Effects of thiamine and its precursor HMP on growth of periphyton in a mountain stream of the Yangtze River Basin, China

Thiamine (vitamin B-1) and its precursor 2-methyl-4-amino-5-hydroxymethylpyrimidine (HMP) have been shown to exert an important influence on primary production in marine environments, but little is known about their influence in fresh waters. We evaluated the ecological effects of thiamine and one of its precursors HMP in streams within the Qiyun catchment, a headwater mountain system of a Yangtze River tributary in Southern China, using nutrient-diffusing substrates. Thiamine limitation of periphyton was observed in April 2019, and HMP limitation was observed in both April and May 2019. Co-limitation by thiamine plus phosphorus and by HMP plus nitrogen occurred in May. The effect sizes of thiamine and HMP on growth of periphyton were similar to each other. Physiochemical features of the stream showed different potential influences on the effects of thiamine and HMP. Water temperature was negatively associated with the effect of thiamine, whereas ammonium showed a negative relationship with the effect of HMP. These findings extend our knowledge of nutrient limitation by thiamine and one thiamine precursor in freshwater environments.