Slow 514 nm absorption phases and oxygen exchange transients in Ulva

1. The slow 514-nm spectral changes in Ulva were studied using bright continuous 650-nm light. Transient and steady-state absorption changes were compared with changes in net rate of O2 exchange in a system designed to measure both parameters simultaneously. 2. Time courses of the 514-nm absorption change show three phases following the onset of light: one rapid increase and two slower (larger than or equal to 1 s) transient increases. Upon cessation of the light three transient absorption phases also follow: a rapid decrease and two slower (greater than 1 s) transient increases. Parallel transient phases (but opposite in sign) were found at 480 nm. 3. The kinetics of the slow 514-nm absorption transients correlate with the characteristic induction transients in net O2 exchange. 4. Similar difference spectra and the restoration kinetics of the light-on and light -off transient phases indicate that the slow 514-nm absorption changes reflect the same component(s) and process(es). 5. The experimental results are discussed in terms of the electrochromic hypothesis for the 515-nm absorption shift. We interpret the slow 514-nm absorption changes in Ulva as a reflection of relatively slow ionic readjustments across the photosynthetic membranes.