Effects of concurrent low temperature and low nitrogen supply on polar andtemperate seaweeds

Antarctic and arctic marine waters have similar near-freezing temperatures, but differ greatly in dissolved inorganic nitrogen (DIN) availability. Ant arctic algae have high DIN supply year-round; arctic algae are N-limited du ring the summer. Temperate algae experience low temperatures and low DIN su pply on a seasonal basis, but never concurrently. Nitrogen supply influence s the ability of algae to achieve the high enzyme activities necessary for cold acclimation. The present study compared N-allocation strategies of ant arctic, arctic, and temperate seaweeds grown under N-replete and N-limited conditions at near-freezing temperature. Sporophytes of the antarctic endem ic, Niman-tothallus grandifolius, did not store NO3-, had small pools of ni trogenous compounds, and were unable to sustain growth for longer than 1 mo under N-limitation. However, N-starved plants with negative growth rates e xhibited chlorophyll fluorescence ratios (F-v/F-m) similar to those of N-re plete plants, and photosynthetic rates remained positive, suggesting that P SII reaction centres (RCII) were functioning efficiently. In contrast, the arctic endemic, Laminaria solidungula, maintained relatively high growth ra tes during 9 mo of N-starvation. The arctic kelp utilised both internal NO3 - pools and organic nitrogenous components, such as protein and chlorophyll , to support growth. Despite declines in the density of RCII and photosynth etic capacity, N-limited L. solidungula continued to accumulate carbon rese rves. Like the arctic plants, temperate L. saccharina from the Atlantic coa st of Maine had internal reserves of NO3- and organic compounds that provid ed the initial N-source for growth under low external N-supply. The interna l N-reserves were depleted fairly rapidly, however, and the temperate kelp showed simultaneous reductions in growth rate, photosynthetic capacity, and F-v/F-m after only 3 mo under low N-supply. Overall, the arctic species al one has an N-allocation strategy for surviving long periods of concurrent l ow temperature and low N-supply, The antarctic species appears to be primar ily adapted to maintaining photosynthesis and growth under low light and lo w temperature, rather than low DIN supply. The temperate species is poorly adapted to survive prolonged periods of both low N and low temperature, eve n though ecotypes of this species extend into the Arctic.