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

Abstract

Antarctic and arctic marine waters have similar near-freezing temperatures, but differ greatly in dissolved inorganic nitrogen (DIN) availability. Antarctic algae have high DIN supply year-round; arctic algae are N-limited during the summer. Temperate algae experience low temperatures and low DIN supply on a seasonal basis, but never concurrently. Nitrogen supply influences the ability of algae to achieve the high enzyme activities necessary for cold acclimation. The present study compared N-allocation strategies of antarctic, arctic, and temperate seaweeds grown under N-replete and N-limited conditions at near-freezing temperature. Sporophytes of the antarctic endemic, Himantothallus grandifolius, did not store NO 3 - , had small pools of nitrogenous compounds, and were unable to sustain growth for longer than 1 mo under N-limitation. However, N-starved plants with negative growth rates exhibited chlorophyll fluorescence ratios (F v /F m ) similar to those of N-replete plants, and photosynthetic rates remained positive, suggesting that PSII reaction centres (RCII) were functioning efficiently. In contrast, the arctic endemic, Laminaria solidungula, maintained relatively high growth rates during 9 mo of N-starvation. The arctic kelp utilised both internal NO 3 - pools and organic nitrogenous components, such as protein and chlorophyll, to support growth. Despite declines in the density of RCII and photosynthetic capacity, N-limited L. solidungula continued to accumulate carbon reserves. Like the arctic plants, temperate L. saccharina from the Atlantic coast of Maine had internal reserves of NO 3 - and organic compounds that provided the initial N-source for growth under low external N-supply. The internal 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 alone has an N-allocation strategy for surviving long periods of concurrent low temperature and low N-supply. The antarctic species appears to be primarily adapted to maintaining photosynthesis and growth under low light and low temperature, rather than low DIN supply. The temperate species is poorly adapted to survive prolonged periods of both low N and low temperature, even though ecotypes of this species extend into the Arctic.