PRELIMINARY INVESTIGATIONS INTO THE STRUCTURE AND ACTIVITY OF RIBULOSE BISPHOSPHATE CARBOXYLASE FROM TWO PHOTOSYNTHETIC DINOFLAGELLATES1

Ribulose bisphosphate carboxylase / oxygenase (Rubisco) from the dinoflagellates Symbiodinium sp. Freudenthal and Amphidinium carterae Hulburt rapidly loses activity following cell lysis. Evidence presented indicates that this is not due to proteolysis. Using the tight binding inhibitor [¹⁴C] carboxyarabinitol bisphosphate as a marker, the Rubisco large subunit (LSu) from Symbiodinium sp. was purified. The subunit molecular weight was 56 kDa, while non-denaturing polyacrylamide gel electrophoresis indicated that the purified protein had a molecular weight significantly less than that expected of the intact hexadecameric protein. No trace of the small subunit was apparent. The initial loss of carboxylase activity following cell lysis may be due to instability of the quaternary structure of the enzyme. Antibodies prepared to the purified LSU cross-reacted with LSus from other dinoflagellates but not with the LSus of higher plants, diatoms, and other chromophytic algae. This suggests that the LSu of at least some dinoflagellates is antigenically different from that of other eukaryotes.     

谷子核酮糖1,5—二磷酸羧化加氧酶大亚基基因的核苷酸序列

谷子（Ｓｅｔａｒｉａ ｉｔａｌｉｃａ）叶绿体ＤＮＡ 含有ｒｂｃＬ基因的３．０ ｋｂ Ｂｇｌ Ⅱ＋ ＸｂａⅠ酶切片段被克隆到ｐＢｌｕｅｓｃｒｉｐｔ ＳＫ （－ ）质粒载体上,并构建了该基因的限制性酶切图谱,测定了该基因的１９９０ ｂｐ 全序列。其中基因的编码区长１４３１ ｂｐ,共编码４７６ 个氨基酸,推测其分子量为５２６７９ Ｄ。其３８９ ｂｐ 的５′上游区含有类似于原核生物的－ １０ ｂｏｘ、－ ３５ ｂｏｘ 和ＳＤ序列。其１７０ ｂｐ ３′下游区含有３ 个茎环结构。对Ｃ４ 植物和Ｃ３ 植物中的ｒｂｃＬ基因序列进行比较,表明在编码区、启动区和３′下游区没有差别。由此认为Ｃ４ 植物中ｒｂｃＬ基因的细胞特异性表达与其序列无关     

ULTRASTRUCTURE OF MALE GAMETOGENSIS IN FUCUS SERRATUS (PHAEOPHYCEAE)1

The main structural features of male gametogenesis in Fucus serratus L. are described. The antheridial parent cells exhibit meristematic characteristics: large nucleus, dense cytoplasm, chloroplasts with few thylakoids. Synaptonemal complexes are frequently observed in the nuclei at the same time that microtubules are very abundant around the centrioles. Meiotic spindles are intranuclear, as are the mitotic spindles of the subsequent synchronous karyokineses. A nuclear membrane is present in all stages observed. Antheridial maturation is accompanied by multiplication of mitochondria and plastids as well as development of two major kinds of “vacuoles.” Cytokinesis proceeds with the progressive fusion of one of these types of vacuoles: this mode of cytoplasmic cleavage is unique among algae. Formation and differentiation of characteristic spermatozoid organelles (flagella, eyespot, proboscis) are also described.     

THE RELEASE OF COPPER-COMPLEXING LIGANDS BY THE BROWN ALGA FUCUS VESICULOSUS (PHAEOPHYCEAE) IN RESPONSE TO INCREASING TOTAL COPPER LEVELS

The growth of Fucus vesiculosus L. germlings in chemically defined culture media containing a range of Cu concentrations (20–1000 nM) was monitored simultaneously with measurement of the Cu speciation in the media by competitive equilibrium-adsorptive cathodic stripping voltammetry. Fucus vesiculosus germlings were found to exude Cu-complexing ligands with conditional stability constants of the order of 1.6 × 10¹¹. Ligand concentrations increased with increasing total dissolved Cu concentrations (Cu_T) until a concentration of 500–800 neq Cu·L⁻¹ was reached. Concentrations of the ligand exceeded Cu_T in treatments containing 20 and 100 nM Cu, were similar to Cu_T in the 500-nM Cu treatment, but were less than Cu_T in the 1000-nM treatment. Therefore, [Cu²⁺] were calculated to be at concentrations of 10⁻¹¹− 10⁻¹⁰ M in the 20- and 100-nM treatments, 10⁻⁹ M in the 500-nM treatment, and 10⁻⁷ M in the 1000-nM treatment. Growth rates were lowest at Cu²⁺ concentration > 10⁻⁹. These results are discussed within the context of the potential roles for exuded copper-complexing ligands.     

ISOLATION AND CHARACTERIZATION OF GOLGI APPARATUS FROM EGGS OF FUCUS (PHAEOPHYCEAE)1

A golgi-rich cell-free fraction has been obtained from eggs of Fucus serratus L. and characterized by enzyme markers (IDP-ase and TPP-ase) and electron microscopy. The results are correlated with cytochemical localization of IDP-ase and TPP-ase in situ.     

PREMATURE CHROMOSOME CONDENSATION OF THE KARYOGAMY-BLOCKED SPERM PRONUCLEUS IN THE FERTILIZATION OF FUCUS DISTICHUS (FUCALES,PHAEOPHYCEAE)1

Mitosis of egg and sperm pronuclei of Fucus distichus subsp. evanescens (C. Agardh)Powell was examined by fluorescence and electron microscopy when migration of the sperm pronucleus and, as a result, karyogamy were blocked by colchicine treatment after plasmogamy. Chromosome condensation was obsewed in both pronuclei Microspectrophotometric studies after staining the nuclei with mithramycin A clearly showed that DNA synthesis ocurred in the egg pronucleus but not in the sperm pronucleus. This means that chromosomes condensed prematurely in the sperm pronucleus (premature chromosome condensation). In some cases, the egg chromosomes became arranged on a metaphase plate, whereas the sperm chromosomes lay scattered near the egg pronucleus. Immuno fluorescence microscopy using anti-β-tubulin antibody confirmed that a normal spindle was formed at the egg pronucleus. A pair of centrioles existed at the two poles of this spindle. The sperm nuclear membrane disappeared, and microtubules radiated to the sperm chromosomes from one pole of the egg spindle.     

RECENT VERSUS RELIC: DISCERNING THE GENETIC SIGNATURE OF FUCUS VESICULOSUS (HETEROKONTOPHYTA; PHAEOPHYCEAE) IN THE NORTHWESTERN ATLANTIC1

Fucus vesiculosus L. is one of the most widespread macrophytes in the northwestern Atlantic, ranging from North Carolina (USA) to Greenland (DK). We investigated genetic diversity, population differentiation, patterns of isolation by distance, and putative glacial refugial populations across seven locations from North Carolina (USA) to Cape Breton Island, Nova Scotia (Canada), with microsatellite analyses. Distinct northern versus southern (Delaware–North Carolina) populations were revealed by microsatellite data. Five of six microsatellite loci were fixed in populations in North Carolina, suggesting a recent founder event or a bottleneck, and the same homozygous genotype was found in herbarium materials collected on the North Carolina coast from more than 60 years ago. An additional set of individuals from the northern limit in Greenland was included in our analysis of mitochondrial intergenic spacer (mt IGS) haplotypes in the northwestern Atlantic. Remarkably, 184 of 188 F. vesiculosus specimens from North Carolina to Greenland shared the same haplotype. Recent colonization of the North American shore from Europe is hypothesized based upon the ubiquity of this common haplotype, which was earlier reported from Europe.     

MOLECULAR SYSTEMATICS OF THE FLORIDEOPHYCEAE (RHODOPHYTA) USING NUCLEAR LARGE AND SMALL SUBUNIT rDNA SEQUENCE DATA

Sequence data are presented for approximately 85% of the nuclear large subunit (LSU) rDNA gene for one member of the Bangiophyceae and 47 members of the Florideophyceae, the latter representing all but one of the currently recognized florideophyte orders. Distance, parsimony, and maximum likelihood analyses of these data were used to generate phylogenetic trees, and bootstrap resampling was implemented to infer robustness for distance and parsimony results. LSU phylogenies were congruent with published nuclear small subunit (SSU) rDNA results in that four higher level florideophyte lineages were resolved: lineage 1, containing the order Hildenbrandiales; lineage 2, recovered only under distance analysis, composed of the orders Acrochaetiales, Balliales, Batrachospermales, Corallinales, Nemaliales, Palmariales, and Rhodogorgonales; lineage 3, containing the Ahnfeltiales; and lineage 4, composed of the orders Bonnemaisoniales, Ceramiales, Gelidiales, Gigartinales, Gracilariales, Halymeniales, Plocamiales, and Rhodymeniales. Analyses were also performed on a combined LSU–SSU data set and an SSU-only data set to account for differences in taxon sampling relative to published studies using this latter gene. Combined LSU–SSU analyses resulted in phylogenetic trees of similar topology and support to those obtained from LSU-only analyses. Phylogenetic trees produced from SSU-only analyses differed somewhat in particulars of branching within lineages 2 and 4 but overall were congruent with the LSU-only and combined LSU–SSU results. We close with a discussion of the phylogenetic potential that the LSU has displayed thus far for resolving relationships within the Florideophyceae.     

REPRODUCTIVE SUCCESS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) IN THE BALTIC SEA

Marine organisms colonizing brackish habitats such as the Baltic Sea must cope with the negative effects of low salinities on reproductive success because these may reduce gamete viability and/or increase polyspermy. Reproductive characteristics of the marine seaweed Fucus vesiculosus L. were studied in several brackish habitats, particularly in the northern Baltic Sea, to understand its ability to reproduce where few other marine species survive. Polyspermy and fertilization success were variable at the boundary of the continuous distribution of F. vesiculosus in the Baltic Sea, and polyspermy was high (10%–30%) when fertilization was successful. A strong female bias (80%–86%, ca. 5.5:1) was found at the northernmost limit of Baltic F. vesiculosus. Electrophysiological studies showed that many eggs have a high input resistance (519 ± 150 MΩ[mean ± SE, n = 14] at Drivan, 1995), which may be helpful in preventing polyspermy in this brackish habitat. The polyspermy block remains sodium-dependent in the northern Baltic. Sperm bound quickly to northern Baltic eggs in natural water, but fertilization was delayed compared to marine F. vesiculosus. A subset of northern Baltic eggs studied during an optimal reproductive period (7–11 July 1995) had a membrane potential (E_m) of ca. −100 mV and an effective fertilization potential (FP) of ca. 2 min with a plateau of −25 mV, but repolarized too rapidly for the FP to be protective. Pronuclear migration and cell wall secretion occurred more slowly in Baltic than in marine zygotes. The reproductive success of theseboundary populations may be dependent upon windows of opportunity when there are favorable combinations of the levels of salinity, water motion, population density, and sex ratio. These factors and the short duration of the reproductive season in the northern Baltic Sea may result in reproductive failure in some years.     

THE FUCUS (PHAEOPHYCEAE) SPERM RECEPTOR FOR EGGS. I. DEVELOPMENT AND CHARACTERISTICS OF A BINDING ASSAY1

To explore the molecular basis of egg-sperm recognition in the brown alga , Fucus serratus L., we developed an in vitro binding assay involving egg plasma membrane vesicles (PMVs) and proteins contained in a KCl extract of sperm. Binding between the two components was measured using biotinylated PMVs followed by the addition of streptavidin conjugated to alkaline phosphatase and the appropriate substrate. Biotinylation did not affect the ability of egg PMVs to inhibit fertilization in a species-preferential manner. Binding of labeled egg PMVs to the sperm KCl extract was saturable and competable with unlabeled PMVs but was not species-specific. Protease treatment of the KCl extract abolished binding, whereas periodate had no effect, suggesting that sperm protein rather than carbohydrate was involved. Preincubation of the sperm extract with sulfated polysaccharides (e.g. fucoidan and ascophyllan) inhibited binding of egg plasma membranes. Sulfation seems to be important for this effect since desulfated fucoidan was far less effective at blocking binding. Polysaccharides which inhibited binding also inhibited fertilization. Overall, the results indicate that at least some aspects of binding between Fucus sperm and eggs are mediated by a protein(s) derived from sperm which recognizes sulfated glycoconjugates on the egg plasma membrane .     

马槟榔甜味蛋白N-和C-末端部分氨基酸顺序的测定

从马槟榔(Capparis masaikai)种子中曾分离出二种甜味蛋白,称为Mabinlin Ⅰ和Ⅱ,分子量分别为11.6kD和10.4kD。用Edmail降解和反相HPLC鉴定PTH-aa方法测定证明:二者的N末端均因焦谷氨酸环化而封闭。采用1mol/l HCl-甲醇溶液35℃处理24小时的方法可使之开环。二种蛋白N端肽8个氨基酸残基顺序均为:Pyroglutamic acid-Pro-Arg-Gly-Pro-Ala-Leu-Arg-。羧肽酶酶解的动态分析证明MaI羧端的氨基酸顺序为-Phe-Gln-Leu-Ala-Ser,MaⅡ为-Phe-Gln-Leu-Ala。其差别仅在于MaⅡ缺一个Ser,这一结果表明MaⅠ和MaⅡ的主要差异不是在端肽。     

INHIBITION OF FERTILIZATION IN FUCUS (PHAEOPHYCEAE) BY A MONOCLONAL ANTIBODY THAT BINDS TO DOMAINS ON THE EGG CELL SURFACE1

Fertilization in the brown alga Fucus serratus L. involves interactions between sperm and eggs and in species-specific. Recent results with monoclonal antibodies(MAbs FS4 and FS5) that recognize glycoproteins on the Fucus egg cell surface have shown that different sets of glycoproteins are organized into distinct domains. Thus, regions on the egg surface are FS4⁺FS5^? and FS4^?FS5⁺, and some smaller regions are FS4⁺FS⁺. To determine functional differences between these domains, MAbs FS4 and FS5 were included in a fertilization assay, Purified FS5 antibody, which is an immunoglobulin class M (IgM), inhibited fertilization, whereas FS4 IgM had no effect, Fragment antigen-binding fragment, which suggests that the effects were not due to steric hindrance by large IgM molecules or to cross-linking of receptors. FS5 inhibited fertilization in both F. serratus and F. vesiculosus; therefore, its effect was not species-specific, Thus we show that there is functional heterogeneity of the Fucus egg cell surface and compare our results to previous reports on inhibitory effects of lectins, Overall, the evidence suggests that there may be two levels of interaction between gametes in Fucus involoving nonspecific and species-specific binding.     

EVOLUTIONARY ANALYSIS OF THE LARGE SUBUNIT OF CARBOXYLASE (rbcL) NUCLEOTIDE SEQUENCE AMONG THE GRASSES (GRAMINEAE)

The full nucleotide sequences of the chloroplast encoded large subunit of ribulose-1,5-bisphosphate carboxylase (rbcL) are available for nine grass species and partial sequence data for one species. Relative rate tests of the “molecular clock” hypothesis suggest that rbcL evolved more rapidly in the lineage leading to Zea than in those leading to the other species. The estimated overall substitution rate for rbcL among these grasses is about 5 times 10^?10 substitutions per site per year, or about one-half the synonymous rate. The nine full sequences were analyzed by the UPGMA, Wagner parsimony, maximum likelihood, and Fitch-Margoliash methods. The latter three methods produced trees with the same topology. This topology largely agrees with current taxonomic evidence regarding the relationships among these grasses. UPGMA produced a topology that conflicts more substantially with available taxonomic evidence. Statistical comparison of the three alternative topologies for the subfamilies Panicoideae, Pooideae and Bambusoideae failed to support one of these topologies over the others, reflecting the taxonomic ambiguities surrounding the relationships among these taxa. Phylogenetic analyses based on the partial sequences of all 10 species gave conflicting results with regard to the relationship between Hordeum and Triticum, both members of the tribe Triticeae. This indicates that rbcL sequences contain too little information to resolve relationships among genera within this tribe. Overall, the results suggest that rbcL sequence data can provide some new information concerning grass phylogeny, but that the amount of available data from this gene is too small to differentiate statistically among alternative topologies for the grasses. Conflicting results from parsimony, maximum likelihood, and Fitch-Margoliash methods proved useful in exploring the validity of assumptions underlying these methods.     

PATERNAL LEAKAGE OF MITOCHONDRIAL DNA IN A FUCUS (PHAEOPHYCEAE) HYBRID ZONE1

Eukaryotic mitochondria are mostly uniparentally (maternally) inherited, although mtDNA heteroplasmy has been reported in all major lineages. Heteroplasmy, the presence of more than one mitochondrial genome in an individual, can arise from recombination, point mutations, or by occasional transmission of the paternal mtDNA (=paternal leakage). Here, we report the first evidence of mtDNA paternal leakage in brown algae. In Denmark, where Fucus serratus L. and Fucus evanescens C. Agardh have hybridized for years, we found eight introgressed individuals that possessed the very distinct haplotypes of each parental species. The finding of heteroplasmy in individuals resulting from several generations of backcrosses suggests that paternal leakage occurred in earlier generations and has persisted through several meiotic bottlenecks.     

PURIFICATION AND CHARACTERIZATION OF GLYCOLATE OXIDASE FROM THE BROWN ALGA SPATOGLOSSUM PACIFICISM (PHAEOPHYTA)1

Glycolate oxidase was purified to apparent homogeneity from the brown alga Spatoglossum pacificum Yendo. The 1326-fold purified glycolate oxidase enzyme exhibited a specific activity of 22. 4 micromoles glyoxylate formed ·min^?1·mg protein^?1. The molecular weight of the native enzyme was estimated to be 230,000 by gel filtration. The subunit molecular weight of the enzyme was determined to be 49,000 by sodium dodecyl sulfate–polyacrylamide gel electrophoresis, suggesting that the native enzyme is a tetramer. There were two absorption peaks at 345 and 445 nm, indicating that glycolate oxidase is a flavoprotein. This enzyme had a high isoelectric point (pI 9.6) and a high pH optimum (pH 8.3). The K_m values for glycolate and l -lactate were 0.49 and 5.5 mM, respectively. This enzyme also had a broad specificity for other straight-chain α-hydroxy acids but not for β-hydroxyacids. Cyanide, azide, N-ethylmaleimide, and p-chloromercuribenzoic acid did not affect the enzyme, whereas 2-pyridylhydroxymethanesulfonic acid strongly inhibited it. These properties of glycolate oxidase from the brown alga S. pacificum are similar to the properties of the glycolate oxidasesfrom higher plants. Polyclonal antibodies raised against the polypeptide fragment of Spatoglossum glycolate oxidase could recognize glycolate oxidase from Spinacia oleracea L., although the cross-reactivity was weak. The N-terminal sequence of two internal polypeptide fragments of the enzyme from S. pacificum showed a high degree of similarity to that of glycolate oxidase from higher plants. These results suggest that glycolate oxidase from higher plants and brown algae share the same ancestral protein.     

THE FUCUS (PHAEOPHYCEAE) SPERM RECEPTOR FOR EGGS. II. ISOLATION OF A BINDING PROTEIN WHICH PARTIALLY ACTIVATES EGGS1

An in vitro binding assay involving egg plasma membrane vesicles (PMVs) of Fucus serratus L. and proteins contained in a KCl extract of sperm has been used to identify a sperm protein involved in egg binding. High-performance gel filtration (HPGF) separated the sperm KCl extract into several major fractions, and a protein (apparent M, 60 kDa) was identified as being involved in binding to the egg PMVs. This protein ran on denaturing sodium dodecyl sulfate (SDS)gels with an apparent molecular weight of 27 kDa. This suggests that either the native form of the protein is a dimer or the molecular weight on HPGF is an artifact caused by high ionic strength buffer promoting hydrophobic interactions. When KCl-sol-uble proteins were separated by SDS-polyacrylamide gel electrophoresis (PAGE), blotted onto nitrocellulose, and incubated with biotinylated egg PMVs, these bound to a band at 27 kDa, confirming the role of this protein. Addition of the Fucus sperm extract or HPGF fractions containing the binding protein to eggs in the absence of sperm induced the release of polysaccharides onto the egg cell surface. This labeling was patchy, in contrast to the uniform release of polysaccharides observed when sperm were added to eggs. The monoclonal antibody (MAb) FS17 was raised against the 27-kDa sperm protein. It labeled the sperm body and both flagella by immunofluorescence, though the sperm had to he permeabilized to observe labeling, suggesting that the epitope recognized is not exposed at the cell surface. Addition of FS17 to the KCl extract in the binding assay reduced subsequent binding of egg PMVs. Removal of the 27-kDa protein recognized by FS17 from the sperm extract prevented the binding of egg PMVs in the binding assay and the triggering of the patchy release of polysaccharides when added to eggs. Overall the results suggest that the 27-kDa sperm protein is involved in binding to the egg plasma membrane and can trigger partial activation of the egg .     

PHYLOGENETIC RELATIONSHIPS AMONG LINEAGES OF THE CERAMIACEAE (CERAMIALES,RHODOPHYTA) BASED ON NUCLEAR SMALL SUBUNIT rDNA SEQUENCE DATA1

Phylogenetic relationships among 69 species of the Ceramiales (51 Ceramiaceae, six Dasyaceae, seven Delesseriaceae, and five Rhodomelaceae) were determined based on nuclear SSU rDNA sequence data. We resolved five strongly supported but divergent lineages among the included Ceramiaceae: (i) the genus Inkyuleea, which weakly joins other orders of the Rhodymeniophycidae rather than the Ceramiales in our analyses; (ii) the tribe Spyridieae, which is sister to the remainder of the included ceramialean taxa; (iii) the subfamily Ceramioideae, weakly including the tribe Warrenieae; (iv) the subfamily Callithamnioideae; and (v) the subfamily Compsothamnioideae, which emerges as sister to the Dasyaceae/Delesseriaceae/Rhodomelaceae complex, thus rendering the Ceramiaceae sensu lato unequivocally paraphyletic, as has been argued separately on anatomical grounds by Kylin and Hommersand. Our data support a restricted concept of the Ceramiaceae that includes only one of the five lineages (Ceramioideae) that we have resolved. In addition to failing to ally with the Ceramiales in our molecular analyses, species of Inkyuleea differ substantially from other Ceramiaceae sensu lato in details of pre‐ and postfertilization development. The genus Inkyuleea is here assigned to the Inkyuleeaceae fam. nov., which we provisionally retain in the Ceramiales. Species of Spyridia also differ from the remaining Ceramiaceae in their postfertilization development, and, in light of our molecular data, the genus Spyridia is assigned to the Spyridiaceae. The Callithamnioideae is strongly monophyletic (100% in all analyses), which, in combination with key anatomical differences, supports elevation to family status for this lineage as the Callithamniaceae. Similarly, the Compsothamnioideae is solidly monophyletic in our molecular trees and has a unique suite of defining anatomical characters that supports family status for a complex that we consider to include the tribes Compsothamnieae, Dasyphileae, Griffithsieae, Monosporeae, Ptiloteae, Spermothamnieae, Sphondylothamnieae, Spongoclonieae, and Wrangelieae, for which the reinstated family name Wrangeliaceae is available.     

EVOLUTION OF THE FUCACEAE (PHAEOPHYCEAE) INFERRED FROM nrDNA-ITS

Sequences of the internal transcribed spacer region (ITS-1, 5.8S, and ITS-2) of nuclear ribosomal DNA were obtained from 16 species representing all six genera of Fucaceae ( Ascophyllum, Fucus, Hesperophycus, Pelvetia, Pelvetiopsis, and Xiphophora ) plus one outgroup ( Hormosira ). Parsimony analysis indicated that the family Fucaceae is monophyletic and that the northern hemisphere taxa are highly divergent from the only southern hemisphere genus, Xiphophora. The genus Pelvetia is not monophyletic because the European P. canaliculata is more closely related to Fucus, Hesperophycus, and Pelvetiopsis than to other Pelvetia species. We establish Silvetia, gen. nov. and transfer the 3 Pacific species of Pelvetia to the new genus. Fucus is monophyletic and not ancestral in the Fucaceae. The ITS sequences identified two strongly supported lineages within Fucus, one with F. serratus sister to the clade containing F. gardneri, F. distichus, and F. evanescens and a second including F. vesiculosus, F. spiralis, F. ceranoides, and F. virsoides. The ITS was not useful for resolving relationships within each of these clusters and between populations of F. vesiculosus. Within-individual variation in ITS sequences is high in Fucus, a derived genus, compared to Ascophyllum, a more ancestral genus. Mapping of the two characters that form the basis of Powell's model for speciation in the Fucaceae showed that 1) number of eggs per oogonium has not followed a gradual reduction and that 2) monoecy/dioecy has changed several times during evolution of this family.     

INHIBITION OF CARBON UPTAKE AND STIMULATION OF NITRATE UPTAKE AT LOW SALINITIES IN FUCUS DISTICHUS (PHAEOPHYCEAE)1

Inorganic carbon and nitrate uptake were examined in whole plants of Fucus distichus L. (Powell) incubated in dilutions of synthetic ocean water and media with different concentrations of Na⁺, K⁺ and Cl^?. Reduction in salinity from normal seawater (33 ppt) decreased carbon uptake rate but increased nitrate uptake rate by 50% each. Substitution of K⁺ for Na⁺ at constant ionic strength decreased nitrate uptake. Substitution of K⁺ or mannitol for Na⁺ decreased carbon uptake. Neither the uptake of nitrate or carbon was changed by substituting SO₄^2- for Cl^?. Ionophores, valinomycin and monensin, inhibited both nitrate uptake and carbon fixation from 20 to 70% of control rates. The stimulation of nitrate uptake at low salinity may be beneficial to plants in estuarine tidal environments in which nitrate is supplied by the fresh water source.