Interactions between iron oxidizing and reducing bacteria in iron-rich pelagic aggregates (iron snow)

Fe-cycling bacteria are often found coexisting in nature and a variety of interactions occur between acidophilic Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). Ironrich pelagic aggregates (iron snow) are dominated by acidophilic Fe-cycling microbes, best represented by the FeOB (Acidithrix, Ferrovum) and the FeRB (Acidiphilium). This thesis aimed to elucidate the metabolic mechanisms between different FeOB and FeRB shaping iron snow formations. First, the whole genomes of representative FeOB (Acidithrix sp. C25) and FeRB (Acidiphilium sp. C61, Acidocella sp. C78) strains paired with a two-year metatranscriptome profiling monitored the activities of the iron snow microbiome, including the uncultivated Ferrovum. Both approaches revealed the presence and expression of genes involved in the synthesis of the infochemical PEA in FeOB, and the motility and polysaccharide hydrolysis enabling FeRB to join and colonize FeOB. A follow-up protein-based 13C labeling approach confirmed that chemolithoautotrophic Ferrovum fixed CO2 to produce organic carbon under both conditions and FeRB Acidiphilium and Acidocella took up the carbon under oxic conditions. Taken together, these results demonstrate the importance of chemolithoautotrophic Ferrovum (FeOB) in feeding FeRB within iron snow, implying cooperative interactions between them. Finally, the addition of PEA induced aggregation in all tested Acidiphilium spp., but not in the iron snow isolate Acidocella sp. C78. Comparative transcriptomics indicated the upregulated expression patterns of genes associated with flagellar motility in Acidphilium sp. C61. The specific aggregation and motility induction effect of PEA on FeRB promotes rapid co-colonization onto the surface of the iron snow particles. Collectively, these specific inter-species relationships between FeOB and FeRB are advantageous to fulfill their metabolic dependencies through the formation and aggregates stability of iron snow.