||The higher capacity to harvest photons at incipient light saturation (Ek) in oxygenic organisms is paralleled by a higher chromophore density per unit area of membrane. Thus, the kDa protein corresponding to 1 mol of chlorophyll and (lightharvesting) carotenoid in light-harvesting and reaction centre pigment–protein complexes in oxygenic organisms lacking phycobilin antennae is 2.0–6.3, BNID 105057 whereas the value for phycobilin antennae is 5.8–15.7 (Raven 1984a, 1984b Alberte 1989 Ting et al. 2002), BNID 105058. For rhodopsin ion pumps the corresponding value is 26 (Raven 1984b). This makes rhodopsins much more costly in terms of energy and nitrogen to synthesise than the light-harvesting and photochemical machinery of oxygenic organisms (Raven 1984a, 1984b Alberte 1989 Ting et al. 2002). Even when the other electron transfer and proton pump components in oxygenic organisms (e.g. cytochrome b6–f complex, cytochrome c6 or plastocyanin, ferredoxin or flavodoxin) are considered (see Raven et al. 1999), the nitrogen and energy costs of rhodopsins are still significantly higher per unit chromophore than the oxygenic organism’s machinery.