Hong et al . (Reports, 5 May 2017, p. 527) suggested that previous studies of the biogeochemically significant marine cyanobacterium Trichodesmium showing increased growth and nitrogen fixation at projected future high CO ₂ levels suffered from ammonia or copper toxicity. They reported that these rates instead decrease at high CO ₂ when contamination is alleviated. We present and discuss results of multiple published studies refuting this toxicity hypothesis.