Photosynthetic metabolism and major organic compounds in the marine brown alga Desmarestia menziesii from King George Island (Antarctica)

Abstract

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 μmol O 2 g −1 DW h −1, respectively) and minimal values in January (42 and 16 μmol O 2 g −1 DW h −1, respectively). Similarly, a peak of photosynthetic efficiency ( α) in November [(7.5 μmol O 2 g −1 DW h −1 ( μmol 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 μmol photon m −2 s −1) and low in December (25 μmol photon m −2 s −1), whereas compensation points ( I c) gradually decreased from 45 μmol photon m −2 s −1 in October to values close to 10 μmol 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.