, and 50 m) at eight different freshwater lakes in Austria in June 2018. Water samples were filtered, and microorganisms were cultured on two different media types, TSA (a general growth medium) and KBC (a medium semi-selective for bacteria in the genus Pseudomonas). Mean concentrations (colony forming units per mL, or CFU/mL) of bacteria cultured on TSA ranged from 19,800 (Wörthersee) to 210,500 (Gosaulacke) CFU/mL, and mean concentrations of bacteria cultured on KBC ranged from 2590 (Ossiachersee) to 11,000 (Vorderer Gosausee) CFU/mL. There was no significant difference in sampling distance from the shore for concentrations of microbes cultured on TSA (p = 0.28). A wireless bathymetry sensor was tethered to the drone to map temperature and depth across the sampling domain of each of the lakes. At the 50 m distance from the shore, temperature ranged from 17 (Hinterer Gosausee, and Gosaulacke) to 26 °C (Wörthersee), and depth ranged from 2.8 (Gosaulacke) to 11.1 m (Grundlsee). Contour maps of concentrations of culturable bacteria across the drone sampling domain revealed areas of high concentrations (hot spots) in some of the lakes. The percentage of ice-nucleation active (ice+) bacteria cultured on KBC ranged from 0% (0/64) (Wörthersee) to 58% (42/72) (Vorderer Gosausee), with a mean of 28% (153/544) for the entire sample set. Future work aims to elucidate the structure and function of entire microbial assemblages within and among the Austrian lakes.