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

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.

Cadmium tolerance and adsorption by the marine brown alga Fucus vesiculosus from the Irish Sea and the Bothnian Sea.

Cadmium (Cd) uptake capacities and Cd tolerance of the marine alga Fucus vesiculosus from the Irish Sea (salinity 35 psu) and from the Bothnian Sea (northern Baltic, 5 psu) were quantified. These data were complemented by measurements of changes in maximal photosynthetic rate (P(max)), dark respiration rate and variable fluorescence vs. maximal fluorescence (F(v):F(m)). At concentrations between 0.01 and 1 mmol Cd l(-1), F. vesiculosus from the Bothnian Sea adsorbed significantly more (about 98%) Cd compared with F. vesiculosus from the Irish Sea. The photosynthetic measurements showed that the Bothnian Sea F. vesiculosus were more sensitive to Cd exposure than the Irish Sea algae. The algae from the Irish Sea showed negative photosynthetic effects only at 1 mmol Cd l(-1), which was expressed as a decreased P(max) (-12.3%) and F(v):F(m) (-4.6%). On the contrary, the algae from the Bothnian Sea were negatively affected already at Cd concentrations as low at 0.1 mmol Cd l(-1). They exhibited increased dark respiration (+11.1%) and decreased F(v):F(m) (-13.9%). The results show that F. vesiculosus from the Bothnian Sea may be an efficient sorption substrate for Cd removal from Cd contaminated seawater and this algae type may also have applications for wastewater treatment.

Seasonal variations in nitrate reductase activity and internal N pools in intertidal brown algae are correlated with ambient nitrate concentrations.

Nitrogen metabolism was examined in the intertidal seaweeds Fucus vesiculosus, Fucus serratus, Fucus spiralis and Laminaria digitata in a temperate Irish sea lough. Internal NO(3) (-) storage, total N content and nitrate reductase activity (NRA) were most affected by ambient NO(3) (-), with highest values in winter, when ambient NO(3) (-) was maximum, and declined with NO(3) (-) during summer. In all species, NRA was six times higher in winter than in summer, and was markedly higher in Fucus species (e.g. 256 +/- 33 nmol NO(3) (-) min(-1) g(-1) in F. vesiculosus versus 55 +/- 17 nmol NO(3) (-) min(-1) g(-1) in L. digitata). Temperature and light were less important factors for N metabolism, but influenced in situ photosynthesis and respiration rates. NO(3) (-) assimilating capacity (calculated from NRA) exceeded N demand (calculated from net photosynthesis rates and C : N ratios) by a factor of 0.7-50.0, yet seaweeds stored significant NO(3) (-) (up to 40-86 micromol g(-1)). C : N ratio also increased with height in the intertidal zone (lowest in L. digitata and highest in F. spiralis), indicating that tidal emersion also significantly constrained N metabolism. These results suggest that, in contrast to the tight relationship between N and C metabolism in many microalgae, N and C metabolism could be uncoupled in marine macroalgae, which might be an important adaptation to the intertidal environment.