Soda pans of the Pannonian steppe are unique environments regarding their physical and chemical characteristics: shallowness, high turbidity, intermittent character, alkaline pH, polyhumic organic carbon concentration, hypertrophic condition, moderately high salinity, sodium and carbonate ion dominance. The pans are highly productive environments with picophytoplankton predominance. Little is known about the planktonic bacterial communities inhabiting these aquatic habitats, therefore amplicon sequencing and shotgun metagenomics were applied to reveal their composition and functional properties. Results showed a taxonomically complex bacterial community which was distinct from other soda lakes regarding its composition, e.g. the dominance of class Alphaproteobacteria was observed within phylum Proteobacteria. The shotgun metagenomic analysis revealed several functional gene components related to the harsh and at the same time hypertrophic environmental conditions, e.g. proteins involved in stress response, transport and hydrolase systems targeting phytoplankton-derived organic matter. This is the first detailed report on the indigenous planktonic bacterial communities coping with the multiple extreme conditions present in the unique soda pans of the Pannonian steppe.

Astatic soda pans of the Pannonian Steppe are unique environments with respect to their multiple extreme physical and chemical characteristics (high daily water temperature fluctuation, high turbidity, alkaline pH, salinity, polyhumic organic carbon concentration, hypertrophic state and special ionic composition). However, little is known about the seasonal dynamics of the bacterial communities inhabiting these lakes and the role of environmental factors that have the main impact on their structure. Therefore, two soda pans were sampled monthly between April 2013 and July 2014 to reveal changes in the planktonic community. By late spring in both years, a sudden shift in the community structure was observed, the previous algae-associated bacterial communities had collapsed, resulting the highest ratio of Actinobacteria within the bacterioplankton (89%, with the dominance of acIII-A1 lineage) ever reported in the literature. Before these peaks, an extremely high abundance (> 10,000 individuum l−1) of microcrustaceans (Moina brachiata and Arctodiaptomus spinosus) was observed. OTU-based statistical approaches showed that in addition to algal blooms and water-level fluctuations, zooplankton densities had the strongest effect on the composition of bacterial communities. In these extreme environments, this implies a surprisingly strong, community-shaping top-down role of microcrustacean grazers.

6Astatic soda pans of the Pannonian Steppe are unique environments with respect to their multiple extreme physical 7 and chemical characteristics (high daily water temperature fluctuation, high turbidity, alkaline pH, salinity, 8 polyhumic organic carbon concentration, hypertrophic state and special ionic composition). However, little is 9 known about the seasonal dynamics of the bacterial communities inhabiting these lakes and the role of 10 environmental factors that have the main impact on their structure. Therefore, two soda pans were sampled 11 monthly between April 2013 and July 2014 to reveal changes in the planktonic community. By late spring in both 12 years, a sudden shift in the community structure was observed, the previous algae-associated bacterial 13 communities had collapsed, resulting the highest ratio of actinobacteria within the bacterioplankton (89%, with 14 the dominance of acIII-A1 lineage) ever reported in the literature. Before these peaks, an extremely high 15 abundance (>10,000 individuum l -1 ) of microcrustaceans (Moina and Arctodiaptomus) was observed. OTU-based 16 statistical approaches showed that in addition to algal blooms and water-level fluctuations, zooplankton densities 17 had the strongest effect on the composition of bacterial communities. In these extreme environments, this implies 18 a surprisingly strong, community-shaping top-down role of microcrustacean grazers. 19 20

Three alkaliphilic and halotolerant bacterial strains, designated ZV-19, R4-8 and S4-12, were isolated from the water of soda pans located in the Kiskunság National Park, Hungary. Cells of all three strains were Gram-staining-negative, rod-shaped, motile and non-endospore-forming. They were facultatively anaerobic, and oxidase- and catalase-positive. Their major isoprenoid quinone was Q-8, and their predominant fatty acids were C18 : 1ω7c, C16 : 1ω7c and C16 : 0. The DNA G+C content was 54.5 mol% in strain ZV-19 and 45.8 mol% in strain R4-8. The 16S rRNA gene based phylogenetic analysis showed that all three strains were members of the genus Nitrincola (family Oceanospirillaceae, class Gammaproteobacteria). Strain ZV-19 showed 96.6 and 95.5 % sequence similarities and 19±3 and 18±3 % DNA-DNA relatedness to Nitrincolalacisaponensis DSM 16316 and Nitrincolaalkalisediminis JCM 19317, respectively. Strains R4-8 and S4-12 exhibited 97.9 and 98.6 % sequence matches and 34±4 and 13±8 % DNA-DNA hybridization values with N. lacisaponensis DSM 16316 and N. alkalisediminis JCM 19317, respectively. According to the phenotypic, chemotaxonomic and phylogenetic data, the strains studied represent two novel species, Nitrincola alkalilacustris sp. nov. with the type strain ZV-19 (=DSM 29817=NCAIM B 02612) and Nitrincola schmidtii sp. nov. with the type strain R4-8 (=DSM 100788=NCAIM B.02626). An emended description of the genus Nitrincola is also presented.

In April 2014, dual bloom of green algae and purple bacteria occurred in a shallow, alkaline soda pan (Kiskunság National Park, Hungary). The water was only 5 cm deep, in which an upper green layer was clearly separated from a near-sediment purple one. Based on microscopy and DNA-based identification, the upper was inhabited by a dense population of the planktonic green alga, Oocystis submarina Lagerheim, while the deeper layer was formed by purple, bacteriochlorophyll-containing bacteria, predominated by Thiorhodospira and Rhodobaca . Additional bacterial taxa with a presumed capability of anoxygenic phototrophic growth belonged to the genera Loktanella and Porphyrobacter . Comparing the bacterial community of the purple layer with a former blooming event in a nearby soda pan, similar functional but different taxonomic composition was revealed. Members from many dominant bacterial groups were successfully cultivated including potentially new species, which could be the result of the application of newly designed media. Electronic supplementary material The online version of this article (10.1007/s00792-019-01098-4) contains supplementary material, which is available to authorized users.