KARST SPRING MICROBIAL DIVERSITY DIFFERS ACROSS AN OXYGEN-SULPHIDE ECOCLINE AND REVEALS POTENTIAL FOR NOVEL TAXA DISCOVERY

Strong geochemical gradients of dissolved oxygen and sulphide establish habitats where specialized bacterial and archaeal taxonomic groups occupy specific redox-sensitive niches, primarily based on metabolic and ecological requirements. In contrast, knowledge of microeukaryote diversity and their ecology in redox-stratified habitats is poor, as species-specific occupation of such geochemical gradients has not been well established. Here we assessed total microbial diversity from rRNA genes retrieved from two morphologically distinct microbial mats formed along an oxygen-sulphide gradient in the outflow channel from the Zveplenica sulphidic karst spring, Slovenia. Microbial mats contained diverse bacteria and archaea associated with chemolithoautotrophic and primary productivity, and overall microeukaryotic diversity was higher under oxygenated conditions. The oxygenated mats were comprised of undescribed and undifferentiated fungi, Annelida, Nematoda, Apicomplexa, and Gastrotricha, some being represented by novel lineages. Under anoxic conditions, diversity was dominated by Ciliophora, Nematoda, and Fungi-Ascomycota, also affiliated with novel lineages. Colonization of the distinct mat types related to ecological tolerance of specific geochemical conditions, and the associations between bacterial and archaeal diversity with distinct microeukaryotes may be related to grazing options and food web structure within the karst system.