Comparing the Relative Importance of Water-Borne Cues and Direct Grazing for the Induction of Defenses in the Brown Seaweed Fucus vesiculosus

Some seaweed species have been shown to release water-borne cues after herbivore attack, for example, to attract natural enemies of the herbivore. These cues may also be sensed by neighboring seaweeds and used to adjust their defenses in anticipation of a possible herbivore attack. Several studies indicated information transfer between seaweed individuals in the past, including the brown seaweed Fucus vesiculosus. Previous work showed induction of defenses in F. vesiculosus in response to water-borne cues released by isopod-grazed conspecifics. In contrast, another study on induced responses after exposure to cues from isopod-grazed neighbors using the same seaweed species yielded contradictory results. This study reassessed the ability of F. vesiculosus individuals to sense water-borne cues released by isopod-grazed neighbors in a series of experiments that monitored F. vesiculosus palatability in response to direct grazing by Idotea baltica and water-borne cues from isopod-grazed neighbors relative to unmanipulated seaweed pieces. Two-choice feeding assays were conducted with both fresh and reconstituted seaweed pieces. Direct grazing by I. baltica induced a chemical defense in F. vesiculosus, confirming results of previous studies. In contrast, evidence for increased herbivore resistance in seaweed pieces that were located downstream of isopod-grazed F. vesiculosus could not be provided. The lack of defense induction in response to grazing of conspecific neighbors may be explained by the environmental conditions and the scattered distribution of F. vesiculosus individuals in the intertidal zone of Helgoland, which may render resource investment in the emission and/or response to water-borne cues at this site unprofitable.     

Invasion of a Habitat-Forming Seaweed: Effects on Associated Biota

Fucus evanescens is a brown alga of arctic origin that has invaded European coasts. The epiphytic community of F. evanescens in southern Sweden was compared with that of the native Fucus vesiculosus, to examine to what extent an invading seaweed can modify local biodiversity. F. evanescens was much less fouled than F. vesiculosus, supporting both less biomass and fewer species of epiphytes. Multivariate analysis of the most common epiphyte taxa showed that the epiphytic community composition of F. evanescens was not entirely separated from that of F. vesiculosus, but host species contributed significantly to explain the variation in community composition. The biomass of free-living invertebrates was also lower on F. evanescens, although the pattern differed between taxonomic groups. While the biomass of amphipods was lower on F. evanescens, there was no significant difference in biomass of isopods or gastropods between the Fucus species. The good correlation between biomass of epiphytes and free-living animals suggests that the epiphytes play an important role in providing a suitable habitat for many species of free-living epifauna. The study shows that the invasion of F. evanescens affects the environmental conditions for many species associated with the Fucus community but that the direct effect on biodiversity is probably low.     

D-VITAMINS AND THEIR PRECURSORS AS GROWTH REGULATORS IN AXENICALLY CULTIVATED MARINE MACROALGAE1

Light petroleum extracts from thalli of Fucus vesiculosus (L.) and Laminaria digitata (Huds.) Lamour. stimulate growth of axenically cultured Fucus spiralis L. and Enteromorpha compressa (L.) Grev. in amounts per litre culture medium corresponding to one to 10 mg wet weight of the brown algae. This indicates the presence of a lipophilic, very active growth regulator in the blades. Extracts from the meristematic zone of Laminaria are much less active. Vitamins D₃, D₂ and 1,25-(OH)₂-D₃ at 1·10^?8– 1·10^?7 M increase the fresh weight of Enteromorpha compressa and Fucus spiralis up to 100% or more. The dry weight of Nemalion helminthoides (Veil, in With.) Batt. is also increased under the same conditions. Ergosterol and cholesterol are also growth-stimulating but higher concentrations (e.g. 1·10^?6 M) are necessary. The effects of the D vitamins manifest themselves without any auxins being added to the medium.     

Characterization of microsatellite loci in the marine seaweeds,Fucus serratus and F. evanescens (Heterokontophyta; Fucaceae)

Fucus serratus and F. evanescens commonly occur on Northern European shores. Nine microsatellite loci were developed for F. serratus (8–22 alleles, observed heterozygosities = 0.367–0.850) and one for F. evanescens (seven alleles, observed heterozygosity = 0.804). Cross‐amplification was apparent, as five F. serratus loci were polymorphic in F. evanescens and 2–5 were polymorphic in F. vesiculosus, F. distichus, and F. spiralis.     

Longitudinal profiles of carbon dioxide fixation capacities in marine macroalgae

Fucus serratus L., Fucus spiralis L., and Fucus vesiculosus L. (Fucales, Phaeophyceae) as well as Laminaria digitata (Huds.) Lamour., Laminaria hyperborea (Gunn.) Fosl., and Laminaria saccharina (L.) Lamour. (Laminariales, Phaeophyceae) have been investigated for the distribution of enzymic CO₂ fixation capacities via phosphoenolpyruvate carboxykinase (EC 4.1.1.32) (PEP-CK) and via ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) (RubP-C) in different regions of the thalli. The maximum of PEP-CK activity is found to be confined to the growing regions of the algae, while the activity of RubP-C achieves its highest values in the entirely differentiated parts of the fronds. These findings are confirmed by the results of photosynthetic and light-independent (dark) carbon assimilation as determined by in vivo ¹⁴CO₂ fixation. The physiological significance of these differential patterns of carboxylation patterns is discussed with respect to the ontogenetic stage and the chemical constitution of the different thallus parts.     

Hybridization and introgression in a mixed population of the intertidal seaweeds Fucus evanescens and F. serratus

The introduced Fucus evanescens (hermaphroditic) and the native F. serratus (dioecious) have been in secondary contact along the Danish coast of the Kattegat Sea for 60–100 years and dioecious hybrids have been observed at Blushøj for several years. Hybridization in Fucus is unusual because it appears to always involve a hermaphroditic and a dioecious parental pair. We determined the degree and spatial patterns of introgression for 286 individuals using 10 microsatellite loci and cpDNA. Hybrids accounted for nearly 13% of the population, yet parental species were well differentiated (F_ST = 0.633). The presence of F. evanescens chloroplasts in 100% of F₁ hybrids revealed asymmetrical hybridization. Fucus evanescens cpDNA was observed in 50% of introgressed and 5.4% of pure F. serratus, but no F. serratus cpDNA was found in F. evanescens. In contrast, nuclear DNA introgression was symmetrical with an equal amount (≈1.5%) of genes introgressed into each parental species. Survivorship and viability data suggest selection against hybrids in the hybrid zone.     

SPECIES DELIMITATION,TAXONOMY, AND BIOGEOGRAPHY OF DICTYOTA IN EUROPE (DICTYOTALES,PHAEOPHYCEAE)1

Taxonomy of the brown algal genus Dictyota has a long and troubled history. Our inability to distinguish morphological plasticity from fixed diagnostic traits that separate the various species has severely confounded species delineation. From continental Europe, more than 60 species and intraspecific taxa have been described over the last two centuries. Using a molecular approach, we addressed the diversity of the genus in European waters and made necessary taxonomic changes. A densely sampled DNA data set demonstrated the presence of six evolutionarily significant units (ESUs): Dictyota dichotoma (Huds.) J. V. Lamour., D. fasciola (Roth) J. V. Lamour., D. implexa J. V. Lamour., D. mediterranea (Schiffn.) G. Furnari, D. spiralis Mont., and the newly described D. cyanoloma sp. nov., which was previously reported as D. ciliolata from the Mediterranean Sea. Species distributions, based on DNA‐confirmed occurrence records, indicate that all species are geographically confined to the NE Atlantic Ocean with the exception of D. dichotoma and D. implexa, which also occur in South Africa and Bermuda, respectively. To investigate potential hybridization between D. dichotoma and D. implexa, which were previously shown to be sexually compatible in culture, we compiled and analyzed sets of mitochondrial, plastid, and nuclear markers to detect putative hybrids or introgression in natural populations. Failure to detect natural hybrids indicates that effective pre‐ and postzygotic isolation mechanisms are at play in natural populations and supports the by‐product hypothesis of reproductive isolation.     

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.     

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.     

Waterborne signaling primes the expression of elicitor-induced genes and buffers the oxidative responses in the brown alga Laminaria digitata

As marine sessile organisms, seaweeds must respond efficiently to biotic and abiotic challenges in their natural environment to reduce the fitness consequences of wounds and oxidative stress. This study explores the early steps of the defense responses of a large marine brown alga (the tangle kelp Laminaria digitata) and investigates its ability to transmit a warning message to neighboring conspecifics. We compared the early responses to elicitation with oligoguluronates in laboratory-grown and harvested wild individuals of L. digitata. We followed the release of H₂O₂ and the concomitant production of volatile organic compounds. We also monitored the kinetics of expression of defense-related genes following the oxidative burst. Laboratory-grown algae were transplanted in kelp habitats to further evaluate their responses to elicitation after a transient immersion in natural seawater. In addition, a novel conditioning procedure was established to mimic field conditions in the laboratory. Our experiments showed that L. digitata integrates waterborne cues present in the kelp bed and/or released from elicited neighboring plants. Indeed, the exposure to elicited conspecifics changes the patterns of oxidative burst and volatile emissions and potentiates this kelp for faster induction of genes specifically regulated in response to oligoguluronates. Thus, waterborne signals shape the elicitor-induced responses of kelps through a yet unknown mechanism reminiscent of priming in land plants.     

Rhizoid production in excised Dictyota dichotoma

Excised segments of fronds of Dictyota dichotoma (Huds.) Lamour. show a marked basipetal increase in rhizoid production when grown in culture. Rhizoid production is inhibited by bright light and apparently enhanced by red light (λ 600–690 mμ).     

IMMUNOLOCALIZATION OF CENTER IN THE FLAGELLAR APPARATUS OF MALE GAMETES OF ECTOCARPUS SILICULOSUS (PHAEOPHYCEAE) AND OTHER BROWN ALGAL MOTILE CELLS1

Centrin or a centrin homologue was localized using immunofluorescence in the flagellar basal body region in zoids of five brown algal species: Ectocarpus siliculosus (Dillw.) Lyngb., Scytosiphon lomentaria (Lyngb.) Link, Laminaria digitata (Huds.) Lamour., Sphacelaria rigidula (Kütz.) Prud'homme van Reine, and Fucus serratus L. The antigen is restricted to short rods extending along the basal body(ies) and towards the nucleus, which always remains firmly linked to the flagellar apparatus in isolated cytoskeletons. To identify these antigenic sites, pre- and postembedding immunogold electron microscopy was applied to male gametes of E. siliculosus. At least three different structures associated with the basal bodies were antigenic: a fibrous structure connecting the proximal end of the posterior basal body to the nucleus (nucleus-basal body connector), a striated band that links the two basal bodies to each other and is located in the angel formed by them, and amorphous material interconnecting the basal bodies in their most proximal regions. In addiction, specific labeling occurs along the external surface and within the lumen of both basal bodies and in the flagellar transitional region. The possible function of these centrin-containing structures is discussed.     

Distribution of histamine in the thallus ofFurcellaria lumbricalis

Histamine occurs in all regions of the thallus of male, female and tetrasporophyte plants ofFurcellaria lumbricalis (Huds.) Lamour. The mean values varied from 60 μg to 500 μg (g fresh weight)^?1. There were few consistent differences either between the same region of different types of plant or different regions of the thallus of one type of plant. Fluorescence staining of sections of the thalli ofF. lumbricalis, with o-phthalaldehyde, revealed that cells of the cortex and medulla have histamine associated with them. The histamine appears to be intracellular since only intact cells exhibit fluorescence. Cells in sections of female plants ofPolyides rotundus (Huds.) Grev., which do not contain histamine, do not exhibit fluorescence dependent upon o-phthalaldehyde.     

Chemical settlement inhibition versus post-settlement mortality as an explanation for differential fouling of two congeneric seaweeds

It has been proposed that seaweed secondary metabolites, e.g. brown algal phlorotannins, may have an ecologically important function as a chemical defence against epiphytes, by acting against colonisation of epiphytic organisms. We tested whether the low epiphytic abundance on the invasive brown seaweed Fucus evanescens, compared to the congeneric F. vesiculosus, is due to a more effective chemical defence against epiphyte colonisation. A field survey of the distribution of the common fouling organism Balanus improvisus (Cirripedia) showed that the abundance was consistently lower on F. evanescens than on F. vesiculosus. However, contrary to expectations, results from experimental studies indicated that F. vesiculosus has a more effective anti-settlement defence than F. evanescens. In settlement experiments with intact fronds of the two Fucus species, both species deterred settlement by barnacle larvae, but settlement was lower on F. vesiculosus both in choice and no-choice experiments. Phlorotannins from F. vesiculosus also had a stronger negative effect on larval settlement and were active at a lower concentration than those from F. evanescens. The results show that Fucus phlorotannins have the potential to inhibit settlement of invertebrate larvae, but that settlement inhibition cannot explain the lower abundance of the barnacle Balanus improvisus on F. evanescens compared to F. vesiculosus. Assessment of barnacle survival in the laboratory and in the field showed that this pattern could instead be attributed to a higher mortality of newly settled barnacles. Observation suggests that the increased mortality was due to detachment of young barnacles from the seaweed surface. This shows that the antifouling mechanism of F. evanescens acts on post-settlement stages of B. improvisus.     

Recent changes in the seaweed community of Western Prince Edward Island: implications for the seaweed industry

Prior to 1980, seaweed beds of western Prince Edward Island, Canada, were dominated by Chondrus crispus Stackhouse, associated with 37 other species of common algae. Furcellaria lumbricalis (Huds.) Lamour. occurred in 4.6% of the samples from the Pleasant View bed but never reached measurable levels of cover. By 1991 F. lumbricalis was reported in 67% of the samples from this bed. Furcellaria lumbricalis biomass was negatively correlated with C. crispus biomass and positively correlated with Phyllophora Grev. spp. biomass. F. lumbricalis biomass and cover peaked in subtidal areas below 4 m at 359 ± 299 SD fresh g 0.25 m^–2. Harvesters avoid concentrations of F. lumbricalis, resulting in a 5% content of this species in the commercial harvest. Systematic harvesting located concentrations of F. lumbricalis in seven of 10 commercial beds. Proposed causes for change in species dominance from C. crispus to F. lumbricalis range from general environmental change, natural succession to excessive human perturbation.     

Effect of brown algae metabolites on the synthesis of O-glycosyl hydrolases by bacteria degrading the thallus of <Emphasis Type="Italic">Fucus evanescens</Emphasis>

The effect of fucoidan, extractive substances from Fucus evanescens and the Laminaria cichorioides protein (an inhibitor of endo-1→3-β-D-glucanase) on the degradation of F. evanescens thallus and on the growth of bacteria involved in this process was studied. The complex of O-glycosyl hydrolases and the level of their enzymatic activity in bacteria cultivated under various conditions depended significantly on the composition of the growth medium. The highest taxonomic diversity was observed for bacteria isolated from the thallus degraded in the control medium (sea water). These bacteria were characterized by very low levels of activity of the enzymes degrading polysaccharides (fucoidan, laminaran, and pustulan). In the presence of 1→3-β-D-glucanase inhibitors, the taxonomic diversity of the microorganisms degrading F. evanescens thallus was decreased, and the activity of O-glycosyl hydrolases (particularly, of fucoidan hydrolases) was increased.     

Kairomonal range of generalist predators in specialized habitats: responses to multiple phloeophagous species emitting pheromones vs. host odors

We evaluated responses of the predominant predators of pheromone-producing bark beetles (Coleoptera: Scolytidae) to chemical cues associated with other phloeophagous species that colonize the same trees. This study considered the range of chemical signals exploited by a category of predators that may be viewed either as specialists, because they feed almost exclusively within trees killed by bark beetles, or as generalists, because they feed on a diverse fauna of primary and secondary insects within this habitat. It also evaluated one aspect of a broader model of predator-prey coevolution, that proposes altered semiochemistry as a source of partial escape from predators that exploit kairomones. The predators, Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae), were attracted to cues associated with feeding on bark-phloem disks by two scolytids that produce adult pheromones, Ips pini (Say) and Ips grandicollis (Eichhoff). These predators were not attracted to beetles that feed on lower stems or roots and are not known to produce adult pheromones,Dendroctonus valens LeConte, Hylastes porculus Erickson (Coleoptera: Scolytidae), and Hylobius pales (Herbst) (Coleoptera: Curculionidae). The predator Tenebroides collaris (Sturm) (Coleoptera: Trogositidae) was attracted to I. pini and I. grandicollis, and also to D. valens, H. porculus, and H. pales. Ips pini was attracted to conspecifics only, but I. grandicollis was attracted both to its conspecifics and to volatiles associated with feeding lower stem and root insects. Lower stem and root insects were not or only weakly attracted to cues associated with their conspecifics. These results are consistent with a dynamic coevolved interaction between T. dubius and P. cylindrica and Ips spp.     

Benthic marine algal flora of the north shore of Prince Edward Island (Gulf of St. Lawrence), Canada

A seaweed community along 60 km of shoreline in the southern Gulf of St. Lawrence has been studied for 5 years. Because of severe ice action along the shores, the algal flora is largely restricted to subtidal levels, which are accessible only during the ice-free months, April to early December. A total of 52 Rhodophyta, 50 Phaeophyta, 19 Chlorophyta and 5 Cyanophyta was identified, mostly occurring between mean low water and 10 m depth. The normally intertidal dominants Focus vesiculosus L. and Chondrus crispus Stackh. flourish at depths of 0–2.5 m and 2.5–5 m, respectively. Chondrus and Furcellaria lumbricalis (Huds.) Lamour. co-dominate the algal benthos at 5–7.5 m, and F. lumbricalis becomes prevalent at 7.5–10 m. Phyllophora truncata (Pallas) A. Zin. and Phyllophora pseudoceranoides (Gmel.) Newr. & A. Tayl. are the main perennials at 10–15 m, and often at 15–20 m as well. Below 15 m, the flora diminishes markedly, and at 20–25 m consists chiefly of Agarum cribrosum (Mert.) Bory, crustose corallines and Polysiphonia urceolata (Lightf. ex Dillw.) Grev. Laminarians other than Chorda occur only as scattered plants or small patches and do not exceed 1 m length. The algal community is relatively undiversified and is especially deficient in green algae. Despite higher surface-water temperatures during summer, the flora is essentially a cold-water one as is indicated by both composition and low R/P and (R+C)/P ratios of 1.06 and 1.45, respectively.