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

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

Sample preparation effects on stable C and N isotope values: a comparison of methods in Arctic marine food web studies

“…Søreide et al 2006). Considering pretreatment of POM samples for estimating representative food web baseline values it may be appropriate to remove lipids, since algae also vary in their lipid content (Doucette & Fryxell 1985, Mock & Gradinger 2000. The increase in δ 15 N values after lipidremoval for Ice-POM was only slightly larger than the analytical precision level for δ 15 N values in this study, and should thus not lead to any ecologically significant implications for inferences of carbon sources.…”

“…Hobson et al 1995, 2002, Hop et al 2002, Tamelander et al 2006), but not always (Dunton et al 1989, Iken et al 2005. Algae also vary in their lipid content and particularly at the end of the growth season they may build up large lipid stores in response to nutrient stress (Fahl & Kattner 1993, Lindqvist & Lignell 1997, Mock & Gradinger 2000. However, lipid extraction of samples of particulate organic matter (POM) prior to stable isotope analysis has only been done in 2 recent Arctic marine food web studies (Søreide et al 2006, Tamelander et al 2006.…”

“…However, these samples were collected during a spring bloom situation before nutrients became limiting and, thus, the algae had good conditions, favouring growth rather than lipid accumulation (Mock & Gradinger 2000). Lipid storage in algae is related to unfavourable growth conditions, such as nutrient limitation, low temperature and low irradiance (Fahl & Kattner 1993, Lindqvist & Lignell 1997, Mock & Gradinger 2000. Algae with C:N ratios > 7.0 are normally nutrient limited (Mayzaud et al 1989 and references therein) and such high C:N ratios were only found in our Ice-POM samples (Table 3).…”

Increased irradiance reduces food quality of sea ice algae

“…Microscopic analyses of sea ice algae from Rijpfjorden revealed furthermore the occurrence of droplets that probably represent storage lipids in diatoms sampled in June, but not in April. Contrary to these field data, experimental studies by Mock & Gradinger (2000) showed no increase in storage lipids, but a dominance of glycolipids under decreasing nutri- Potential reasons for an increased allocation of photosynthetically fixed carbon to neutral (storage) lipids can be nutrient limitation (Lizotte & Sullivan 1992), increased irradiance (Smith et al 1989), or generally the transition to the stationary growth phase. In particular, a lack of silicate has been proven to increase the lipid content of diatoms substantially (Taguchi et al 1987).…”

“…Experimental studies addressing the role of nutrients and light in the lipid and fatty acid composition of ice algae revealed an increase in storage lipid production under N-limitation (Mock & Kroon 2002a), as well as a growing desaturation of chloroplast membrane lipids under light limitation (Mock & Kroon 2002b). Especially low-light acclimated sea ice algae react to nutrient depletion and decreasing irradiances by increasing glycolipid production (Mock & Gradinger 2000).…”