Photosynthesis of marine algae in fluctuating light. I. Adjustment of rate in constant and fluctuating light regimes

The adjustment of photosynthetic rates to fluctuating and constant light was investigated in Hormosira banksii (Turn.) Dene., Carpophyllum maschalocarpum, (Turn.) Grev. and Ecklonia radiata (C. Ag.) J. Ag. by monitoring either CO2 exchange in emergent material with an infrared gas analyser (IRGA) or O-2 exchange in submersed material with an O-2 electrode. Using the IRGA, mean photosynthetic rates ((P) over bar) in fluctuating light showed several kinetic patterns taking 2-4 h to become fully adjusted after pretreatments of low light or darkness. In continuous low light the adjustment period was similar but the pattern was less complex. The time for (P) over bar to reach a steady value in fluctuating light can be substantially reduced by a 30 min pretreatment of the material in the higher light. Errors due to O-2 storage in the bladders of Hormosira precluded study of the adjustment in immersed material of this species. In Ecklonia monitoring with an O-2 electrode and microcomputer revealed adjustment periods of (P) over bar to fluctuating light of at least 30-40 min. Classical induction contributes to the adjustments of (P) over bar to fluctuating light but other processes such as post-illumination CO2 fixation and photoadaptive responses appear to play an important role. These long-term changes in photosynthesis complicate studies of photosynthesis-irradiance responses. Capacity of these macroalgae to adjust photosynthetic performance may be significant for carbon gain in dynamic light.