Chemically defended benthic macroalgae that dominate shallow, hard bottom communities along the western Antarctic Peninsula support very high densities of mesograzers, particularly amphipods but also small gastropods. Previous studies have demonstrated that the macroalgae and amphipods form a mutualistic relationship. The chemically defended macroalgae provide the amphipods with a refuge from predation while the macroalgae benefit from the amphipods greatly reducing surface fouling by smaller algae. One of the three most important macroalgae in terms of overstory cover, Himantothallus grandifolius, forms huge blades that can carpet the benthos. Field observations suggest that gastropods may be higher in relative abundance in proportion to amphipods on H. grandifolius than on other overstory macroalgae. The present study documents the finding that natural abundances of gastropods on H. grandifolius maintained in mesocosms reduce fouling by microscopic algae, primarily diatoms. However, amphipods are probably also important in keeping the macroalga clean of diatoms in nature. In a smaller scale experiment, three gastropod species were differentially effective at reducing diatom coverage on H. grandifolius. The hypothesis that gastropods benefit from associating with H. grandifolius in potentially gaining a refuge from sea-star predation was also tested but not supported by the experimental results.

A revision of the check-list of the red (Rhodophyta), green (Chlorophyta) and brown (Ochrophyta, Phaeophyceae) seaweeds of Britain has been compiled and revealed 644 taxa, mostly at species level but including infraspecific taxa: 348 reds, 110 greens and 186 browns. There were 30 additions to the list, notably five newly described species and five new non-native additions. In addition to the check-list, an updated list of non-native species was compiled based on a stringent review of the available evidence. We have listed 31 non-native species (~5% of the flora), although the numbers may be considerably higher and probably include species that are effectively naturalized and perceived to be part of the indigenous flora. The check-list is discussed in relation to the impact of molecular techniques on the identification, taxonomy and phylogeny of species, and points to the need for a comprehensive, in-depth study on the identity and relationships of the seaweeds of Britain.

Kelps, fucoids and other large brown seaweeds are common and important features of temperate coastal zones. The British Isles is a centre for seaweed diversity in the NE Atlantic, but, despite numerous surveys, an incomplete picture of the distribution remains. Survey data and herbarium specimens were used to examine the environmental preference of 15 species of large brown seaweeds, covering the orders Laminariales (kelps), Fucales (wracks) and one species of Tilopteridales. Habitat suitability models were developed to estimate broad-scale distribution and area of habitat created by these species around the British Isles. Topographic parameters were important factors limiting distributions. Generally, temperature did not appear to be a limiting factor, probably because the British Isles lies in the centre of the NE Atlantic distribution for most species, and not at climatic tolerance limits. However, for the recent migrant Laminaria ochroleuca, temperature was found to be important for the model, thus range expansion could continue northwards provided dispersal is possible. In contrast, the widespread Alaria esculenta showed a negative association with warmer summer temperatures. The total potential habitat around the British and Irish coastline is more than 19,000 km2 for kelps and 11,000 km2 for wracks, which represents a significant habitat area similar in scale to British broadleaf forest. We conclude that large brown algal species need to be managed and conserved in a manner that reflects their scale and importance.

The stimulating effect of blue light on the carbon-concentrating mechanism of Ectocarpus siliculosus was investigated by measuring the intracellular pools of inorganic carbon (Ci) in red light (low photosynthetic rates) and in red light plus blue light (high photosynthetic rates). Ci accumulated in red light to about twice the levels found in the presence of blue light, but the permeability of the cell membranes for Ci, measured as the rate of influx of tracer carbon, was similar in both light conditions. This was also true after the seawater pH was adjusted to pH 9·5, when bicarbonate should be the only inorganic carbon species to enter the cells. Activation of photosynthesis after prolonged red light caused the accumulated Ci to decline to levels similar to those observed in blue light. The time course of this decline was rapid and coincided with the time course that was expected for the emptying of a carbon pool whose existence had been demonstrated earlier in measurements of photosynthetic oxygen evolution in the absence of external carbon. The accumulation of Ci in red light required active photosynthesis, but it also occurred in the presence of inhibitors of carbonic anhydrase activity. It is suggested that blue light mobilizes the Ci pool by causing changes in intracellular pH and that the size of the pool depends on the activity of metabolic CO2-sink activities that are also under the control of blue light.

Blue light stimulates photosynthesis in brown algae by activating a mechanism for carbon acquisition. As this is connected with liberation of CO2 from an internal pool, it had previously been suggested that a C4 pathway might operate, with CO2 mobilization from an intermediate mediated by the action of blue light. The possibility that such a C4 mechanism functions was tested in Ectocarpus siliculosus by investigating metabolite pools and carbon fixation products. Extracts were analysed by HPLC with an ion exchange column. With the exception of citrate/isocitrate all metabolite levels were far too low to serve as the intermediate pool. No changes in the concentrations of any of the metabolites were found after activation by blue light following red light pre-irradiation. During short-term incubations with 14C inorganic carbon, the major label was found in phosphoglycerate. There was also clear labelling of peaks containing malate plus pyruvate and in citrate/isocitrate. In addition, glycerate contained high levels of radioactivity in some culture strains of Ectocarpus siliculosus. In chase experiments, the radioactivity of all metabolites continued to increase, indicating the presence of considerable amounts of intracellular inorganic carbon. Transient labelling was not observed for any of the fixation products. When Ectocarpus was pre-irradiated with red light and then stimulated by blue light, fixation rates increased with a time course that correlated with the activation of oxygen evolution. Photosynthetic oxygen evolution was investigated in vivo after the addition of possible intermediates to carbon-free seawater. Most of the exogenous applied substrates did not enhance photosynthesis, either in red light or after blue light stimulation. A strong enhancement seen after the addition of oxaloacetate was most likely due to release of CO2 during the breakdown of the metabolite in the medium. Taken together, the data argue strongly against the operation of a C4 pathway in Ectocarpus. It is suggested that the internal pool mobilized by blue light contains inorganic carbon.

The polarized apical cells of Sphacelaria rigidula display a well-organized cortical F-actin cytoskeleton. This consists of bundles of actin filaments (AFs), assuming definite patterns of organization in different regions of the cell cortex. At the tip region of the apical dome the AFs appear randomly oriented, showing a diffuse fluorescence. Immediately below, at the base of the apical hemisphere, the AFs form a ring-like band around the plasmalemma transverse to the polar cell axis. The rest of the cell cortex is traversed by AFs showing an axial or slightly inclined or helical orientation. Examination of the apical cells of S. rigidula in appropriate thin sections revealed that the wall has a multi-layered structure. In the tip region of the apical dome the cell wall bears randomly oriented cellulose microfibrils (MFs), while in the basal part of the apical dome it is reinforced by a layer of densely arranged transverse MFs. As the cell grows at the apex, the transverse MFs are continuously displaced towards the cell base. Below the transverse MF layer, an additional layer with axial or slightly oblique MFs starts being depositing internally, on the tubular part of the cell. Externally to them, the layer of transversely oriented MFs remains visible. The above observations were confirmed in apical cells of S. tribuloides. MF orientation in the innermost wall layer of the apical cells coincides with that of the cortical AFs observed by fluorescence. This mutual alignment between AFs and MFs in a cell that lacks cortical microtubules (MTs) suggests that the AFs are involved in the oriented deposition of MFs. Experimental disruption of AFs with cytochalasin B caused abnormal MF deposition, a fact strongly supporting the above hypothesis. The transverse MFs forming at the base of the apical dome define the diameter and consequently the cylindrical shape of the apical cells. It is suggested that in the brown algal cells examined the AFs play a morphogenetic role similar to that of cortical microtubules in higher plant cells.

The marine brown alga Myriotrichia clavaeformis (Dictyosiphonales) is the host of a large icosahedral virus with a double-stranded DNA genome, MclaV-1. This pathogen replicates exclusively in prospective reproductive organs of the alga. We studied the assembly of virus particles by transmission electron microscopy. Replication of viral DNA starts in the nucleus, which hypertrophies and later disintegrates. Capsid assembly begins in the cytoplasm by budding from virus-detaching bodies. After nuclear breakdown, assembly continues in a mixed cyto-/nucleoplasm on membrane cisternae, which probably originate from the endoplasmic reticulum. Virus particles thereby acquire as an integral capsid component a membrane to which proteins are apposed. Material inside the capsids partly condenses to form an additional layer in the core shell. DNA is packaged after capsid formation, giving rise to an electron-opaque nucleoprotein core. M. clavaeformis infected by MclaV-1 is the second brown algal host-virus system in which virus assembly has been studied in detail. Together with previous observations, our results allow conclusions on general mechanisms of virus assembly in brown algae. Some features of virus formation in brown algae show similarities with large icosahedral DNA viruses infecting animal cells.

The brown macroalga Laminaria saccharina exhibits a type of HCO3− utilization that could be almost completely inhibited either by proton buffers or by acetazolamide, an inhibitor of extracellularly operating carbonic anhydrase. This means of HCO3− utilization featured properties similar to direct HCO3− uptake in that photosynthetic rates were proportional to the HCO3− concentration of the seawater over a wide pH range (pH 7·0–9·5). Despite this, it must be characterized as carbonic anhydrase-catalysed external HCO3− dehydration and not as direct HCO3− uptake. A mechanism is suggested involving a CO2-concentrating capability located at the cell membrane. This mechanism, which might be common in brown algae, is suggested to have an adaptational advantage in colder regions of the sea (as compared with the direct HCO3− uptake of green macroalgae). This means of HCO3− utilization is inhibited even by fairly low concentrations of buffer, with consequences for the interpretation of earlier experimental studies on L. saccharina (and possibly other brown algae). These consequences relate both to ecology (e.g. determination of inorganic C affinity) and physiology (e.g. assessing mechanisms for inorganic C uptake).

The organization of F-actin in vegetative cells of the brown alga Sphacelaria rigidula was studied after staining with a modified rhodamine-phalloidin (Rh-Ph) protocol. The interphase vegetative cells display a well-organized F-actin cytoskeleton, consisting of cortical, endoplasmic and perinuclear arrays of actin filaments (AFs). The organization of these AFs changes slightly during mitosis, while they almost disappear at cytokinesis. The perinuclear AF population becomes more obvious during prophase, especially at the poles. At metaphase, an actin spindle is organized, co-localized with the microtubule spindle, while at anaphase an interzonal AF population appears, which persists at early telophase. At advanced telophase the image changes to a rather diffuse actin meshwork in the mid-area between the daughter nuclei. During post-telophase-early cytokinesis this AF system becomes gradually disassembled and a conspicuous actin plate is formed on the cytokinetic plane. This plate becomes more compact with the progress of cytokinesis, and seems to overcoat the patches of the forming diaphragm, and finally the daughter plasmalemmata. Treatment with cytochalasin B disturbs mitosis, not allowing the cells to proceed to anaphase, and prevents cytokinesis. These observations show that AFs are ubiquitous cytoskeletal elements of the vegetative cells of brown algae. The probable role of AFs during the cell cycle and particularly the unique actin configuration involved in cytokinesis are discussed.

Detailed studies on the sporophytes and gametophytes of Dictyopteris australis and Dictyopteris muelleri from Australia have demonstrated the usefulness of several reproductive characters in species discrimination. In D. australis, spherical sporangia that project above the thallus surface and are clustered around reflexed bundles of paraphyses contrast markedly with the angular sporangia of D. muelleri, which are embedded in the thallus and are scattered in broad fertile zones on the upper branches. In D. muelleri, tetrads of released spores enveloped by the inner sporangial wall are temporarily retained above the thallus surface on the ends of mucilaginous stalks. Sporangial stalks have not been reported in any other species of the brown algae, although egg stalks are produced by several fucalean species. Differences in oogonial structure can also be used to distinguish D. australis and D. muelleri. The spherical oogonia of D. australis, which project above the thallus surface, are conspicuously different from the angular embedded oogonia of D. muelleri. Detailed comparative studies on other species of Dictyopteris are now required to ensure that reproductive characters are used not only to define species of Dictyopteris but also to effectively circumscribe the genus.

A new large DNA virus (PlitV-1) infects the marine filamentous brown alga Pilayella littoralis. It was collected in Alaska and infects other P. littoralis isolates of different geographic origin. The virus has an icosahedral capsid of c. 161 nm in diameter, enclosing an electron-dense core. The genome consists of double-stranded DNA and is approximately 280000 base pairs in size. The virus is latent in somatic cells of the host and is propagated only upon induction of the host's reproductive organs. It causes deformed sporangia, resulting in infertility, and is structurally similar to other brown algal viruses. PCR amplification of a genomic sequence coding for part of a structural glycoprotein of the Ectocarpus siliculosus virus EsV-1 produced a fragment of similar size to that obtained with EsV-1.

Examination of type material, comparative studies on vegetative characters and data from published accounts of sporangia, gametangia and spermatozoids are used here to further develop and strengthen generic and species concepts in Zonaria and Homoeostrichus. Zonaria is characterized by octosporangia which lack a stalk cell and occur among whitish paraphyses, by the presence of cortical cells in indusiate sporangial and oogonial sori, and by slightly elongate sterile cells bordering the antheridial sori. Other characters (mucilage in sporangial and oogonial sori, biflagellate sperm with a row of many tall spines along the anterior flagellum) may also be diagnostic of Zonaria but have not been demonstrated in the generitype. Eight species are retained in Zonaria. Generic placement of Zonaria stipitata, described on vegetative characters, requires confirmation. Homoeostrichus, as typified by Homoeostrichus sinclairii, is defined by the presence of brown paraphyses in nonindusiate sporangial and oogonial sori and bordering antheridial sori, a multicellular sporangial stalk, oogonia without a stalk cell, and uniflagellate spermatozoids. Zonaria harveyana and Homoeostrichus olsenii are similar in sporangial structure, but they cannot be assigned to either Zonaria or Homoeostrichus as now defined and consequently are transferred to the new genus, Exallosorus, as Exallosorus harveyanus (Pappe ex Kützing) comb. nov. and Exallosorus olsenii (Womersley) comb. nov. Exallosorus is distinguished from Zonaria and Homoeostrichus using sporangial characters: tetrasporangia have a stalk cell and are closely packed into indusiate sori which lack paraphyses and mucilage. Problems in obtaining reliable species identification for the five southern Australian species of Zonaria are partially resolved. This study demonstrates again the essential contribution of reproductive characters to the definition of taxa and supports the position that decisions to erect, subsume or transfer members of the Dictyotales should be based on more than vegetative characters.

A comparative study of fertile gametophytes has been carried out on six species of the Dictyotales from Australia. Patterns of gametangial distribution and structure are similar in Zonaria angustata, Zonaria diesingiana, Zonaria spiralis and Zonaria turneriana, but there are some marked differences apparent between both Homoeostrichus olsenii and Homoeostrichus sinclairii and the Zonaria species. Sterile cortical cells and mucilage between the cylindrical oogonia are characteristic of the indusiate female sori of Zonaria, which differ markedly from the comparable structures of H. olsenii, in which all cortical cells have been transformed into oogonia and the walls of adjacent oogonia are contiguous. In contrast, the large spherical oogonia of H. sinclairii are unique in the Dictyotales, lacking a stalk cell and occurring among brown paraphyses in a large non-indusiate sorus on each terminal branch segment. Antheridial sori of the six species exhibit far less structural variation than the oogonial sori. Male sori are surrounded by undifferentiated cortical cells in H. olsenii, by tall brown paraphyses and short paraphyses in H. sinclairii and by sterile cells in Zonaria spp. Short paraphyses form only around the male sori of H. sinclairii in the Dictyotales; they differ in structure from the brown paraphyses but are similar to the antheridia, from which they may have been derived. Gametangial characters have proved invaluable in defining the genus Zonaria and in separating H. olsenii from H. sinclairii, but they cannot be used to delimit the species of Zonaria. Furthermore, the differences observed in the gametangia have provided new characters for phylogenetic analyses, as well as providing further supporting evidence for the recognition of Zonaria and Homoeostrichus as distinct entities.