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Abstract.The relationships between photoinhibition and photoprotection in high and low-light-grown Ulva were examined by a combination of chlorophyll-fluorescencemonitoring techniques. Tissues were exposed to a computer-controlled sequence of 5-min exposures to red light, followed by 5-min darkness, with stepwise increases in photon flux. Coefficients of chlorophyll fluorescence quenching (1-qe and NPQ) were calculated following a saturating pulse of white light near the end of each 5-min light treatment. Dark-adapted chlorophyll fluorescence parameters (Fo and Fv/FM) were calculated from a saturating pulse at the end of each 5-min dark period. Lowlight-grown Ulva showed consistently higher 1-qp, i.e. higher reduction status of Q (high primary acceptor of photosystem II), and lower capacity for nonphotochemical quenching (NPQ) at saturating light than did highlight-grown plants. Consequently, low-light plants rapidly displayed photoinhibitory damage (increased F0) at light saturation in seawater. Removal of dissolved inorganic carbon from seawater also led to photoinhibitory damage of high-light-grown Ulva at light saturation, and addition of saturating amounts of dissolved inorganic carbon protected low-light-grown plants against photoinhibitory damage. A large part of NPQ was abolished by treatment with 3 mM dithiothreitol and the processes so inhibited were evidently photoprotective, because dithiothreitol treatment accelerated photoinhibitory damage in both low-and high-light-grown Ulva. The extent of photoinhibitory damage in Ulva was exacerbated by treatment with chloramphenicol (1 mM) without much effect on chlorophyll-quenching parameters, evidently because this inhibitor of chloroplast protein synthesis reduced the rate of repair processes.Abbreviations: CAP = chloramphenieol; DIC = dissolved inorganic carbon in seawater (i.e. COz+ HCO3 +CO~ ); DTT= dithiothreitol; 1-qp=reduction status of Q, the primary acceptor of PSII, measured as described; FM, Fo, Fv = dark-adapted maximum, minimum and variable fluorescence, respectively; NPQ=nonphoto-chemical fluorescence quenching, measured as described; PFD= photon flux density Correspondence to: C.B. Osmond; FAX: 61(62)490756