Michael Herdman

Generic Assignments, Strain Histories and Properties of Pure Cultures of Cyanobacteria

Summary: On the basis of a comparative study of 178 strains of cyanobacteria, representative of this group of prokaryotes, revised definitions of many genera are proposed. Revisions are designed to permit the generic identification of cultures, often difficult through use of the field-based system of phycological classification. The differential characters proposed are both constant and readily determinable in cultured material. The 22 genera recognized are placed in five sections, each distinguished by a particular pattern of structure and development. Generic descriptions are accompanied by strain histories, brief accounts of strain properties, and illustrations; one or more reference strains are proposed for each genus. The collection on which this analysis was based has been deposited in the American Type Culture Collection, where strains will be listed under the generic designations proposed here.

Phylum BX. Cyanobacteria

The oxygenic photosynthetic procaryotes comprise a single taxonomic and phylogenetic group (see master phylogenetic tree of the Bacteria). In the last edition of the Manual, two separate groups were described, but it is now apparent that members of the Prochlorales simply represent different, unrelated genera which fall into the main cluster of the Cyanobacteria (see Oxygenic Photosynthetic Bacteria, below). The principal character that defines all of these oxygenic photosynthetic procaryotes is the presence of two photosystems (PSII and PSI) and the use of H2O as the photoreductant in photosynthesis. Although facultative photo- or chemo-heterotrophy may occur in some species or strains, all known members are capable of photoautotrophy (using CO2 as the primary source of cell carbon).

Genome Size of Cyanobacteria

Summary: The genome sizes of 128 strains of cyanobacteria, representative of all major taxonomic groups, lie in the range 1.6 × 109 to 8.6 × 109 daltons. The majority of unicellular cyanobacteria contain genomes of 1.6 × 109 to 2.7 × 109 daltons, comparable in size to those of other bacteria, whereas most pleurocapsalean and filamentous strains possess larger genomes. The genome sizes are discontinuously distributed into four distinct groups which have means of 2.2 × 109, 3.6 × 109, 5.0 × 109 and 7.4 × 109 daltons. The data suggest that genome evolution in cyanobacteria occurred by a series of duplications of a small ancestral genome, and that the complex morphological organization characteristic of many cyanobacteria may have arisen as a result of this process.

Deoxyribonucleic Acid Base Composition of Cyanobacteria

Summary: The DNA base compositions of 176 strains of cyanobacteria were determined by thermal denaturation or by CsCl density gradient centrifugation. A summary of all data now available for this prokaryotic group is presented and the taxonomic and evolutionary implications are discussed.

Akinetes of the Cyanobacterium Nostoc PCC 7524: Macromolecular Composition, Structure and Control of Differentiation

Summary: Synchronized akinete differentiation occurred following the transition from exponential to non-exponential (linear) growth, the major trigger being energy limitation. Young akinetes first accumulated cyanophycin, then developed a multilayered extracellular envelope and a thickened wall. The dry weight, chlorophyll a, glycogen and carbon contents of mature akinetes were greater than those of vegetative cells, while their contents of DNA, RNA, protein, phycocyanin and nitrogen were similar to those of vegetative cells. Akinetes were resistant to desiccation and low temperatures, but not to temperatures above the maximum for vegetative cell growth. In N2-grown cultures heterocyst differentiation ceased at the end of exponential growth, while cell division continued, and akinetes first appeared in a regular pattern at a fixed distance (9 cells) from the nearest heterocyst. Exogenous NH4 + inhibited the differentiation of heterocysts and, in their absence, akinetes developed in irregular positions. The regular spatial pattern imposed on akinete differentiation by heterocysts was, like the heterocyst spatial pattern itself, independent of N2 fixation. Similar changes in both patterns induced by 7-azatryptophan suggested that they share a common mechanism of control.

Prochlorococcus marinus Chisholm et al. 1992 subsp. pastoris subsp. nov. strain PCC 9511, the first axenic chlorophyll a2/b2-containing cyanobacterium (Oxyphotobacteria).

The formal description of Prochlorococcus marinus Chisholm et al. 1992, 299 was based on the non-axenic nomenclatural type, strain CCMP 1375T. The purification and properties of the axenic strain PCC 9511, derived from the same primary culture (SARG) as the type species, are reported here. Prochlorococcus PCC 9511 differs from the latter in possessing horseshoe-shaped thylakoids, exhibiting a low chlorophyll b2 content and lacking phycoerythrin, but shares these phenotypic properties with Prochlorococcus strain CCMP 1378. This relationship was confirmed by 16S rRNA sequence analyses, which clearly demonstrated that the axenic isolate is not co-identic with the nomenclatural type. Strain PCC 9511 has a low mean DNA base composition (32 mol% G+C) and harbours the smallest genome of all known oxyphotobacteria (genome complexity 1.3 GDa = 2 Mbp). Urea and ammonia are the preferred sources of nitrogen for growth, whereas nitrate is not utilized. Several different organic phosphorus compounds efficiently replace phosphate in the culture medium, indicative of ecto-phosphohydrolase activity. In order to distinguish strain PCC 9511 from the nomenclatural type, a new subspecies is proposed, Prochlorococcus marinus Chisholm et al. 1992 subsp. pastoris subsp. nov.

rDNA analyses of planktonic heterocystous cyanobacteria, including members of the genera Anabaenopsis and Cyanospira.

The taxonomic coherence and phylogenetic relationships of 11 planktonic heterocystous cyanobacterial isolates were examined by investigating two areas of the rRNA operon, the 16S rRNA gene (rrnS) and the internal transcribed spacer (ITS) located between the 16S rRNA and 23S rRNA genes. The rrnS sequences were determined for five strains, including representatives of Anabaena flos-aquae, Aphanizomenon flos-aquae, Nodularia sp. and two alkaliphilic planktonic members of the genera Anabaenopsis and Cyanospira, whose phylogenetic position was previously unknown. Comparison of the data with those previously published for individual groups of planktonic heterocystous cyanobacteria showed that, with the exception of members assigned to the genus Cylindrospermopsis, all the planktonic strains form a distinct subclade within the monophyletic clade of heterocystous cyanobacteria. Within this subclade five different phylogenetic clusters were distinguished. The phylogenetic groupings of Anabaena and Aphanizomenon strains within three of these clusters were not always consistent with their generic or specific assignments based on classical morphological definitions, and the high degree of sequence similarity between strains of Anabaenopsis and Cyanospira suggests that they may be assignable to a single genus. Ribotyping and additional studies performed on PCR amplicons of the 16S rDNA or the ITS for the 11 planktonic heterocystous strains demonstrated that they all contain multiple rrn operons and ITS regions of variable size. Finally, evidence is provided for intra-genomic sequence heterogeneity of the 16S rRNA genes within most of the individual isolates.

[22] Cellular differentiation: Hormogonia and baeocytes

Publisher Summary This chapter focuses on cellular differentiation in Hormogonia and Baeocytes. Hormogonia are produced by many, but not all, heterocystous cyanobacteria. The term hormogonium (Greek: hormos, chain; goneta, generation) has been traditionally employed in the phycological literature to describe any motile filament released from an immotile, ensheathed parental trichome of both heterocystous and aheterocystous cyanobacteria. Pleurocapsalean cyanobacteria reproduce by multiple fission. As both hormogonia (as defined here) and baeocytes are produced by rapid cell division in the absence of growth, they can be considered to be analogous structures, even though they are respectively filamentous and unicellular. Like baeocytes, hormogonia serve for dispersal. Those of many strains are motile while others, unlike the parental filament, contain gas vacuoles. Again, they may be released from a sessile parental colony and glide, float, or be washed by water currents to a new location. In general, hormogonia, like baeocytes, are formed abundantly after transfer of stationary phase cultures (which normally contain only vegetative trichomes) to fresh medium.

Metabolic control of phycocyanin degradation in the cyanobacterium Synechocystis PCC 6803: a glucose effect

SUMMARY: Following transfer to medium lacking a nitrogen source, cells of Synechocystis PCC 6803 continued to divide, giving a doubling of cell number after 40 h. Phycocyanin degradation commenced immediately after the transfer, with a rapid phase lasting 5 h in which 50% of the phycocyanin disappeared and a slow second phase in which the phycocyanin content decreased to 10% of its initial value by 24 h. In the presence of glucose, a utilizable carbon source for this facultatively heterotrophic cyanobacterium, phycocyanin was degraded initially at a rate 60% of that observed in the absence of the sugar and proteolysis was almost completely inhibited after 6 h when only 27% of the phycocyanin had been lost; cell division ceased at this time. Photosynthetic O2 evolution decreased rapidly in the presence of glucose and the cells consumed O2 in the light after 7 h, indicative of a switch to oxidative metabolism; net O2 uptake in the absence of glucose occurred only after approximately 25 h. Inhibition of phycocyanin degradation by glucose required metabolism of the sugar, probably via the oxidative pentose phosphate cycle, and appeared to result from irreversible inactivation of the protease.

Regulation of protein phosphorylation in the cyanobacterium Anabaena strain PCC 7120

SUMMARY: Protein kinase activities have been detected in cell-free extracts of the cyanobacterium Anabaena PCC 7120. At least 12 polypeptides in the soluble fraction were phosphorylated in vitro at the expense of [y -32P]ATP and the pattern of phosphorylation was shown to be regulated by intermediary metabolites and other effectors, at physiological concentrations. Glucose 6-phosphate exerted a regulatory effect on a phosphopolypeptide of M r 56000 (p56) by stimulating a protein phosphatase, whereas ribulose 5-phosphate inhibited the corresponding protein kinase. In addition, DTT and the calmodulin antagonist trifluoperazine influenced the phosphorylation state of several different polypeptides, indicative of control by redox conditions and a calmodulin-like mediator, respectively. Furthermore, it was established that the phosphorylation of p56 required Mg2+ (> 100 μM) whereas that of a polypeptide of M r 16000 occurred in the absence of Mg2+ and was inhibited by high concentrations (> 1 mM) of this cation. Several of the phosphopolypeptides detected in vitro corresponded in mobility on SDS-PAGE to species phosphorylated in vivo.