In summer, the concentrations of dissolved organic nitrogen compounds are often higher than that of inorganic nitrogen. Under such conditions it would be advantageous if phytoplankton species could utilize organic nitrogen sources, including free or combined amino acids, in addition to inorganic nitrogen. This study focused on histidine, the degradation of which potentially yields three nitrogen atoms for each molecule of histidine. In this work, histidase from Dunaliella tertiolecta, a deaminating enzyme catalysing the first steps of histidine degradation, was purified 4000-fold and partially characterized. The molecular weight of the native enzyme was estimated to be 155 kDa, corresponding to four subunits of 38 kDa. D. tertiolecta histidase is stable in the presence of dithiothreitol and is inactivated by cyanide. Histidinol phosphate, histidine and Mn2+ are effective protectors against cyanide inactivation. The enzyme did not exhibit classical Michaelis-Menten kinetics but showed a relationship between the rate of catalysis (V) and the concentration of substrate (S) that was characteristic of negative allosteric behaviour. A Hill coefficient of 4 was measured for histidine concentrations higher than 22.5 mM. Guanine, xanthine and cytosine nucleotides are inhibitors of D. tertiolecta histidase.

The degree of synchrony of light/dark-adapted Chlamydomonas reinhardtii cell-wall-less cells was analysed by flow cytometry. The cultures exhibited the classical growth curve reported for synchronously dividing cells. However, analysis of the nuclear DNA content revealed a partial asynchrony of cell division. This allowed us to determine that cell division lasts only 1 h 15 min. The changes in the distribution of nuclei in G1 and S phases did not exhibit the DNA peaks expected for highly synchronous cells. However, these peaks were observed for the G2+M nuclei and reflected the number of cell divisions. Modification of culture conditions including temperature, photoperiod, quantum irradiance and culture density affected the degree of synchrony but did not completely abolish the asynchrony. An overlap between the divisions of cells dividing one, two or three times could account for the partial asynchrony observed. In fact, it is possible that cells perform their divisions synchronously but that the timing and/or the duration of the cell divisions is different for the three kinds of cells. The synchrony seems to be perturbed when the cells move from the G1 to the S phase. The transduction of the release signal, allowing cells to pass through the restriction point of the G1 phase, might be too slow and insufficiently homogeneous between the cells to allow perfect synchronization. Finally, if highly synchronized cells are needed to study Chlamydomonas reinhardtii cell cycle, other synchronization procedures should be considered.

The ultrastructure of Peridinium cinctum, was examined by serial sectioning with particular emphasis on the detailed construction of the flagellar apparatus. The pusular system of P. cinctum included two sac pusules in open connection with the flagellar canals; disorganized material was found inside the pusules and near the flagellar pores, indicating excretory activity of the pusules. Several cells had replicated basal bodies, which were oriented toward the cell apex. The functional basal bodies formed an angle of about 65° and were linked by a layered connective, a structure so far seen only in peridinioids. A transverse microtubular root looped around the transverse flagellar canal and nucleated several rows of microtubular extensions that lined most of the transverse sac pusule. A single-stranded root was associated with the surface of the longitudinal basal body almost at a right angle to the basal body triplets. A fibrous connective between the transverse striated root and the longitudinal microtubular root was absent. The striated collars encircling the flagellar pores had a complex structure and were linked by a large connective; a smaller fibrous connective was also found in one cell. An extension of the longitudinal striated collar lined part of the longitudinal sac pusule; it showed a tubular substructure and was perhaps contractile. Bacteria were present in the peripheral cytoplasm of the cell, enveloped by several unit membranes and surrounded by vacuoles; near the sac pusules they appeared disorganized with aggregated ribosomes, possibly the result of digestion.

The production of potent toxins by bloom-, scum- and mat-forming cyanobacteria, in fresh-, brackish and marine waters, appears to be a global phenomenon. Cyanobacterial toxins can also be produced by cyanobacteria from terrestrial sources. The range and number of known cyanobacterial toxins are increasing apace as associated poisoning incidents are investigated, and increasingly powerful analytical methods are applied to complement toxicity-based studies on both natural samples and laboratory isolates of cyanobacteria. Water quality management to reduce toxic cyanobacterial mass developments, and schemes to mitigate the potential effects of cyanobacterial toxins, require an understanding of the occurrence and properties of the toxins and of the exposure routes via which the toxins present risks to health. Here, we review advances in the recognition of cyanobacterial toxins and their toxicity, and of the exposure routes with reference to human health, namely via skin contact, inhalation, haemodialysis and ingestion (the oral route).

Exposure of Fucus spiralis germlings to precise copper concentrations (0 to 844 nM Cu2+) in chemically defined medium demonstrated a relationship between ultrastructural changes and growth retardation with increasing copper concentration. Electron-translucent vesicles, present in ova, which normally disappear after fertilization, accumulated in germlings exposed to Cu2+ above 10·6 nM, suggesting that copper may inhibit a metabolic pathway involved in cell wall formation which is initiated by fertilization. No membrane damage was observed during the exposure period. During a post-exposure period in copper-free medium, recovery occurred (rhizoid extension, apical hair formation) in germlings previously exposed to concentrations below 106 nM Cu2+ and electron-translucent vesicles became granular and disappeared. It is proposed that the electron-translucent vesicles contain a cell wall precursor and that copper inhibits its incorporation into the cell wall, preventing growth and development of the zygote.

That plastids were once free-living cyanobacteria is now taken for granted by many, and for good reasons, for there is a wealth of data – in particular from the comparison of plastid and cyanobacterial genomes – that support this view. There is currently no seriously entertained alternative hypothesis to the view that plastids descend from cyanobacteria. But that was not always the case. Well into the 1970s there was a generally favoured alternative hypothesis, namely that early in evolution plastids arose de novo from within a non-plastid bearing cell (an autogenous origin) rather than through invasion by a cyanobacterium into a non-plastid-bearing cell with subsequent intracellular coexistence and reduction to an organelle (an endosymbiotic origin). Interestingly, the shift from autogenous to endosymbiotic hypotheses during the 1970s was a reversal of state for during the first two decades of this century, the endosymbiont hypothesis for the origins of plastids (and mitochondria, which will not be further discussed here) was very popular among biologists. It fell into disfavour shortly after the First World War, for reasons that are very difficult to summarize briefly, and remained scorned for 50 years (see Sapp, 1994, for an historical account in English, and Höxtermann, 1998, for a succinct historical account in German). So where did the first version of the endosymbiont hypothesis come from? In a nutshell, it came from Konstantin Sergejewiz Merezkovskij (usually written as Constantin Mereschkowsky), a Russian botanist of little standing who worked at a rather small and by no means prominent university in Kasan and who published a very remarkable paper in 1905. We are not aware of any true precedent for his paper, which draws upon three lines of evidence known at the time.

Morphometric analysis and phylogenetic analysis of 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 were carried out on 26 specimens of marine and freshwater Hildenbrandia from North America. Nineteen marine specimens were collected from Alaska to Costa Rica on the Pacific coast and from Newfoundland to Connecticut on the Atlantic coast. Seven freshwater samples were collected from Texas, Costa Rica, St Lucia and Puerto Rico. Three groups of samples were distinguished by morphometric analysis: one containing all freshwater samples (H. angolensis Welwitsch ex West et West), one consisting of a marine sample with parallel tetrasporangial divisions (H. occidentalis Setchell ex Gardner) and one group with non-parallel tetrasporangial divisions (H. rubra (Sommerfelt) Meneghini. These groupings were partially incongruent with those obtained by analyses of the molecular data. Parsimony and distance analyses of the rbcL gene resulted in trees in which Atlantic and Pacific clades were largely resolved. However, an Alaskan sample was included in the Atlantic group, which may indicate a trans-Arctic invasion event. The freshwater samples were paraphyletic for the rbcL gene, among the marine collections, which supports the concept of multiple invasions establishing the freshwater populations in North America. The 18S rRNA gene sequence data indicate that the freshwater samples are monophyletic with the exception of the unresolved position of the H. occidentalis sample. The freshwater samples form a monophyletic clade when multiple outgroups are used. The rbcL data appear to be mutationally saturated above approximately 17% divergence, which makes interpretation of phylogenetic signal among distant groups difficult. This may be a result of the asexual reproduction of the alga.

Several macroalgae from the Swedish West Coast (11 green, 5 red and 11 brown algal species) and two Ulva spp. from the Mediterranean were tested for mechanisms of HCO3− utilization. In all except one (Palmaria palmata), photosynthesis supported by inorganic carbon (Ci) at pH 8.7 was rapidly and substantially (>50%) inhibited by acetazolamide (AZ), an inhibitor of extracellular carbonic anhydrase activity. This suggests that extracellular dehydration followed by uptake of the CO2 formed is an important component of the HCO3− utilization mechanism(s) of these algae. In representatives of most of the green algal genera, e.g. Enteromorpha, Ulva, Chaetomorpha and Monostroma, a specific inhibitor response could be induced. This response, a sensitivity to the anion exchange inhibitor 4,4′-diisothiocyanatostilbene-2,2′-disulphonate (DIDS) in the presence of AZ, is characteristic of an additional mechanism of HCO3− utilization, a direct import of HCO3− across the plasma membrane, which has been previously described only for some Ulva spp. and Enteromorpha intestinalis. The DIDS sensitivity could be induced in several ways, all involving a potential shortage of the supply of CO2 for photosynthesis: through photosynthesis at high pH, through growth in strong light and nutrient-rich seawater, or through photosynthesis in the presence of AZ. Induction of photosynthesis at high pH (which is a specific feature of green macroalgae) was not always accompanied by a development of this specific DIDS sensitivity; it occurred in Chaetomorpha melagonium but not in C. linum. For Ulva lactuca cultivated in natural seawater (in continuous moderate irradiance), induction of DIDS sensitivity by high pH treatment was successful all year round, except during a period in late August. The results thus suggest that green macroalgae capable of developing photosynthesis at high pH are also capable of developing a DIDS-sensitive mechanism for HCO3− uptake, but that some additional conditions (other than low CO2 availability) must be fulfilled. The ability to dehydrate extracellular HCO3− still remained after induction of the direct HCO3− uptake, and the two mechanisms for HCO3− utilization may operate independently of one another. In fact the response to DIDS was more pronounced after the addition of AZ. The possibility that the results are related to a plasticity of the HCO3−-utilizing mechanisms of green macroalgae is discussed. Neither red nor brown macroalgae showed any sensitivity to DIDS in the presence of AZ.

The global carbonate cycle has been controlled and maintained by life processes for at least 3.5 billion years. Within the exogenic carbonate cycle and in very different environments cyanobacteria appear in ‘key positions’ in that they actively and passively influence carbonate cycling. In the carbonate cycle, cyanobacteria play an important and sometimes decisive role. Cycling of carbon and carbonate is linked to biological processes. Some build up specific carbonate structures, some destroy carbonate substrates and others do both simultaneously. All these processes take place from the high mountains down to the sea in various terrestrial and freshwater as well as marine environments. The photosynthetic activity of cyanobacteria, their extracellular polymeric substances and possibly also their adherent heterotrophic bacteria are responsible for the construction of various carbonate structures and the ability to penetrate carbonate material. Boring activity of euendoliths results in biological corrosion and disintegration of carbonate surfaces. Grazing organisms on carbonate surfaces colonized by epi- and endolithic cyanobacteria produce specific biokarst forms and specific grains which can contribute to nearshore sedimentation. Biological corrosion and abrasion together constitute bioerosion. The results of all these processes are calcareous crusts, typical traces and biokarst forms which in many cases have a high fossilization potential, and therefore can be powerful ecological, palaeoecological and facies indicators in recent as well as in fossil environments.

Ultrastructural observations on the vegetative filaments, motile cells and processes leading to zoospore differentiation in Chrysonephos lewisii are presented. Vegetative filaments are embedded in a mucilaginous envelope, preserved by Alcian blue fixation, which favours their aggregation and also the attachment of bacteria, diatoms and various types of debris. The filaments are provided with an external wall consisting of microfibrils positive to the PATAg test and ConA-colloidal gold labelling. Inside the filaments the vegetative cells differentiate into zoospores. This accounts for the ultrastructural similarity between vegetative cells and zoospores, although zoospores are typically provided with a flagellar apparatus. They share an external cell covering or theca, vacuolar or cytoplasmic ‘scale-like structures’, protruding stalked pyrenoids, scattered nucleoids of plastid DNA and absence of a photoreceptor-eyespot complex. The flagellar apparatus of zoospores is characterized by two distinct basal plates, the central pair of flagellar microtubules originating some distance above the transitional plate and basal body angle less than 90°. These results provide strong taxonomic evidence that this genus should be placed in the Sarcinochrysidales sensu stricto, rather than remaining in Chrysomeridales. The key role of the Golgi apparatus in the synthesis of different structural elements – depending upon the functional stages of the cells – and in the processes of exocytosis during zoosporogenesis is also discussed.

Molecular phylogenetic relationships among sexually isolated subpopulations (syngens) of the morphologically defined volvocacean flagellate Pandorina morum (O. F. Müller) Bory are inferred from sequences of the internal transcribed spacer regions (ITS-1 and ITS-2) of the nuclear ribosomal DNA repeats. The resolved phylogenetic lines are composed of strains with very similar sets of traditional characters, such as zygote aggregation pattern, chromosome number and cell division timing, and are correlated with the distribution pattern of the xanthophyll loroxanthin [(3R,3′R,6′R)-β,ε-carotene-3,19,3′-triol]. Thus, loroxanthin, a phylogenetic character within the Volvocales, is also of value at the infraspecific level in P. morum, a morphologically uniform species complex, within the Volvocales.

We recently reported (Gueneau et al., 1998) the existence of a GC-rich region downstream of the trnR gene which was conserved in the plastidial genome of three algae: Thalassiosira weissflogii, Odontella sinensis (Bacillariophyceae) and Porphyra purpurea (Rhodophyceae). We identified this GC-rich region as a possible vestigial ORF and noted additional high similarity to an open reading frame (ORF) in a plastid from a fourth alga, Pavlova lutherii (Haptophyceae); sequence data from P. lutherii did not extend to the TΨC tRNA arm. We now report that this region instead encodes a recently recognized type of RNA known as tmRNA (Fig. 1A).

The dependence of the cellular (biogenic) and frustule-associated (mineralized) silica content of the diatoms Navicula salinarum and Thalassiosira weissflogii on salinity and aluminium conditions was studied in order to make it possible to manipulate silicification in vitro and maximize it to levels required for physico-chemical frustule characterization by physisorption, X-ray scattering analysis and NMR, which is our ultimate objective. Enrichments with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T. weissflogii. Aluminium additions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silica matrix of the frustule. In contrast, lowering the salinity from 28 practical salinity units (PSU) to 20 and 15 resulted in a significant increase in the biogenic silica content per cell of both species, which is in line with an increase in density of the chemically derived silica under low salt conditions. The silica content per cell was variable during culture growth; increase in cell densities (during exponential growth) was accompanied by a decrease in contents of biogenic silica per cell. Electron microscopy and energy dispersive X-ray analysis (EDXA) support the chemical analyses and also suggest higher biogenic silica contents at lower salinities. The results indicate that besides silicate concentration and pH, the concentration of salts is an important inorganic factor that affects the silica polymerization inside the silica deposition vesicle of diatoms.

In the paper:

Studies on the Dasyaceae. 3. Towards a phylogeny of the Dasyaceae (Ceramiales, Rhodophyta), based on comparative rbcL gene sequences and morphology, by Y. S. D. M. de Jong, A. W. G. van der Wurff, W. T. Stam & J. L. Olsen (Eur. J. Phycol., 33: 187–201, 1998), the cladogram in Fig. 2 was inadvertently printed with mixed bold and narrow lines. The bold lines have no significance.