Physiological responses of intertidal marine brown algae to nitrogen deprivation and resupply of nitrate and ammonium

Abstract

Intertidal macroalgae Fucus and Laminaria experience seasonally fluctuating inorganic N supply. This study examined the effects of long-term N deprivation, recovery following N resupply, and effects of elevated ammonium and nitrate exposure on N acquisition in intertidal algae using manipulations of N supply in tank culture. Over 15 weeks of N deprivation, internal N and nitrate reductase activity (NRA) declined, but maximum quantum yield of PSII was unaffected in Fucus serratus and Fucus vesiculosus. Low NRA was maintained despite no external nitrate availability and depletion of internal pools, suggesting a constitutive NRA, insensitive to N supply. Nitrate resupplied to N-starved thalli was rapidly taken up and internal nitrate pools and NRA increased. Exposure to elevated (50 microM) nitrate over 4 days stimulated nitrate uptake and NRA in Laminaria digitata and F. serratus. Exposure to elevated ammonium suppressed NRA in L. digitata but not in F. serratus. This novel insensitivity of NRA to ammonium in Fucus contrasts with regulation of NRA in other algae and higher plants. Ammonium suppression of NRA in L. digitata was not via inhibition of nitrate uptake and was independent of nitrate availability. L. digitata showed a higher capacity for internal nitrate storage when exposed to elevated ambient nitrate, but NRA was lower than in Fucus. All species maintained nitrate assimilation capacity in excess of nitrate uptake capacity. N uptake and storage strategies of these intertidal macroalgae are adaptive to life in fluctuating N supply, and distinct regulation of N metabolism in Fucus vs Laminaria may relate to position in the intertidal zone.

Nitrate Assimilation: The Role of In Vitro Nitrate Reductase Assay as Nutritional Predictor

“…The use of NR in an ecological context is particularly relevant for marine environments where nitrogen is often limiting, thereby providing relevant information regarding the physiological nitrogen status of organisms (Hernández et al, 1993). Nitrate reductase is considered as a key enzyme in nitrogen metabolism, through being, not only the rate-limiting enzyme in inorganic nitrogen assimilation, but also the major regulatory step in nitrogen metabolism (Crawford, 1995;Berges, 1997;Lartigue & Sherman, 2005;Young et al, 2009). Changes in NR activity, both in the field or in laboratory, have been examined in very few macroalgae.…”

Photosynthetic response of the macroalga,Gracilaria lemaneiformis(Rhodophyta), to various N and P levels at different temperatures

“…and Table ) compared to an unenriched control. The nutrients increase accordingly increased the substrate concentration of a series of related physiological processes of the seaweed , which, in turn, enhanced its assimilation in a certain scope and accelerated the photosynthetic activity of the seaweed due to the stimulated increase in activities of Rubisco and carbonic anhydrase of many macrophytes, such as U. rigida , Ulva lactuca , and Porphyra . Whether the mechanisms are also owned by G. lemaniformis need to be further investigated.…”

Emersion Induces Nitrogen Release and Alteration of Nitrogen Metabolism in the Intertidal Genus Porphyra

“…Leakage of internal pools of inorganic N nitrate (NO 3 − ) and nitrite (NO 2 − ) is one possibility, though these are present at very small pool sizes (0.1 µmol g −1 FW) [3], arguing that this form alone cannot explain the much greater total observed loss [2]. Ammonium (NH 4 + ) is, likewise, present at very low concentrations (e.g., 1 µmol g −1 FW) [4]. Unlike NO 3 − and NO 2 − , however, ammonium may be volatilized as NH 3 , especially if photosynthesis reduces internal pH [5].…”