Seasonal photosynthetic characteristics of Ascoseira mirabilis (Ascoseirales, Phaeophyceae) from King George Island, Antarctica

Abstract

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 (Pmax) showed maximum values in September (12 to 25 μmol O2 g-1 fr wt h-1), and decreased towards the summer to values ranging between 2.0 and 5.0 μmol O2 g-1. In the distal section, however, a second optimum occurred in December (25 μmol O2 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 μmol O2 g-1 fr wt h-1, respectively). Gross photosynthesis exhibited high values between September and December. Concomitant with the seasonal decrease of photosynthetic efficiency (α) from mean values of 1.2 μmol O2 g-1 fr wt h-1 (μmol photons cm-2 s-1)-1 in September to 0.3 μmol O2 g-1 fr wt h-1 (μmol photons cm-2 s-1)-1 in January, the initial light saturating point (Ik) gradually increased from 19 to 60 μmol 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 Pmax, gross photosynthesis, α 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