Photosynthesis and organic composition in Desmarestia menziesii from a population at Potter Cove (King George Island) were measured during the Antarctic spring-summer. Light saturated net photosynthesis (P-max), and dark respiration showed a similar seasonal variation with maximum values in plants collected in November (250 and 200 mu mol O-2 g(-1) DW h(-1), respectively) and minimal values in January (42 and 16 mu mol O-2 g(-1) DW h(-1), respectively), Similarly, a peak of photosynthetic efficiency (alpha) in November [(7.5 mu mol O-2 g(-1) DW h(-1) (mu mol photons m(-2) s(-1))(-1)] was coupled with a peak of Chl a (6.7 mg g(-1) DW). Saturation points (I-k) for photosynthesis were high in January (100 mu mol photon m(-2) s(-1)) and low in December (25 mu mol photon m(-2) s(-1)), whereas compensation points (I-C) gradually decreased from 45 mu mol photon m(-2) s(-1) in October to values close to 10 mu mol photon m(-2) s(-1) in summer (December to February). Carbon (C) and nitrogen (N) as well as C:N ratios did not vary significantly along the study period with means close to 35% C, 3.8% N of dry weight and 9.3 (C:N ratio), respectively, which clearly indicates no nutrient limitation for algal metabolism. In contrast, major organic compounds showed marked seasonal changes. High protein contents close to 22% of DW were determined in September and October, decreasing towards summer to values of 12% of DW, whereas total amino acids (free and protein-bound) had a peak in November (18% of DW) and remained low between December and February (below 12% of DW). Mannitol content was relatively low between September and December increasing to values close to 8% of DW in February. Laminaran was high in September (7.2% of DW) and remained low and relatively constant (ca. 2.5% of DW) from October onwards. The significant effect of amino acids, mannitol, laminaran, protein contents inferred from regression analyse point up to a relationship between monthly allocation of organic compounds and high metabolic activities during spring-summer, i.e., reserve carbohydrates may compensate, at least partially, for C losses due to the high respiratory activity. (C) 1998 Elsevier Science B.V.
