Structural aspects of akinete germination in the cyanobacterium Anabaena variabilis

Electronmicroscopical investigations of light activated akinetes in different phases before outgrowth of the germinating cell showed two alterations in the akinete envelope, obviously in connection with the germination process. After induction of germination the akinetes show formation of an expanding more or less electron dense layer between the outer cell wall layer (outer membrane, L_IV) and the condensed part of the akinete coat (the transformed sheath of the vegetative cell). Between this new formed layer and the mentioned part of the akinete coat thick laminar layers are deposited which contain alternately electron dense and electron transparent strata. The expanding layer is assumed to be a mucous layer which acts as swelling body causing, after bursting of the layered shell, the expulsion of the germinating cell in the manner characteristic for Anabaena variabilis.     

Environmental influences on akinete germination of Anabaena circinalis and implications for management of cyanobacterial blooms

Certain cyanobacteria, including the noxious bloom-forming species Anabaena circinalis Rabenhorst, produce thick-walled reproductive structures (akinetes) which may serve as a resting stage and ensure survival during adverse growth conditions. The effect of certain environmental variables (temperature, salinity and desiccation) on akinete germination of A. circinalis was investigated under laboratory conditions, to determine the conditions under which germination was inhibited. The overall aims were to provide a broader understanding of the life history and ecology of this species and to assess suppression of akinete germination as a potential management strategy for control of cyanobacterial blooms in the lower Murray River, Australia. The results indicated a marked threshold of temperature and salinity tolerance for germination of A. circinalis, but the latter was not within a range that could be successfully manipulated in a natural ecosystem. However, it was found that desiccation of akinetes for moderately short periods can significantly impair their capacity to germinate. It is, therefore, speculated that allowing periodic drying of shallow wetlands adjacent to the Murray River and in other areas may reduce the size of the inoculum for population growth by reducing viability of akinetes in surface sediments.     

Geographical segregation of the neurotoxin-producing cyanobacterium Anabaena circinalis

Blooms of the cyanobacterium Anabaena circinalis are a major worldwide problem due to their production of a range of toxins, in particular the neurotoxins anatoxin-a and paralytic shellfish poisons (PSPs). Although there is a worldwide distribution of A. circinalis, there is a geographical segregation of neurotoxin production. American and European isolates of A. circinalis produce only anatoxin-a, while Australian isolates exclusively produce PSPs. The reason for this geographical segregation of neurotoxin production by A. circinalis is unknown. The phylogenetic structure of A. circinalis was determined by analyzing 16S rRNA gene sequences. A. circinalis was found to form a monophyletic group of international distribution. However, the PSP- and non-PSP-producing A. circinalis formed two distinct 16S rRNA gene clusters. A molecular probe was designed, allowing the identification of A. circinalis from cultured and uncultured environmental samples. In addition, probes targeting the predominantly PSP-producing or non-PSP-producing clusters were designed for the characterization of A. circinalis isolates as potential PSP producers.     

Characterization of akinete differentiation in the cyanobacterium Anabaena azollae

Differentiation of akinetes was investigated in the filamentous cyanobacterium Anabaena azollae Stras. In this organism all pre-existing vegetative cells are capable of developing into akinetes. Standard sporulation medium (SSM) was used to synchronously induce the formation of akinetes, while cultures in Allen and Arnon (AA/8) medium were used as controls.This paper describes the changes in photosynthetic pigments and total soluble proteins in these cultures over a 25-day period encompassing akinete differentiation. Heterocyst frequencies and nitrogenase activity were also monitored during the same period in both media. SDS-PAGE results indicated that specific proteins were synthesized in a manner correlated with akinete differentiation. The results demonstrate that in cultures undergoing akinete development, some of the photosynthetic pigments are maintained, nitrogen-fixation and heterocyst differentiation are suppressed, and the cells synthesize a variety of specific proteins.     

Effect of canavanine and other amino acid analogues on akinete formation in the cyanobacterium Anabaena cylindrica

Addition of the arginine analogue, canavanine, to cultures of nitrogen-fixing Anabaena cylindrica at the onset of akinete formation, resulted in the development of akinetes randomly distributed within the filament, in addition to those adjacent to heterocysts. The total frequency of akinetes increased up to five-fold. A feature of akinetes is their increased content of cyanophycin granules (an arginine-aspartic acid polymer) and addition of canavanine to cultures at an earlier stage resulted in entire filaments becoming agranular and containing agranular akinetes. The effects on akinete pattern appeared to be specific for canavanine since other amino acid analogues, although increasing the frequency of akinetes (approximately two-fold), had no effect on their position relative to heterocysts. In ammonia-grown, stationary phase cultures of A. cylindrica, akinetes were observed adjacent to proheterocysts and in positions more than 20 cells from any heterocyst. These observations indicate that nitrogen fixation and heterocysts are not essential for akinete formation in A. cylindrica, although the availability of a source of fixed nitrogen does appear to be a requirement.These results suggest that during exponential growth some aspect of the physiology of vegetative cells suppresses their development into akinetes and that the role of the heterocyst may not be one of direct stimulation of adjacent vegetative cells to form akinetes, but the removal or negation of the inhibition within them. A model for akinete formation and the involvement of canavanine is given.     

The influence of light and nutrients on buoyancy, filament aggregation and flotation of Anabaena circinalis

At Chaffey Dam, New South Wales, Australia, Anabaena circinalis filaments accumulated at the surface as diurnal surface layer thermal stratification developed. Previously buoyant, homogeneously distributed colonies accumulated in the top 2 m, but a proportion lost buoyancy. Similarly, a percentage of A.circinalis suspended in bottles lost buoyancy at depths experiencing >30% surface irradiance (Io). Nutrient addition reduced the proportion of filaments that lost buoyancy following a full day of high irradiance. The greatest axial linear dimension (GALD) was measured for A.circinalis deployed in bottles at three depths in the reservoir. GALD increased in samples exposed to 1 and 30% Io by the following day. The rank order of GALD from smallest to largest grouped samples exposed to 70, 30 and 1% Io, suggesting that increasing GALD is a function of irradiance. The increased GALD of biomass units was attributed to aggregation of filaments in low light. The enlargement of biomass units increased the mean floating velocity, supporting the theory that filament aggregation may be a strategy, utilized by light-limited filaments, to increase light exposure. High irradiance increased the carbohydrate content of cells and decreased the floating velocity of filaments.      

The fine structure of akinete formation and germination in Cylindrospermum

Summary Akinete formation and germination were studied in a species of Cylindrospermum using the electron microscope. The differentiation of a vegetative cell into an akinete is characterized by cell enlargement, sheath condensation, deposition of several spore envelope layers, including a dense fibrillar layer and deposition of large cyanophycin granules. The mature akinete in addition to the multilayered envelope retains internally a large number of cyanophycin granules, a photosynthetic thylakoid system, polyhedral bodies, lipid deposits and nucleoplasmic regions. Germination of the akinete can take place in several modes differing in detail. Most frequently the spore envelope remains intact and the germling which may or may not have divided emerges through a pore at one end of the envelope. The photosynthetic thylakoid system appears to increase by the fusion of small vesicles found in the cytoplasm. Alpha-granules are numerous and cyanophycin is nearly absent in the germling.     

Effects of UV-light andγ-rays on the survival,akinete formation and akinete germination inStigeoclonium pascheri

The damage produced by UV light to any of the three different stages of the life cycle of the parent generation of the green algaS. pascheri, i.e. akinetes, germinating akinetes and vegetative cells remained up to the stage of germination of akinetes of the first generation and no deleterious effect was reported thereafter. Lower doses of γ-rays (25–75 Gy) increased the percentage germination of akinetes and germinating akinetes of parent generation. The percentage germination of akinetes, germinating akinetes, survival of colonies originated from vegetative cells and sporulation of cells of the parent generation decreased with increasing doses from 100 to 300 Gy. The γ-induced effect to any of the three different stages was not transferred to the subsequent stage of algal generation.     

Extinction of cells of cyanobacterium Anabaena circinalis in the presence of humic acid under illumination

Laboratory experiments targeting the effect of humic acid (HA) on the cell lysis of cyanobacterium Anabaena circinalis have been performed. Light irradiation was found to be an important factor for the cell lysis phenomenon, whereas intracellular hydrogen peroxide (H₂O₂) might be a chemical factor for the process. An exogenous H₂O₂ concentration of 1.0 mg l⁻¹ was determined as the threshold for cell survival. Our results indicated that HA or its possible product(s) of photochemical reaction can induce damage to intracellular catalase under artificial illumination, which leads intracellular H₂O₂ to be accumulated to an abnormally high concentration, eventually resulting in cell death. Moreover, H₂O₂ released into the culture from dead cells can damage other cells, which in turn brings about the population extinction.     

Identification of an Na(+)-dependent transporter associated with saxitoxin-producing strains of the cyanobacterium Anabaena circinalis

Blooms of the freshwater cyanobacterium Anabaena circinalis are recognized as an important health risk worldwide due to the production of a range of toxins such as saxitoxin (STX) and its derivatives. In this study we used HIP1 octameric-palindrome repeated-sequence PCR to compare the genomic structure of phylogenetically similar Australian isolates of A. circinalis. STX-producing and nontoxic cyanobacterial strains showed different HIP1 (highly iterated octameric palindrome 1) DNA patterns, and characteristic interrepeat amplicons for each group were identified. Suppression subtractive hybridization (SSH) was performed using HIP1 PCR-generated libraries to further identify toxic-strain-specific genes. An STX-producing strain and a nontoxic strain of A. circinalis were chosen as testers in two distinct experiments. The two categories of SSH putative tester-specific sequences were characterized by different families of encoded proteins that may be representative of the differences in metabolism between STX-producing and nontoxic A. circinalis strains. DNA-microarray hybridization and genomic screening revealed a toxic-strain-specific HIP1 fragment coding for a putative Na(+)-dependent transporter. Analysis of this gene demonstrated analogy to the mrpF gene of Bacillus subtilis, whose encoded protein is involved in Na(+)-specific pH homeostasis. The application of this gene as a molecular probe in laboratory and environmental screening for STX-producing A. circinalis strains was demonstrated. The possible role of this putative Na(+)-dependent transporter in the toxic cyanobacterial phenotype is also discussed, in light of recent physiological studies of STX-producing cyanobacteria.     

Geographical Segregation of the Neurotoxin-Producing Cyanobacterium Anabaena circinalis

Blooms of the cyanobacterium Anabaena circinalis are a major worldwide problem due to their production of a range of toxins, in particular the neurotoxins anatoxin-a and paralytic shellfish poisons (PSPs). Although there is a worldwide distribution of A. circinalis, there is a geographical segregation of neurotoxin production. American and European isolates of A. circinalis produce only anatoxin-a, while Australian isolates exclusively produce PSPs. The reason for this geographical segregation of neurotoxin production by A. circinalis is unknown. The phylogenetic structure of A. circinalis was determined by analyzing 16S rRNA gene sequences. A. circinalis was found to form a monophyletic group of international distribution. However, the PSP- and non-PSP-producing A. circinalis formed two distinct 16S rRNA gene clusters. A molecular probe was designed, allowing the identification of A. circinalis from cultured and uncultured environmental samples. In addition, probes targeting the predominantly PSP-producing or non-PSP-producing clusters were designed for the characterization of A. circinalis isolates as potential PSP producers.     

Identification of an akinete marker gene in Anabaena variabilis

Cyanobacteria that form akinetes as well as heterocysts present a rare opportunity to investigate the relationships between alternative differentiation processes and pattern formation processes in a single bacterium. Because no akinete marker gene has been identified, akinete formation has been little studied genetically. We report the first identification of an akinete marker gene.     

Spatial and temporal variability in filament length of a toxic cyanobacterium (Anabaena affinis)

1. This study examines the distribution of Anabaena affinis filament lengths under natural conditions as a function of depth and season, and in the laboratory as a function of growth phase. Because Anabaena affinis is only toxic when consumed, both its filament length and position in the water column are important determinants of its potential impact on zooplankton populations. 2. Star Lake (Norwich, Vermont, U.S.A.), a natural, eutrophic pond, remained thermally stratified throughout the Anabaena bloom. Filament number and length differed significantly with both sampling date and water depth. Most filaments occurred at 0.5 m, particularly at the height of the bloom. Throughout the entire water column average filament length decreased from approximately 0.53 mm in May to 0.14 mm in July. The shortest filaments occurred at the 2.5 m depth. Filament length distributions (combined for all depths) for 29 May, 12 June and 3 July, corresponding to the beginning, middle and end of the bloom, respectively, differed significantly among the three dates. These patterns most likely reflect variable growth conditions, both during the season and in the water column. 3. In the laboratory, Anabaena filament length was affected by medium composition and growth phase. Filaments were significantly longer when grown in MBL than in ASM medium. Also, the average length of Anabaena filaments grown in MBL changed significantly as cultures aged; by day 13 filament length (2.01 ± 0.38 mm, mean ± SD) was twice that on day 0 (0.97 ± 0.71 mm). As cell concentration continued to increase, mean filament length gradually decreased.     

光对种子萌发的影响机理研究进展

种子萌发是植物成功实现天然更新的关键环节, 需要适宜的温度、水分或光照条件。对于需光性种子, 光照是决定其萌发与否或萌发率高低的主要因素。光对植物种子萌发的影响不仅是一个复杂的生理过程, 也是受到调控的信号传递和基因表达过程。该文系统总结了影响种子萌发的光照属性、光与水/热耦合作用和种子的光属性(光敏色素)与种子萌发的关系, 明确了光调控种子萌发的生态意义; 重点综述了种子内光敏色素调控种子萌发的生理反应模式和光敏色素的光信号转导途径。试图为全面评估光对种子萌发的影响和将来开展更深入的研究提供参考。     

Sporulation in the filamentous cyanobacterium Anabaena cylindrica

Sporulation in the filamentous cyanobacterium Anabaena cylindrica involves the transformation of a vegetative cell into a thick-walled resistant structure. Because this process occurs at predictable loci in each filament and involves a significant increase in cell size, the course of sporulation in a culture can be quantitatively determined. Sporulation occurs during the late logarithmic phase of a culture, a time of slow but unbalanced growth. Under the conditions imployed here, sporulation is not a synchronous event either between or within filaments. The information in this paper provides an estimate of the rate of spore differentiation and supports the previous notion that in the formation of strings of more than one spore, a gradient of spore maturation exists.     

Molecular Phylogeny of Anabaena circinalis and Its Identification in Environmental Samples by PCR

Although the cyanobacterium Anabaena circinalis occurs worldwide, Australian isolates are believed to exclusively possess the saxitoxin group neurotoxins (paralytic shellfish poisons). Identification of A. circinalis in a mixed population is complicated due to limited morphological differences between Anabaena species. Sequence analysis of the DNA-dependent RNA polymerase (rpoC1) gene from 24 Anabaena isolates, including 12 designated A. circinalis, permitted a phylogenetic analysis to be performed. In addition, an A. circinalis-specific PCR was developed and tested successfully on environmental samples.