/Under extreme low pH conditions (pH 6.0), urea alleviated the inhibitory effects of acidification on Synechococcus 7002. At pH 7.0, growth rates with urea were higher than at standard seawater pH (8.0). High urea concentrations (24 mM) caused bleaching and photosynthetic damage at pH 8.0, but this toxicity was mitigated at lower pH (6.0 and 7.0).