Distinct patterns of nitrate reductase activity in brown algae: light and ammonium sensitivity in Laminaria digitata is absent in Fucus species¹

Abstract

Fucus and Laminaria species, dominant seaweeds in the intertidal and subtidal zones of the temperate North Atlantic, experience tidal cycles that are not synchronized with light:dark (L:D) cycles. To investigate how nutrient assimilation is affected by light cycles, the activity of nitrate reductase (NR) was examined in thalli incubated in outdoor tanks with flowing seawater and natural L:D cycles. NR activity in Laminaria digitata (Huds.) Lamour. showed strong diel patterns with low activities in darkness and peak activities near midday. This diel pattern was controlled by light but not by a circadian rhythm. In contrast, there was no diel variation in NR activity in Fucus serratus L., F. vesiculosus (L.) Lamour., and F. spiralis L. either collected directly from the shore or maintained in the outdoor tanks. In laboratory cultures, transfer to continuous darkness suppressed NR activity in L. digitata, but not in F. vesiculosus; continuous light increased NR activity in L. digitata but decreased activity in F. vesiculosus. Furthermore, 4 d enrichment with ammonium (50 lmol AE L )1 pulses), resulted in NR activity declining by >80% in L. digitata, but no significant changes in F. serratus. Seasonal differences in maximum NR activity were present in both genera with activities highest in late winter and lowest in summer. This is the first report of NR activity in any alga that is not strongly regulated by light and ammonium. Because light and tidal emersion do not always coincide, Fucus species may have lost the regulation of NR by light that has been observed in other algae and higher plants.

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

“…To examine the effect of tank storage on N metabolism, whole thalli of Fucus serratus L. and Fucus vesiculosus (L.) Lamour were collected in March 2001 from the intertidal region of Strangford Lough at Portaferry (54°23′N, 5°34′W). Thalli were either transferred to outdoor tanks adjacent to the field collection site, with flow‐through seawater as described previously (Young et al 2007b), or transported to the laboratory and incubated in 20‐l tanks with freshly collected, filtered, aerated seawater at 11°C. Laboratory tanks were exposed to a light regime similar to the ambient at the time of collection (10:14 L:D cycle) at 220 μmol photons m −2 s −1 supplied to one side of the tanks from GROW‐lux fluorescent light tubes (Osram, Langley, Berkshire, UK).…”

“…Samples were collected from all thalli over the following week to monitor NRA, internal thallus nitrate content and total C and N thallus content. Although L. digitata was successfully grown in outdoor tanks (Young et al 2007b, Gomez and Lüning 2001), a laboratory N deprivation experiment was set up with L. digitata alongside that for Fucus , but the L. digitata thalli fragmented and disintegrated within 4 weeks so that experiment was terminated.…”

“…During the first 24 h, additional tissue and water samples were collected at 4, 8 and 24 h to assess short‐term effects of elevated N on NRA in thalli and to measure rate of N uptake by the thalli using the initial thallus mass for calculations. For F. serratus , thallus tips were sampled, while L. digitata were sampled by cutting discs out of the thalli, avoiding the meristematic region and the oldest tissue (Young et al 2007b). Samples were blotted dry and frozen in liquid N 2 .…”

“…An F ‐test showed that the variability of NRA in six tips from each thallus was not significantly different from that of NRA in tips from six different individuals sampled over 2 days ( P > 0.05). Thereafter, all tips were treated as independent for statistical comparisons (Young et al 2007b). Changes in NRA and internal nutrient concentrations in response to experimental manipulations were compared using one‐way anova for N treatment or two‐way anova with N treatment and time as variables (S igmastat v. 3.1; Systat Software Inc, Chicago, IL).…”

The cultivation of European kelp for bioenergy: Site and species selection

“…Nitrate reductase activity is light stimulated in L. digitata leading to greater assimilation during the day than night [111]. It has been shown in S. latissima that 10 mmol L À1 of nitrate is required to maximise growth rate and leads to internal storage for later use [112].…”