Invited Speaker Australian Society for Microbiology Annual Scientific Meeting 2022

Lighting up the dark - microbial dark matter matters in mats (82506)

Brendan P Burns 1 2 , Hon Lun Wong 1 2 3 , Fraser I MacLeod 1 2 , Stephanie Nobs 1 2 , Julia Meltzer 1 2 , Richard A White 2 4 , Pieter T Visscher 2 5
  1. School of Biotechnology and Biomolecular Sciences, The University of New South Wales , Sydney, Australia
  2. Australian Centre for Astrobiology, Sydney, Australia
  3. Department of Aquatic Microbial Ecology, , Biology Centre of the Academy of Sciences of the Czech Republic, České Budějovice, Czech Republic
  4. University of North Carolina , Charlotte, USA
  5. Marine Sciences, University of Connecticut, Groton, USA

Gathaagudu Country (Shark Bay) on the west coast of Australia, harbours one of the most diverse and extensive systems of living microbial mats, proposed to be analogues of some of the earliest ecosystems on Earth. The traditional tree of life is in flux and new discoveries of novel organisms and pathways is affording a dynamic and holistic view of mat ecosystems and the complex network of processes occurring through space and time.  Mat systems have been shown to possess a substantial abundance of uncultivable microorganisms, and we hypothesise that these enigmatic groups, coined as ‘microbial dark matter’ (MDM), play key roles in modern microbial mats. We reconstructed >100 metagenome-assembled genomes affiliated to MDM, spanning across 41 phyla within the bacterial and archaeal domains, including potentially novel organisms in the deep-branching archaeal Asgard group. Despite possessing reduced-size genomes, the MDM are capable of fermenting and degrading organic carbon, suggesting a role in recycling organic carbon in these mats. Several forms of RuBisCo were identified, allowing putative CO2 incorporation into nucleotide salvaging pathways, which may act as an alternative carbon and phosphorus source. Putative viral defensive mechanisms, including core genes of CRISPR pathways, were also delineated. In light of our findings, we propose H2, ribose and CO/CO2 as the main energy currencies of the MDM community in these mat systems. The finding of an abundance of anaerobic microorganisms enriched at the surface where oxygen levels were highest, coupled with peak methane production in the oxic zone, suggests putative surface anoxic niches in these mats. These findings indicate that microbial metabolisms/interactions are tightly coupled in mat systems to maintain balance and ensure survival in often extreme and fluctuating conditions. Our work provides new metagenome-based models into how this dark matter contributes to overall mat function and survival, and in addition we are finding key insights regarding the origin of eukaryotes: the greatest genealogy search of them all.