ABSTRACT: In nature and in the laboratory, phytoplankton cells are exposed to fast and extreme fluctuations in light intensity. These include diel and seasonal changes in irradiance, and changes stemming from vertical mixing over the light field. In algal mass cultures and photobioreactors, similar changes take place as cultures grow denser and as cells are mixed rapidly in the system. To survive supraoptimal, free-radical-generating irradiance levels as well as prolonged exposure to dim light, phytoplankton species are capable of photoacclimation. Under low light, light-harvesting pigments such as phycobilins, chlorophylls, fucoxanthin and peridinin increase all the way to optically becoming black cells. The same pigments decrease under high light, resulting in cells being rather transparent. The opposite takes place with the photoprotective pigments β-carotene and astaxanthin and the elements of the xanthophyll cycle, all of which increase whenever cells are exposed to high irradiance levels, concomitant with enhanced activity of the antioxidant enzymes catalase, superoxydismutase, and peroxidase. These processes are complemented by up to 5–fold changes in RUBISCO per photosystem unit (PSU) levels, and parallel changes in light-saturated photosynthetic rates. Thus, light-harvesting and utilization efficiencies are maximized under low light, whereas photosynthetic carbon assimilation and throughput rates reach their peak values whenever light is sufficiently high. Maximal photosynthesis and growth rates have to be matched by correspondingly high respiration rates. Photoacclimation can be used to optimize biomass and target product yields in biotechnological applications.
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