SEASONAL PHOTOSYNTHETIC CHARACTERISTICS OF ASCOSEIRA-MIRABILIS (ASCOSEIRALES, PHAEOPHYCEAE) FROM KING-GEORGE-ISLAND, ANTARCTICA

Monthly variation in photosynthesis, dark respiration, chlorophyll a content and carbon:nitrogen (C:N) ratios in different lamina sections of adult plants of Ascoseira mirabilis Skottsberg from King George Island, Antarctica, was investigated between September 1993 and February 1994. Light saturated net photosynthesis (P-max) showed maximum values in September (12 to 25 mu mol O-2 g(-1) fr wt h(-1)), and decreased towards the summer to values ranging between 2.0 and 5.0 mu mol O-2 g(-1) fr wt h(-1). In the distal section, however, a second optimum occurred in December (25 mu mol O-2 g(-1) fr wt h(-1)). Dark respiration rates were also highest in October and November and decreased strongly in December to February (6.0 and 1.0 mu mol O-2 g(-1) fr wt h(-1), respectively). Gross photosynthesis exhibited high values between September and December. Concomitant with the seasonal decrease of photosynthetic efficiency (alpha) from mean values of 1.2 mu mol O-2 g(-1) fr wt h(-1) (mu mol photons cm(-2) s(-1))(-1) in September to 0.3 mu mol O-2 g(-1) fr wt h(-1) (mu mol photons cm(-2) s(-1))(-1) in January, the initial light saturating point (I-k) gradually increased from 19 to 60 mu mol photons m(-2) s(-1). Likewise C:N ratios were low in spring (12 to 13) and increased in summer (20). In general, the photosynthetic parameters P-max, gross photosynthesis, alpha and Chl a concentrations were significantly higher in the distal section of the thallus. In contrast, C:N ratios were lower in the distal section of the lamina. The results show that photosynthesis obviously strongly supports growth of the alga in late winter to spring, as it does in some morphologically related brown algae from temperate and polar regions. The question whether growth is additionally powered by remobilisation of reserve carbohydrates remains open.