Photosynthetic and light-independent carbon fixation in Macrocystis, Nereocystis, and some selected Pacific Laminariales

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Abstract
The large kelp Macrocystis integrifolia and Nereocystis luetkeana off the coast of British Columbia have light saturation points at about 200 foot candles (1 foot candle = 10.76391 lx); however, photosynthetic rates remain unimpeded through mugh higher light intensities, up to 7000 foot candles. Short-term labelling experiments indicate that the Calvin cycle is operational in these plants, but, in addition, and particularly in the young growing parts, relatively high activities of phosphoenolpyruvate carboxykinase (PEP-CK)and correspondingly high rates of 14C fixation in the dark are observed. In older, mature parts, the activity of PEP-CK as well as the ability for light-independent 14C assimilation are lower. These observations are supported by data from a variety of other Pacific Laminariales. Pulse–chase experiments in the light and in the dark reveal characteristic differences between young and mature parts of the fronds: apart from qualitative difference in the soluble fraction of assimilates, the young parts show a significantly large proportion of radiocarbon confined to the ethanol-insoluble fraction.

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