MEPS 202:1-11 (2000) - doi:10.3354/meps202001
Changes in photosynthetic carbon allocation in algal assemblages of Arctic sea ice with decreasing nutrient concentrations and irradiance
Thomas Mock*, Rolf Gradinger
ABSTRACT: Photosynthetic carbon assimilation into protein, low-molecular-weight metabolites (LMWM), polysaccharides, total lipids and into 3 lipid classes (neutral 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 carbon production (54.6 ± 0.4%) followed by LMWM (35.0 ± 1.0%), carbohydrates (7.3 ± 0.1%) and proteins (3.0 ± 0.8%). Total lipids were mainly composed of glycolipids (67.4 ± 3.5%) with a relatively lower allocation into phospholipids (28.1 ± 6.7%) and neutral lipids (4.5 ± 3.2%). Nutrient addition (final concentrations: Si(OH)4 = 65.5 ± 0.4 µmol l-1, NO3 = 42.9 ± 0.6 µmol l-1, PO4 = 2.6 ± 0.0 µmol l-1) caused algal community growth of 0.22 ± 0.0 d-1 until nutrients became limiting 10 d later. Si(OH)4:NO3 ratios and NO3:PO4 ratios in the cultures decreased 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 proportions 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 decreased (min. 32.0 ± 0.4%). Increasing relative proportions of carbon production for carbohydrates were only observed at the end of exponential growth (max. 12.9 ± 1.3%). A dramatic increase of lipids was measured under nutrient 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 attained 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 irradiance with simultanous glycolipid production. Glycolipids are the main lipid class in chloroplasts, and especially in thylakoidmembranes, which are strongly developed during low-light acclimation. Excess light energy during stationary algal growth after Day 10 is dissipated in the form of glycolipids and/or neutral lipids. But the latter are probably more significant under high-light conditions.
KEY WORDS: Arctic · Sea ice · Algae · Assimilation · Photosynthetic carbon allocation · Nutrient deficiency · Proteins · Carbohydrates · Lipids · Glycolipids
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