Photoadaptation of sea-ice microalgae in springtime: photosynthesis and carboxylating enzymes

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Abstract
Photoadaptive responses of sea-ice microalgae in springtime were observed in southeastern Hudson Bay (Canadian Arctic). The responses included changes in pigment composition (chlorphyll a, carotenoids), photosynthetic parameters (Pmax, Ik, .alpha., .beta.) and carboxylating enzymes. The complete transition from shade to light adaptation took place over 1 generation time while susceptibility to photoinhibition decreased more rapidly. Activities of carboxylating enzymes were never the rate-limiting step of photosynthesis. At low irradiances, increased pigments in the cells and modifications of the photosynthetic parameters suggest that photosynthesis did depend on the trapping of light energy and on the rate of electron transport. With increased irradiances, light energy harvested by the cells exceeded their energetic requirements, so that photosynthesis was only related to the rate of electron transport. These results emphasize the ability of sea-ice microalgae to photoadapt to the seasonally increasing under-ice irradiance, showing that they are not an obligate shade flora in southeastern Hudson Bay.