Changes in photosynthetic carbon allocation in algal assemblages of Arcticsea ice with decreasing nutrient concentrations and irradiance

Photosynthetic carbon assimilation into protein, low-molecular-weight metab olites (LMWM), polysaccharides, total lipids and into 3 lipid classes (neut ral lipids, glycolipids and phospholipids) was determined in batch-culture experiments with natural assemblages of Arctic ice algae under simulated in situ irradiance. Photosynthate allocation in 3 parallel batch incubations revealed a high contribution of lipid assimilation to total particulate car bon production (54.6 +/- 0.4 %) followed by LMWM (35.0 +/- 1.0 %), carbohyd rates (7.3 +/- 0.1 %) and proteins (3.0 +/- 0.8 %). Total lipids were mainl y composed of glycolipids (67.4 +/- 3.5 %) with a relatively lower allocati on into phospholipids (28.1 +/- 6.7 %) and neutral lipids (4.5 +/- 3.2 %). Nutrient addition (final concentrations: Si(OH)(4) = 65.5 +/- 0.4 mu mol 1( -1), NO3 = 42.9 +/- 0.6 mu mol 1(-1), PO4 = 2.6 +/- 0.0 mu mol 1(-1)) cause d algal community growth of 0.22 +/- 0.0 d(-1) until nutrients became limit ing 10 d later. Si(OH)(4):NO3 ratios and NO3:PO4 ratios in the cultures dec reased from initially 1.5 +/- 0.0 to 0.2 +/- 0.1 and 16.8 +/- 0.2 to 1.2 +/ - 0.5, respectively. During the first few days of incubation, relative prop ortions of carbon production for proteins increased 3-fold (max 11.1 +/- 1. 0 %), those for LMWM 1.5-fold (max. 45.7 +/- 6.4 %), whereas Lipids decreas ed (min. 32.0 +/- 0.4 %). Increasing relative proportions of carbon product ion for carbohydrates were only observed at the end of exponential growth ( max. 12.9 +/- 1.3 %). A dramatic increase of lipids was measured under nutr ient depletion (max. 70.9 +/- 3.6 %) after Day 10, which was the result of glycolipid production, while protein and carbohydrate production decreased to values below 5 % of total particulate carbon production. LMWM also attai ned lower incorporation rates under nutrient depletion (min. 23.5 +/- 1.1 % ). Production of glycolipids during exponential algal growth is attributed to an acclimation to decreasing irradiance as a consequence of an increase in algal biomass. Decreasing particulate carbon:chlorophyll a ratios during the experiment indicate a physiological response to a reduction in irradia nce with simultanous glycolipid production. Glycolipids are the main lipid class in chloroplasts, and especially in thylakoidmembranes, which are stro ngly developed during low-light acclimation. Excess light energy during sta tionary algal growth after Day 10 is dissipated in the form of glycolipids and/or neutral lipids. But the latter are probably more significant under h igh-light conditions.