The optimal temperature and illumination photoperiod requirements for the phototrophic growth of a novel microplantlet suspension culture derived from the macrophytic marine red alga Agardhiella subulata were determined. The optimal growth temperature was 24degreesC. The effects of illumination light-dark (LD) photoperiod (hour of light:hours of darkness within a 24 h cycle) on biomass production was studied within a bubble-column photobioreactor. The 4.5 cm diameter photobioreactor was maintained at near-saturation conditions with respect to light flux (38 mumol photons m(-2) s(-1)), nutrient medium delivery (20% nutrient replacement per day), and CO2 delivery (0.35 mmol CO2 L-1 h(-1)) so that the cumulative effects of photodamage on the cell density versus time curve at photoperiods approaching continuous light could be observed. Biomass production was maximized at 16:8 LD, where biomass densities exceeding 3.6 g dry cell mass L-1 were achieved after 60 days in culture. Biomass production was proportional to photoperiod at low fractional photoperiods (less than or equal to10:14 LD), but high fractional photoperiods approaching continuous light (greater than or equal to20:4 LD) shut down biomass production. Biomass production versus time profiles under resource-saturated cultivation conditions were adequately described by a cumulative photodamage growth model, which coupled reversible photodamage processes to the specific growth rate. Under light-saturated growth conditions, the rate constant for photodamage was k(d) = 1.17 +/- 0.28 day(-1) (+/-1.0 SE), and the rate constant for photodamage repair was k(r) = 5.12 +/- 0.95 day(-1) (+/-1.0 SE) at 24degreesC.