Shannon, K. C. | Search bioRxiv

bioRxiv2025

The allelopathic vitamin B1 antagonist bacimethrin impacts microbial gene expression in a hypereutrophic watershed dominated by cyanobacterial blooms

Freshwater cyanobacterial harmful algal blooms (cyanoHABs), often dominated by Aphanizomenon, Dolichospermum, and Microcystis, are intensifying in eutrophic watersheds globally. A potential control on bacterioplankton dynamics in these systems is the availability of the essential metabolic cofactor thiamin (vitamin B1) and presence of the allelopathic thiamin antagonist, bacimethrin, that competitively inhibits thiamin-requiring enzymes. We examined dissolved concentrations of thiamin chemical congeners and bacimethrin, 16S-amplicon based microbiome compositions, prokaryotic mRNA-based metatranscriptomes, and reference genomes in hypereutrophic Upper Klamath Basin before and during seasonal cyanoHABs. Our objective was to connect bacterioplankton community compositions and gene expression patterns with thiamin congener and bacimethrin availability under different cyanoHAB conditions. Bacimethrin was present in all samples at nearly equimolar concentrations to the thiamin precursor, HMP, suggesting that similar mechanisms influence the availability of both compounds. Additionally, bacimethrin concentrations were positively correlated with cyanoHAB species abundance (cells mL-1) and the expression of microbial thiamin biosynthesis genes. Samples with high cyanoHAB abundances displayed elevated transcription of genes for thiamin biosynthesis, the pentose phosphate pathway, and photosynthesis. Bacterioplankton unable to synthesize thiamin and thus vulnerable to bacimethrin allelopathy, such as Limnohabitans spp., showed reduced gene expression when cyanoHAB abundances were high. Reference genomes of cyanoHAB and many picocyanobacteria strains contained complete thiamin biosynthesis gene pathways, implicating these taxa as major thiamin sources. These results suggest that bacimethrin provides a competitive advantage to bacterioplankton that do not require exogenous vitamin B1 by eliminating the risk of bacimethrin uptake with vitamin B1 transporters, potentially facilitating cyanoHAB dominance in Upper Klamath Basin and broader eutrophic watersheds.

microbiology↗

bioRxiv2023

Connecting Thiamine Availability to the Microbial Community Composition in Chinook Salmon Spawning Habitats of the Sacramento River Basin

Thiamine Deficiency Complex (TDC) is a major emerging threat to global populations of culturally and economically important populations of salmonids. Salmonid eggs and embryos can assimilate exogenous thiamine, and evidence suggests that microbial communities in benthic environments can produce substantial amounts of thiamine. We therefore hypothesize that microbially produced thiamine in both riverine surface water and hyporheic zones could serve to rescue early life stages of salmonids suffering from TDC. The distributions of thiamine and its metabolically related compounds (dTRCs) have never been determined in freshwater systems. Similarly, the microbial cycling of these compounds has never been investigated. Here we determine that all dTRCs are present in femto-picomolar concentrations across diverse salmon spawning habitats in Californias Sacramento River system. We observed that thiamine concentrations in the Sacramento River are orders of magnitude lower than marine environments, indicating substantial differences in thiamine cycling between these two environments. Our data suggest that the hyporheic zone is likely the source of thiamine to the overlying surface water. Temporal variations in dTRC concentration were observed where highest concentrations were seen when Chinook salmon were actively spawning. Significant correlations were identified between the richness of differentially abundant ASVs and dTRC concentrations. The influence of these ASVs on dTRC concentrations provide evidence of dTRC cycling by microbes in the hyporheic zone, which would influence the conditions where embryonic salmon incubate. Together, these results indicate a connection between microbial communities in freshwater habitats and the availability of thiamine to spawning TDC-impacted California Central Valley Chinook salmon.

microbiology↗