Recent observations in marine ecosystems show that the presence of thiamine regulates primary production, but little is known about the ecological effect of thiamine in streams. I conducted nutrient enrichment experiments in the streams in the New York's Adirondack Mountains and in Qiyun Stream, the headwater of a Yangtze River tributary in Southern China, using nutrient diffusing substrates to evaluate the influence of thiamine (vitamin B1) and its precursor on the growth of stream periphyton.

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... Contrasting treatments in the study of Adirondack streams included nutrient additions of thiamine (C12H17ClN4OS· HCl), nitrogen (NH4Cl), and phosphorus (NaH2PO4). Thiamine limitation was observed in 12 of 14 experiments conducted from June through October in 2015-2017, nitrogen limitation was observed in eight experiments, and phosphorus limitation in one experiment, but no co-limitation by these nutrients was observed. The magnitude of response of periphyton to thiamine enrichment varied among seasons, years, and streams. The growth-enhancing effect on periphyton biomass from thiamine or nitrogen addition was typically observed between 8 and 32 days of incubation, though the periphyton accumulation rate declined after 16-24 days of incubation. Contrasting treatments in the study of Qiyun Stream included nutrient additions of thiamine (C12H17ClN4OS· HCl), nitrogen (NH4Cl), phosphorus (NaH2PO4), and a thiamine precursor HMP (C 6 H 9 N 3 O). In these experiments, thiamine limitation and HMP limitation of periphyton was observed in April 2019 and co-limitation by thiamine/ HMP with nitrogen and phosphorus occurred in May 2019. The effect sizes of thiamine and HMP on the growth of periphyton were similar to each other. However, physiochemical features of the stream showed different influences on the effects of thiamine and HMP. Stream-water temperature, and NH 4 + and chlorophyll a concentrations in the stream water each negatively associated with HMP limitation; while SRP and dissolved oxygen showed positive relationships with the effect of HMP. Light intensity was positively associated with thiamine limitation. The effect of thiamine on the growth and composition changes of periphyton was evaluated and compared with that of nitrogen and phosphorus from experiments in an Adirondack Mountain stream. The richness and evenness of the community did not change with the addition of the nutrients, but the growth of some genera was influenced by thiamine and nitrogen. Thiamine promoted the growth of a genus of Chlorophyta, Scenedesmus; and nitrogen promoted that of a genus of Bacillarophyta, Synedra. For the growth forms and cell and colony sizes of the periphyton, thiamine addition increased the biomass of metaphyton and small size groups (ESD ≤ 8 µm), while nitrogen increased the growth of erect growth forms and larger size groups (16 < ESD ≤ 32 µm). Phosphorus did not show any significant effect on the growth and community composition of periphyton.