Daily integrals of photosynthesis by a cyanobacterial bloom in the Baltic Sea, during the summer of 1993, were calculated from the vertical distributions of light, temperature and the organisms in the water column and from photosynthesis/irradiance curves of picoplanktonic and diazotrophic cyanobacteria isolated from the community. The distribution of chlorophyll a in size-classes < 20 μm and > 20 μm was monitored over 9 days that included a deep mixing event followed by calm. Picocyanobacteria formed 70% of the cyanobacterial biomass and contributed 56% of the total primary production. Of the filamentous diazotrophs that formed the other 30%, Aphanizomenon contributed 28% and a Nodularia-containing fraction 16% of the primary production. For the whole population there was little change in standardized photosynthetic O2 production, which remained at about 31 mmol m−2 before and after the mixing event. There were differences, however, between the classes of cyanobacteria: in picocyanobacteria primary production hardly changed, while in Aphanizomenon it increased by 2·6 and in Nodularia it fell below zero. Total phytoplankton photosynthesis was strongly dependent on total daily insolation with the compensation point at a photon insolation of 22·7 mol m−2 d−1. Similar analyses of N2 fixation showed much less dependence on depth distribution of light and biomass: Aphanizomenon fixed about twice as much N2 as Nodularia their; their fixation exceeded their own N demand by about 12%. Together, these species contributed 49% of the total N demand of the phytoplankton population. Computer models based on the measured light attenuation and photosynthetic coefficients indicate that growth of the cyanobacterial population could occur only in the summer months when the critical depth of the cyanobacteria exceeds the depth of mixing.

Repair of DNA damage induced by ultraviolet-B radiation (UVBR) was investigated in the sublittoral red alga Rhodymenia pseudopalmata at different temperatures, using immunofluorescent detection of thymine dimers. Photoreactivation of thymine dimers was completed within about 3 h at 6, 12 and 18 °C in the presence of ultraviolet-A radiation (UVAR) and photosynthetically active radiation (PAR) but no repair was found in dark-incubated fragments after 16–17 h. In plants previously exposed to a low UVBR dose (biologically effective dose: BEDDNA300 = 1·6 kJ m−2; unweighted irradiance = 0·72 W m−2 for 4 h), photoreactivation started within the first hour under UVAR+PAR at 18 °C but repair only started after 1–2 h at 12 and 6 °C. At 6 °C, repair was more efficient after exposure to a high UVBR dose (BEDDNA300 = 3·9 kJ m−2; unweighted irradiance = 0·78 W m−2 for 5 h) than to a lower dose (BEDDNA300 = 1·6 kJ m−2); no such difference was found at 18 °C. It is concluded that R. pseudopalmata is able to repair DNA damage induced by high UVBR doses efficiently.

Distribution and phenology of some filamentous species of Rhodomelaceae in the low littoral and shallow sublittoral zones of an exposed, western Mediterranean rocky shore were studied for 1 year. The spatial distribution of many species shows a pronounced small-scale (metres or tens of metres) variability. Boergeseniella fruticulosa was most abundant in spring and fertile (tetrasporangia only) in spring and summer. Polysiphonia flocculosa was most abundant in spring and summer and reproductive throughout the year. Polysiphonia opaca was most abundant in summer and fertile in spring–summer. Maximal vegetative development and reproduction of Lophosiphonia cristata were observed in summer; conversely, Polysiphonia ceramiaeformis and Polysiphonia subulata were best developed and mostly reproductive in winter and spring. Womersleyella setacea was the least abundant of the studied species and apparently reproduced only by vegetative growth and fragmentation. The reproductive phenology of the species investigated is generally in agreement with reports for other Mediterranean regions. The life history traits of most species are typical of seaweeds in which environmental factors directly govern the onset of the main activities of the organism by meeting the primary physiological needs (‘season responders’).

This work attempts to identify growth conditions for maximal productivity of cell mass and of eicosapentaenoic acid (EPA) in ultrahigh cell density cultures of Nannochloropsis sp. Using flat plate reactors with a narrow (1–2 cm) light path and rigorous stirring exposed to high photon flux densities (1000–3000 μmol photons m−2 s−1), record population densities (1·2–1·4×1010 cells ml−1) were obtained. Consequently, the EPA content of the culture (mg l−1) was higher by some two orders of magnitude than reported hitherto for cultures of much lower cell concentrations. In continuous cultures, highest culture EPA yield coincided with maximal output rate of cell mass. The very high population densities and output rates of cell mass and of culture EPA were possible provided culture medium was replaced at least every 48 h. Inhibitory activity, for which a bioassay was developed, was thereby removed. When nutrients were added frequently in order to prevent nutrient limitation without removing the inhibitory activity in the cultures, cell proliferation ceased after reaching some 30% of the attainable maximal cell number, and the culture gradually deteriorated. Inhibitor-induced culture deterioration was fully reversible when the growth medium was replaced with fresh medium.

When generating kelp hybrid crosses in vitro, small blades are frequently seen in single-sex cultures of microscopic gametophytes as well as in putative hybrid crosses. We demonstrate a polymerase chain reaction (PCR)-based method that quickly determines the presence/absence of possible parental genomes in the observed blades. ITS1 nrDNA fragments that varied in length in a species-specific way were amplified. Amplification products from possible hybrid blades, apogamic blades and parental gametophytes were compared on agarose gels. Using these methods, we were able to identify a number of apogamic blades (i.e. formed directly from gametophytic cells) as well as a true hybrid sporophyte blade in crosses between Alaria marginata (Alariaceae) and Lessoniopsis littoralis (Lessoniaceae), members of two separate families of the Laminariales. To our knowledge this is the first genotypic confirmation of a hybrid kelp.

Twenty-one marine and freshwater specimens of Hildenbrandia from Europe were compared by analyses of morphometric data and sequences of the rbcL chloroplast gene (which codes for the large subunit of the ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme) and the nuclear 18S ribosomal RNA (rRNA) gene. Eleven freshwater specimens, all corresponding to H. rivularis, were collected from the British Isles, Germany, France, Austria, Spain and Italy. Six marine specimens, representing H. rubra, were obtained from Northern Ireland, Wales, France, Norway, Sweden and the Netherlands, while four marine specimens, corresponding to H. crouanii, were collected from Scotland and Germany. Morphometric analyses of vegetative and reproductive characters distinguished three groups of specimens, which correspond to the three species. Parsimony and distance analyses of the rbcL and 18S rRNA genes were performed on these samples and sequences of North American representatives were included to determine their inter-relationships. The rbcL gene sequence analyses separated the marine and freshwater specimens, with the H. rivularis and H. angolensis representatives forming a well- supported monophyletic clade. The rbcL sequence analyses did not resolve the groupings of marine specimens determined in the morphometric analysis. Analyses of the 18S rRNA gene sequences also supported separation of the marine samples from freshwater samples, although the bootstrap and decay support was not as strong as for the rbcL gene. H. rubra formed a monophyletic clade that was weakly supported. In terms of biogeographic trends, the fairly low intraspecific divergence of H. rivularis (0–1·9% for the rbcL gene and 0–3·6% for the 18S rRNA gene) and the positioning of these samples in the gene trees supports the concept that they are derived from marine populations.

Three sand-dwelling species of the marine dinoflagellate genus Sinophysis were examined from intertidal and subtidal North German Wadden Sea sand and are illustrated by light and scanning electron microscopy. Two are described as new: S. stenosoma sp. nov. and S. grandis sp. nov. Sinophysis stenosoma sp. nov. is compressed laterally with a cell size of 37–56 μm long and 21–33 μm wide, a length/width ratio of 1·43–1·89, an epitheca width (dorsoventrally) of 5·5–8·0 μm, and a sulcus length of about half the cell length. Sinophysis grandis sp. nov. is compressed laterally with a cell size of 50–59 μm long and 35·0–41·5 μm wide, a length/width ratio of 1·29–1·61, an epitheca width (dorsoventrally) of 12–17 μm, and a sulcus length of about three-quarters the cell length. The third species, Sinophysis ebriolum (Herdman) Balech, is the most common species in the Wadden Sea. All species occur throughout the year and are all sand-dwelling, non-photosynthetic, with a smooth theca. They can be distinguished by size, length/width ratio, epitheca size, relative sulcal length and thecal pore pattern. A key to the five known Sinophysis species is provided.

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.

Intertidal cohesive estuarine sediments frequently support extensive microphytobenthic biofilms, which are usually dominated by epipelic (motile) diatoms. The distribution of different diatom taxa varies along estuarine gradients, but it is not clear what the causative factors are in determining these patterns. In many estuaries, salinity covaries with other environmental gradients, particularly with nutrient concentrations when the river is the main nutrient source. Autecological information is required to aid interpretation of these field distributions. In this study the maximum specific growth rate (μ) and stationary phase biomass of four common taxa of estuarine epipelic diatoms (Navicula phyllepta, N. perminuta, N. salinarum and Cylindrotheca closterium) were measured in axenic unialgal cultures grown under a range of nitrogen and salinity regimes. All three Navicula species and two different clones of C. closterium showed significant differences in their response to a range of ammonium (10–4000 μM) and nitrate (10–2000 μM) concentrations across a salinity range of 10–35‰. Growth rates of N. phyllepta were significantly lower at ammonium concentrations > 400 μM, but the other three taxa showed little evidence of ammonium toxicity, even at concentrations > 1 mM. On the basis of the response of each species, the following putative ranking of preferences for inorganic nitrogen concentrations (lowest to highest, with salinity optima in parentheses) was constructed: N. phyllepta (10–20‰) – N. perminuta (10–30‰) – N. salinarum (20–35‰)/C. closterium (clone 1, 10–25‰, clone 2, 25–35‰). Such a ranking should be interpreted cautiously, as interclonal differences and adaptation to local environmental conditions can alter the preferences of taxa. These autecological data are partially supported by previously published data on field distribution, and suggest that certain taxa may have value as trophic indicator species.

Three marine planktonic species of the diatom genus Chaetoceros were studied by light and electron microscopy, from net samples collected from different tropical and subtropical areas: coasts of Baja California, the Gulf of Tehuantepec (both in the Pacific Ocean), the Gulf of Mexico (the Atlantic Ocean) and the Indian Ocean. Two of the species, Chaetoceros pseudodichaeta and C. pseudoaurivilli, were originally described by Ikari in 1926 from Japanese waters. This is the second record of them. C. pseudodichaeta is superficially similar to C. dichaeta but detailed morphology differs, especially that of the intercalary setae, which are four-sided in cross-section, with long spines at the edges, and a pattern of two striae between two costae. Chaetoceros pseudoaurivilli is a distinctive species with a characteristic dome- shaped protuberance on terminal valves and the presence of various rimoportulae on, at least, terminal valves of the chain. Finally, C. pseudosymmetricus is a species only reported twice in the Indian Ocean. Its distinctive character is the heteropolarity of terminal setae: one is coarse, long and strongly curved, and the other is delicate, shorter and smoothly curved. It also has a single rimoportula on terminal valves only. The three species are regarded as rare, and consequently our knowledge of their distribution is rather poor. Additional taxonomic comments are provided for the three species.

The marine diatom Skeletonema costatum was grown at different pH values in a semicontinuous cultivation system which kept the pH at a pre-set value. Growth rate was nearly constant at pH 6·5–8·5 (average 2·4 divisions day−1) and declined at pH > 9. Organic carbon (C) production decreased from an average of 5·3 mg l−1 at pH 6·5–8·5 to 2 and 1 mg l−1 at pH 9·0 and 9·4, respectively. Storage β-1,3-linked glucan decreased from a high of 7·1 mg l−1 at pH 6·5 to 0·2 mg l−1 at pH 9·4, and, concomitantly, the percentage of total organic C as glucan C decreased from 60% to 10%. Cellular concentration of amino acids varied from about 62 to 8 mM over the pH range 8·0–9·4. At pH 9·4 the concentration of glutamine was below the detection limit. The extracellular production of carbohydrates in the pH range 6·5–8·5 was only 3·7% (average) of total C production and increased at pH 9·0 and 9·4. The decrease in growth rate at pH > 9·0 might be caused by a decrease in the rate of some important biochemical reactions, as well as by a change in cell membrane properties in this pH range.

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.