The anti-greenhouse gas dimethylsulfide(DMS)is mainly emitted by algae and accounts for more than half of the total natural flux of gaseous sulfur to the atmosphere,strongly reducing the solar radiation and thereby the temperature on Earth.However,the relationship between phytoplankton biomass and DMS emissions is debated and inconclusive.Our study presents field observations from 100 freshwater lakes,in concert with data of global ocean DMS emissions,showing that DMS and algal biomass show a hump-shaped relationship,i.e.DMS emissions to the atmosphere increase up to a pH of about 8.1 but,at higher pH,DMS concentrations decline,likely mainly due to decomposition.Our findings from lake and ocean ecosystems worldwide were corroborated in experimental studies.This novel finding allows assessments of more accurate global patterns of DMS emissions and advances our knowledge on the negative feedback regulation of phytoplankton-driven DMS emissions on climate.