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.     

Classification of macroalgae as fuel and its thermochemical behaviour

A preliminary classification of five macroalgae from the British Isles; Fucus vesiculosus, Chorda filum, Laminaria digitata, Fucus serratus, Laminaria hyperborea, and Macrocystis pyrifera from South America, has been presented in terms of a Van Krevelen diagram. The macroalgae have been characterised for proximate and ultimate analysis, inorganic content, and calorific value. The different options for thermal conversion and behaviour under combustion and pyrolysis have been evaluated and compared to several types of terrestrial biomass including Miscanthus, short rotation Willow coppice and Oat straw. Thermal treatment of the macroalgae has been investigated using thermogravimetry (TGA) and pyrolysis-gc-ms. Combustion behaviour is investigated using TGA in an oxidising atmosphere. The suitability of macroalgae for the different thermal processing routes is discussed. Ash chemistry restricts the use of macroalgae for direct combustion and gasification. Pyrolysis produces a range of pentosans and a significant proportion of nitrogen containing compounds. High char yields are produced.     

INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.     

Restricted use of nitrate and a strong preference for ammonium reflects the nitrogen ecophysiology of a light‐limited red alga

Ammonium and nitrate are important sources of inorganic nitrogen for coastal primary producers. Nitrate has higher energy requirement for uptake and assimilation, compared with ammonium, suggesting that it might be a more efficient nitrogen source for slow‐growing, light‐limited macroalgae. To address this hypothesis, we examined the nitrogen ecophysiology of Anotrichium crinitum, a rhodophyte macroalgae common in low‐light habitats in New Zealand. We measured seasonal changes in seawater nitrate and ammonium concentrations and the concentration of nitrate and ammonium stored internally by A. crinitum. We determined the maximal uptake rates of nitrate and ammonium seasonally and grew A. crinitum in the laboratory with these nitrogen sources under two ecologically relevant saturating light levels. Our results show that field‐harvested A. crinitum has a high affinity for ammonium and although it will grow when supplied exclusively with nitrate, internal nitrate pools are low and it is unable to take up nitrate without several days of acclimation to saturating light. Our data predict that A. crinitum would be able to sustain growth with ammonium as the sole source of nitrogen, a strategy that would help it survive under low‐light conditions that prevail in the field.     

Antioxidant and pro-oxidant activities of the brown algae, Laminaria digitata, Himanthalia elongata, Fucus vesiculosus, Fucus serratus and Ascophyllum nodosum

The ability of Laminaria digitata, Himanthalia elongata, Fucus vesiculosus, Fucus serratus and Ascophyllum nodosum to scavenge peroxyl radicals was investigated by kinetic studies in a model system. The thermal initiated oxidation of methyl linoleate was performed at 60°C in heptanol, with or without antioxidants. When they reached 1% of the substrate, seaweed extracts exhibited antioxidant activities by extending the induction period, but they did not suppress the rate of oxygen uptake as did vitamin E. A synergistic effect occurred when both algal extracts (1.5 g L^-1) and vitamin E (0.4 mmol L^-1) were present, and the effectiveness of the combined antioxidants during the whole induction period was vitamin E effectiveness. The synergistic effect of L. digitata, however, was subject to seasonal variations: samples collected in summer were effective synergists, whereas samples collected in winter displayed a marked negative synergism. The phospholipid fractions of F. vesiculosus, F. serratus and A. nodosum, including pigments, accounted for only 6% of the total lipid fraction, and did not exhibit a large synergistic effect. The main phospholipid was not phosphatidyl ethanolamine as usually related, but phosphatidyl inositol. Fucoxanthin had some antioxidant activity per se under our experimental conditions, but did not act as a synergist of vitamin E. The most potent synergists were recognized as chlorophyll a and related compounds by the application of liquid-liquid partition and chromatography for the identification of active components. This revised version was published online in June 2006 with corrections to the Cover Date.     

Isolation and Analysis of the Cell Walls of Brown Algae: Fucus spiralis, F. ceranoides, F. vesiculosus, F. serratus, Bifurcaria bifurcata and Laminaria digitata

Mabeau, S. and Kloareg, B. 1987. Isolation and analysis of thecell walls of brown algae: Fucus spiralis, F. ceranoides, F.vesiculosus, F. serratus, Bifurcaria bifurcata and Laminariadigitata.—J. exp. Bot. 38: 1573–1580. Cell walls were isolated from six marine brown algae, Fucusspiralis, F. ceranoides, F. vesiculosus, F. serratus, Bifurcariabifurcata (Fucales, Phaeophyta) and Laminaria digitata (Laminariales,Phaeo-phyta). Yields of isolated cell walls ranged from 35–45%of thallus dry weight. Walls were composed mainly of alginatesand sulphatcd fucans, the proportions of which correlated withspecies zonation in the intertidal region. This result is consistentwith the hypothesis that the sulphated fucans are associatedwith the adaptation of macroalgae to the intertidal environment.Comparing the chemical composition of isolated cell walls withthat of whole plants, we conclude that alginic acid is mainlypart of the fibrillar wall while a significant proportion ofthe sulphated fucans probably belongs to the intercellular spacematrix. Since ascophyllan-like polysaccharides were more abundantin the fucan extracts from the isolated cell walls than fromthe whole plants, it is suggested that differences in the structureof fucans might be related to differences in their localizationthroughout the tissue. Key words: Cell walls, Phaeophyta, sulphated fucans     

BIOLOGICAL AND CHEMICAL CHARACTERISTICS OF THE GIANT DIATOM ETHMODISCUS (BACILLARIOPHYCEAE) IN THE CENTRAL NORTH PACIFIC GYRE

Cells of the giant diatom Ethmodiscus Castr. gathered from the upper 15 m were examined for O₂ evolution, nitrate reductase activity (NRA), C and N composition, internal NO concentrations, , and ¹⁵NO, ¹⁵NH , and ³²Si uptake in a series of cruises in the central N. Pacific gyre. The δ¹⁵N (2.56–5.09 ‰), internal NO concentrations (0.0– 11.5 mM NO⁻), and NRA (6.7 ± 4.7 × 10⁻⁴μM NO cell ⁻¹·h⁻¹) were consistent with recent exposure to elevated nitrate concentrations and utilization of deep NO as a primary N source. These results are similar to other diatoms that migrate vertically to the nutricline as part of their life cycle. Rate measures (Si[OH]₄ uptake, NRA, and O₂ evolution) indicated surface doubling times from 45 h to 75 h. Both NO and NH uptake in surface waters were low and inadequate to supply N needs at surface NO and NH concentrations. Our results suggest a partitioning in nutrient acquisition, with N acquired at depth and C and Si acquired at the surface. Doubling rates were two to three times higher than predicted from cell volume and C content models. These data are consistent with the observed elemental content being lower than expected because of the dominance of cell volume by the vacuole. Our calculations suggest that Ethmodiscus contributes little to the biogeochemistry of the upper water column via upward nutrient transport. Although reported as a paleo-upwelling indicator, thisevidence suggests that Ethmodiscus has adapted to the nutrient-poor open ocean by a vertical migration strategy and has biological characteristics inconsistent with a upwelling indicator.     

Comparing nitrate storage and remobilization in two rice cultivars that differ in their nitrogen use efficiency

Soil nitrogen (N) is available to rice crops as either nitrate or ammonium, but only nitrate can be accrued in cells and so factors that influence its storage and remobilization are important for N use efficiency (NUE). The hypothesis that the ability of rice crops to remobilize N storage pools is an indicator of NUE was tested. When two commonly grown Chinese rice cultivars, Nong Ken (NK) and Yang Dao (YD), were compared in soil and hydroponics, YD had significantly greater NUE for biomass production. The ability of each cultivar to remobilize nitrate storage pools 24 h after N supply withdrawal was compared. Although microelectrode measurements of the epidermal sub-cellular nitrate pools in leaves and roots showed similar patterns of vacuolar remobilization in both cultivars, whole-tissue analysis showed very little depletion of storage pools after 24 h. However, leaf epidermal cell cytosolic nitrate activities were significantly higher in YD when compared with NK. Before N starvation and growing in 10 mM nitrate, the xylem nitrate activity in YD was lower than that of NK. After 24 h of N starvation the xylem nitrate had decreased more in YD than in NK. Tissue analysis of stems showed that YD had accumulated significantly more nitrate than NK, and the remobilization pattern suggested that this store is important for both cultivars. Changes in nitrate reductase activity (NRA) and expression were measured. Growing in 10 mM nitrate, NRA was undetectable in roots of both cultivars, and the leaf total NRA of equivalent leaves was similar in NK and YD. When the N supply was withdrawn, after 24 h NRA in NK was reduced to 80% but no decrease was found in YD. The proportion of NRA in an active form in YD was significantly higher than that in NK under both nitrate supply and deprivation conditions. Checking NR gene expression showed that leaf expression of OsNia1 was faster to respond to nitrate deprivation than OsNia2 in both cultivars. These measurements are discussed in relation to cultivar differences and physiological markers for NUE in rice.     

Adventitious Bud Formation from Bulb-scale Explants of Lilium speciosum Thunb. in vitro Effects of aminoethoxyvinyl-glycine, 1-aminocyclopropane-1-carboxylic acid,and ethylene

Several representatives of marine brown macroalgae (Phaeophyceae) including Fucus serratus L., Fucus spiralis L. and Fucus vesiculosus L. as well as Laminaria digitata (Huds.) Lamour., Laminaria hyperborea (Gunn.) Foslie and Laminaria saccharina (L.) Lamour. were investigated with particular regard to features of biosynthesis of the storage product mannitol. The respective catalytic system involved in the last step of mannitol formation, mannitol-1-phosphate dehydrogenase, appears to be a cytoplasmic enzyme as may be judged from the degree of correlation with the chloroplast key enzyme ribulose-1,5-bisphosphate carboxylase in different tissues of Laminaria digitata and Laminaria saccharina. Activity of mannitol-1-phosphate dehydrogenase in vitro is not affected by mannitol-l-phosphate or free mannitol, suggesting that mannitol biosynthesis in vivo) is mainly controlled by the environment and/or developmental stage. Certain inorganic ions such as NO₃^- (including K⁺) exert a strong influence on the activity of mannitol1-phosphate dehydrogenase thus suggesting that the intracellular pools of stored NO₃^- and mannitol are confined to spatially separated cellular compartments.     

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

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 μmol · 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.     

Time-course of Nitrate Uptake and Nitrate Reductase Activity in Nitrogen-depleted Dwarf Bean

The rate of nitrate uptake by N-depleted French dwarf bean (Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO₃ -After this initial phase the rale remained constant for many hours. Detached root systems showed the same time-course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO₃^- in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m^-3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m^-3) and tungstate (1 mol m^-3). We suggest that initial potential NRA reflects the activity of pre-existing enzyme, whereas actual NRA depends on enzyme assembly during NO₃^- supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean.     

Influence of ammonium and nitrate supply on growth, dry matter partitioning, N uptake and photosynthetic capacity of Pinus radiata seedlings

Growth and physiological responses of Pinus radiata D. Don seedlings to a combination of N supply regimes (low N = 1.78 mol m⁻³, high N = 7.14 mol m⁻³) and ammonium:nitrate ratios (80:20, 50:50 and 20:80; molar basis) were assessed in a hydroponic experiment run over the course of 105 days. Highly significant (P < 0.001) increases in seedling diameter, height, leaf area and dry mass occurred at lower ammonium:nitrate ratios and were two to fourfold greater than the non-significant (for diameter) to marginally significant (P < 0.05 for other dimensions) increases in these dimensions that occurred with greater N supply. Increases in N supply resulted in a highly significant (P < 0.001) reduction in biomass partitioning to roots and highly significant (P < 0.001) increases in allocation to foliage. The ammonium:nitrate ratio was not found to significantly change biomass partitioning to either foliage, stems or roots. Ammonium and nitrate uptake was significantly influenced by N supply and N form and conformed to ammonium and nitrate concentrations in nutrient solution. Uptake rates of ammonium were twice those of nitrate at comparable concentrations suggesting that P. radiata is in the lower end of the ratio of uptake of ammonium to nitrate reported for conifers (range from 2 to 20 mol mol⁻¹). Despite this, plants growing in high ammonium:nitrate ratios were smaller, exhibited luxurious N consumption and lower N use efficiency. Differences in productivity among treatments were partially explained by greater rates of light-saturated photosynthesis associated with nitrate nutrition.     

Some Observations on the Influence of Salinity on Photosynthetic Activity and Chloride Ion Loss in Several Seaweeds

The photosynthetic rates of Enteromorpha intestinalis, Fucus vesiculosus and Laminaria saccharina from the Baltic Sea were measured in different salinity range of Medium A (with NaHCO₃) and Medium B (without NaHCO₃). Photosynthetic activity in Medium A showed higher values than those in Medium B. The materials pretreated for one day showed a higher O₂ output than those untreated at almost every salinity range. In a chloride loss experiment the pretreated thalli of Porphyra umbilicalis, Fucus vesiculosus and Laminaria digitata were placed for a three minute period in distilled water. The excreted chloride ion content showed a higher value in sublittoral species after hypotonic and hypertonic treatment than in intertidal species.     

Ammonium and nitrate nutrition inPlantago lanceolata L. andPlantago major L. ssp.major

With the aims (1) to test whether the different natural occurrence of twoPlantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form; (3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants ofPlantago lanceolata andP. major ssp.major were grown with an abundant supply of nitrate, ammonium or nitrate+ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio inP. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate. In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone. In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots.In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium. Both GDHA and GSA were higher inP. lanceolata than inP. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen. Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly. It is concluded that the nitrogen metabolism of the twoPlantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form. Suitable parameters for establishing the nitrogen source in the field are thein vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent thein vitro GDHA and GSA of the roots. Grassland Species Research Group. Publ. no 118.     

SEASONAL FLUCTUATIONS IN TISSUE NITROGEN FOR FIVE SPECIES OF PERENNIAL MACROALGAE IN RHODE ISLAND SOUND1

Tissue nitrogen was assessed monthly for 16 months in five species of perennial macroalgae representing three phyla at one location in Rhode Island Sound. The species showed a remarkable similarity in their pattern of seasonal fluctuation in both nitrate and total nitrogen. The period of greatest accumulation (January through March) coincided with the period of highest concentration of inorganic nitrogen in the water, and for most of these algae it was also the time of-least growth. Conversely, the period of lowest tissue nitrogen (50% of the winter value, May through July) coincided with the period of lowest inorganic nitrogen in the water and highest algal growth. The greatest accumulation of nitrate was found in Laminaria saccharina (L.) Lamour. (80 μmol·g dry wt.^?1), four times as much as that measured simultaneously in the other species and 560 times the ambient concentration. By April the concentration of internal nitrate had dropped to nearly undetectable levels, but in August it began to accumulate again—a pattern that was repeated in Chondrus crispus Stackh. In Ascophyllum nodosum (L.) Le Jolis, Fucus vesiculosus L. and Codium fragile subsp. tomentosoides (Van Goor) Silva, the period of negligible internal nitrate level extended from March to December. The greatest concentration of total tissue nitrogen was measured in C. crispus (4.8% dry wt.), double the maximum in L. saccharina (2.3% dry wt.).     

A comparative study of the anti-inflammatory, anticoagulant, antiangiogenic, and antiadhesive activities of nine different fucoidans from brown seaweeds

The anti-inflammatory, antiangiogenic, anticoagulant, and antiadhesive properties of fucoidans obtained from nine species of brown algae were studied in order to examine the influence of fucoidan origin and composition on their biological activities. All fucoidans inhibited leucocyte recruitment in an inflammation model in rats, and neither the content of fucose and sulfate nor other structural features of their polysaccharide backbones significantly affected the efficacy of fucoidans in this model. In vitro evaluation of P-selectin-mediated neutrophil adhesion to platelets under flow conditions revealed that only polysaccharides from Laminaria saccharina, L. digitata, Fucus evanescens, F. serratus, F. distichus, F. spiralis, and Ascophyllum nodosum could serve as P-selectin inhibitors. All fucoidans, except that from Cladosiphon okamuranus carrying substantial levels of 2-O-alpha-D-glucuronopyranosyl branches in the linear (1-->3)-linked poly-alpha-fucopyranoside chain, exhibited anticoagulant activity as measured by activated partial thromboplastin time whereas only fucoidans from L. saccharina, L. digitata, F. serratus, F. distichus, and F. evanescens displayed strong antithrombin activity in a platelet aggregation test. The last fucoidans potently inhibited human umbilical vein endothelial cell (HUVEC) tubulogenesis in vitro and this property correlated with decreased levels of plasminogen-activator inhibitor-1 in HUVEC supernatants, suggesting a possible mechanism of fucoidan-induced inhibition of tubulogenesis. Finally, fucoidans from L. saccharina, L. digitata, F. serratus, F. distichus, and F. vesiculosus strongly blocked MDA-MB-231 breast carcinoma cell adhesion to platelets, an effect which might have critical implications in tumor metastasis. The data presented herein provide a new rationale for the development of potential drugs for thrombosis, inflammation, and tumor progression.     

Distribution of carbonic anhydrase in British marine macroalgae

Summary Thirty-four species of marine macroalgae from around St. Andrews, Scotland, have been assayed for their external activity and thirty-three species for their total activity of carbonic anhydrase. Activity was detected in all the Rhodophyta tested apart from Chondrus crispus, but was absent in Codium fragile, Enteromorpha sp. and Monostroma fuscum (Chlorophyta), and Alaria esculenta, Laminaria digitata, L. saccharina and L. hyperborea (Phaeophyta). Total activity of carbonic anhydrase per unit fresh weight tended to be higher in the Rhodophyta than in the Chlorophyta or Phaeophyta. External activity was present in two of the six Chlorophyta, four of the twelve Phaeophyta and four of the sixteen Rhodophyta tested. On average, when present, external carbonic anhydrase activity represented 2.7% of the total activity. A relationship was found between total carbonic anhydrase activity and habitat. Species from the high intertidal and the low-light subtidal habitats had significantly higher activity than species from the mid and low intertidal, rockpools, or high-light region of the subtidal. External carbonic anhydrase activity did not vary significantly with habitat. There appeared to be no strong relationship between carbonic anhydrase activity and the ability of a species to use HCO _- ³ in photosynthesis under water.     

Nitrogen and phosphorus uptake in seedlings of the seagrass Amphibolis antarctica in Western Australia

The uptake of nitrate, ammonium and phosphate was examined in vitro in seedlings of the seagrass Amphibolis antarctica ((Labill.) Sonder ex Aschers.). Uptake of all three nutrients was significantly correlated with external concentration up to 800 µ g l^–1. The uptake of nitrate (0–200 µ g NO₃-N g dry wt^–1 h^–1) was significantly lower than the uptake of ammonium (0–500 µ g NH₄-N g dry wt^–1 h^–1), suggesting that the seedlings have a higher affinity for this form of nitrogen in the water column.Data were in general agreement with uptake rates recorded for other seagrasses, notably Zostera marina. In comparison to the dominant macroalgae for the same region, seedlings had either similar or higher uptake rates in relation to external concentration, lending support to the hypothesis that seedlings, which do not possess roots, behave like macroalgae in terms of nutrient acquisition from the water column.A comparison with literature data on adult seagrass suggests, however, that seagrasses show lower uptake rates than macroalgae suggesting that the macroalgae, which are totally reliant on the water column for nutrients, are more efficient at uptake than seagrasses which may potentially use the sediment for a nutrient source.     

Unusual regulatory nitrate reductase activity in cotyledons of Brassica napus seedlings: enhancement of nitrate reductase activity by ammonium supply

The effect of supplying either nitrate or ammonium on nitrate reductase activity (NRA) was investigated in Brassica napus seedlings. In roots, nitrate reductase activity (NRA) increased as a function of nitrate content in tissues and decreased when ammonium was the sole nitrogen source. Conversely, in the shoots (comprising the cotyledons and hypocotyl), NRA was shown to be independent of nitrate content. Moreover, when ammonium was supplied as the sole nitrogen source, NRA in the shoots was surprisingly higher than under nitrate supply and increased as a function of the tissue ammonium content. Under 15 mM of exogenous ammonium, the NRA was up to 2.5-fold higher than under nitrate supply after 6 d of culture. The NR mRNA accumulation under ammonium nutrition was 2-fold higher than under nitrate supply. The activation state of NR in shoots was especially high compared with roots: from nearly 80% under nitrate supply it reached 94% under ammonium. This high NR activation state under ammonium supply could be the consequence of the slight acidification observed in the shoot tissue. The effect of ammonium on NRA was only observed in cotyledons and when more than 3 mM ammonium was supplied. No such NRA increase was evident in the roots or in foliar discs. Addition of 1 mM nitrate under ammonium nutrition halved NRA and decreased the ammonium content in shoots. Thus, this unusual NRA was restricted to seedling cotyledons when nitrate was lacking in the nitrogen source.