St. John, G. | bioRxiv Science

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↗

bioRxiv2025

Harmful algal bloom species Microcystis aeruginosa releases thiamin antivitamins to suppress competitors

In environmental ecosystems, vitamin concentrations are often exceedingly low (1, 2) and auxotrophy, or reliance on exogenous vitamin or vitamin precursors, is widespread (3-5). We show here that the widespread harmful algal bloom (HAB) species Microcystis aeruginosa, threatening freshwater aquatic ecosystems globally, releases a complex mixture of thiamin antivitamins, including bacimethrin and methoxythiamin, which induce thiamin deficiency in the benign model green alga Chlamydomonas reinhardtii. Putative biosynthetic genes for bacimethrin were upregulated in M. aeruginosa when grown in co-culture resulting in greater production of bacimethrin. Bacimethrin, methoxythiamin, oxidized forms of thiamin and methoxythiamin, and a novel structural homolog of bacimethrin were all found at elevated levels in the co-culture exometabolome extracts and were all inhibitory to the growth of C. reinhardtii individually at very low concentrations and as a mixture in culture medium extracts. The thiamin-requiring mutant C. reinhardtii, CC-25, was much more sensitive to bacimethrin and methoxythiamin than the wildtype. Thiamin addition largely rescued the inhibitory effects of exposure to antivitamins in both the wildtype and mutant strain. Finally, we determined that bacimethrin is present in aquatic environments and is elevated during Microcystis blooms. Thus, allelopathic suppression of competitors, particularly those that are auxotrophic for thiamin, by M. aeruginosa via the production of antivitamins in environments where thiamin availability is low, could help this species to become dominant and form blooms.

microbiology↗