Akinete Formation Research Articles

Temperature-dependent akinete formation strategies of the harmful cyanobacterium Dolichospermum circinale

Cyanobacteria from the orders Nostocales and certain Stigonematales form akinetes, spore-like dormant cells that allow them to survive adverse environmental conditions. Temperature is known to be one of the key factors affecting akinete formation, but there is currently little known about akinete formation during cell growth over a wide range of temperature conditions and its relation to the overall survival strategy of cyanobacteria. Therefore, in the current study, we conducted a temperature-controlled experiment to analyze the akinete formation of a harmful cyanobacterium Dolichospurmum circinale using a growth chamber. We measured the concentration and size of both vegetative cells and different types of akinetes (free, attached, and empty type) under varying temperatures (5–25 °C). We also analyzed the buoyant ability and vertical migration velocity of trichomes along with changes in the volume of vegetative cells and akinetes. The total akinete concentration and ratio (number of akinetes to total number of cells) were both found to be higher at high temperatures (20–25°C) than they were at low temperatures (5–10°C) (p<0.05). Meanwhile, the rate of formation of akinetes (both free and attached akinetes) was highest at low temperature (10 °C) and decreased with increasing temperature. The rate of empty akinete formation increased with increasing temperature and was highest at 25°C, indicating that most of the akinetes produced under high temperature conditions germinated. The change in vegetative cell size was proportional to the increase in the growth rate in response to increasing temperature (p<0.05). At high temperature, vegetative cells exhibited positive buoyancy and higher vertical migration velocity, while at low temperature, they exhibited negative buoyancy and relatively low migration velocity. Akinete size was larger at low temperature than it was at high temperature. These findings suggest that akinetes play an important role in maintaining populations in the water column, with a link between akinete formation and germination during summer cyanobacteria blooms. This information is expected to contribute to a deeper understanding of the D. circinale life cycle.

Survival and Development Strategies of Cyanobacteria through Akinete Formation and Germination in the Life Cycle

Eutrophic freshwater ecosystems are vulnerable to toxin-producing cyanobacteria growth or harmful algal blooms. Cyanobacteria belonging to the Nostocales order form akinetes that are similar to the seeds of vascular plants, which are resting cells surrounded by a thick membrane. They overwinter in sediment and germinate when conditions become favorable, eventually developing into vegetative cells and causing blooms. This review covers the cyanobacterial akinete of the Nostocales order and summarizes the environmental triggers and cellular responses involved in akinete germination and formation based on data from the literature. It also emphasizes the intimate and dynamic relationship that exists between the germination and formation of akinete in the annual life cycle of cyanobacteria. After comparing many published data, it is found that the tolerance ranges for factors affecting both akinete germination and formation do not differ significantly and are broadly consistent with the tolerance ranges for vegetative cell growth. However, the optimal range varies with different species and strains of cyanobacteria. The life cycle of cyanobacteria, as a result of akinete germination and formation, has a seasonal periodicity and spatial connectivity between the water column and the sediment. However, during the summer growing season, intimate coupling between akinete formation and germination can occur in the water column, and this can contribute to high population densities being maintained in the water column. During this time, shallow sediment could also provide suitable conditions for akinete germination, thereby contributing to the establishment of water column populations. The information summarized in this review is expected to help improve our shared understanding of the life cycle of the Nostocales cyanobacteria while also providing insights into the monitoring and management of harmful algal blooms.

Characterizing the microbial metagenome of calcareous stromatolite formations in the San Felipe Creek in the Anza Borrego Desert.

We describe the metagenome composition, community functional annotation, and prokaryote diversity in calcareous stromatolites from a dry stream bed of the San Felipe Creek in the Anza Borrego Desert. Analyses show a community capable of nitrogen fixation, assimilatory nitrate reduction, biofilm formation, quorum sensing, and potential thick-walled akinete formation for desiccation resistance.

Neocylindrospermum variakineticum gen. & sp. nov. (Nostocales, Cyanobacteria), a novel genus separated from Cylindrospermum using a polyphasic method

ABSTRACT The large genus Cylindrospermum is polyphyletic as currently circumscribed, and its cryptic diversity requires detailed taxonomic evaluation. Two novel cyanobacterial strains were isolated by scraping the surface of wet wood above the water level, in the stream below Mae Phun Waterfall in the northern part of Thailand, and investigated with a polyphasic study combining morphological and molecular analyses. The morphological features of these two novel strains fitted the original description of the genus Cylindrospermum but they differed from known species in cell size, exospore features and akinete formation. Phylogenetic analyses based on 16S rRNA, 16S–23S rRNA ITS region, rpoC1 and nifH genes indicated that the novel strains formed a unique clade with strongly supported values from BI and ML methods. This unique clade was distantly separated from the Cylindrospermum sensu stricto clade and other Cylindrospermum-like clades. Molecular analyses showed that the two novel strains had <96.3% 16S rRNA gene sequence similarity and <83.2% 16S–23S rRNA ITS region sequence similarity to known cyanobacterial genera and other related taxa. Furthermore, ITS secondary structures between the 16S–23S rRNA gene in both strains also exhibited singular folding patterns of D1–D1', Box-B and V3 helix, thereby distinguishing them from the closely related taxa. The establishment of Neocylindrospermum gen. nov. with the type species as N. variakineticum sp. nov. is proposed according to the International Code of Nomenclature for Algae, Fungi, and Plants.

Changes in Envelope Structure and Cell-Cell Communication during Akinete Differentiation and Germination in Filamentous Cyanobacterium Trichormus variabilis ATCC 29413.

Planktonic freshwater filamentous cyanobacterium Trichormus variabilis ATCC 29413 (previously known as Anabaena variabilis) can differentiate heterocysts and akinetes to survive under different stress conditions. Whilst heterocysts enable diazotrophic growth, akinetes are spore-like resting cells that make the survival of the species possible under adverse growth conditions. Under suitable environmental conditions, they germinate to produce new vegetative filaments. Several morphological and physiological changes occur during akinete formation and germination. Here, using scanning electron microscopy (SEM), we found that the mature akinetes had a wrinkled envelope, and the surface of the envelope smoothened as the cell size increased during germination. Thereupon, the akinete envelope ruptured to release the short emerging filament. Focused ion beam–scanning electron microscopy (FIB/SEM) tomography of immature akinetes revealed the presence of cytoplasmic granules, presumably consisting of cyanophycin or glycogen. In addition, the akinete envelope architecture of different layers, the exopolysaccharide and glycolipid layers, could be visualized. We found that this multilayered envelope helped to withstand osmotic stress and to maintain the structural integrity. Furthermore, by fluorescence recovery after photobleaching (FRAP) measurements, using the fluorescent tracer calcein, we found that intercellular communication decreased during akinete formation as compared with the vegetative cells. In contrast, freshly germinating filaments restored cell communication.

The Ecophysiological Performance and Traits of Genera within the Stichococcus-like Clade (Trebouxiophyceae) under Matric and Osmotic Stress.

Changes in water balance are some of the most critical challenges that aeroterrestrial algae face. They have a wide variety of mechanisms to protect against osmotic stress, including, but not limited to, downregulating photosynthesis, the production of compatible solutes, spore and akinete formation, biofilms, as well as triggering structural cellular changes. In comparison, algae living in saline environments must cope with ionic stress, which has similar effects on the physiology as desiccation in addition to sodium and chloride ion toxicity. These environmental challenges define ecological niches for both specialist and generalist algae. One alga known to be aeroterrestrial and euryhaline is Stichococcus bacillaris Nägeli, possessing the ability to withstand both matric and osmotic stresses, which may contribute to wide distribution worldwide. Following taxonomic revision of Stichococcus into seven lineages, we here examined their physiological responses to osmotic and matric stress through a salt growth challenge and desiccation experiment. The results demonstrate that innate compatible solute production capacity under salt stress and desiccation tolerance are independent of one another, and that salt tolerance is more variable than desiccation tolerance in the Stichococcus-like genera. Furthermore, algae within this group likely occupy similar ecological niches, with the exception of Pseudostichococcus.

Cartap hydrochloride induced stress response in Anabaena variabilis ARM 441

Cartap hydrochloride is a moderately hazardous nereistoxin analogue insecticide that is predominantly applied in paddy fields of India, at a recommended dose of 10 μg ml−1 to kill chewing and sucking insect pests of rice crop. Toxicity of cartap hydrochloride was studied on non-target free-living nitrogen fixing cyanobacterium Anabaena variabilis ARM 441 commonly used as algal biofertilizer in rice cultivation. Anabaena sp. could tolerate commercial grade insecticide up to 30 μg ml−1. However, at the recommended dose of 10 μg ml−1, it caused reduction in algal growth, total nitrogen and heterocyst frequency by 47.28, 24.29 and 17.72% respectively, as well as photosynthetic pigments under pure culture conditions. Scanning electron micrographs revealed cell rupture and breakage in filaments due to cartap exposure with the formation of akinetes. Cartap hydrochloride induced stress, since level of superoxide dismutase, peroxidase and catalase were increased by 108.57, 187.5 and 117% respectively. Generation of superoxide radicals and hydrogen peroxide were also increased by 152.48 and 34% respectively. Lipid peroxidation was increased by 31.03%, whereas there was decline in ascorbate content by 48.45%, however the glutathione content was increased by 128.57%. Increase in osmolytes such as proline from 8.6 to 32.8% and sucrose from 61.22 to 90.13% indicates their possible role in overcoming cartap induced oxidative stress and can be helpful in assessing its detrimental effect on Anabaena variabilis ARM 441, since cyanobacterial biofertilizers are purposely used in paddy fields as nitrogen contributors.

Effects of Salinity and Temperature on the Growth, Toxin Production, and Akinete Germination of the Cyanobacterium Nodularia spumigena

Harmful blooms of the cyanobacterium Nodularia spumigena occur in several parts of the world, and this species is notorious for its ability to produce cyanotoxins, such as nodularin. This species is also able to perform akinete differentiation, thus favoring the success of their populations under adverse conditions. In southern Brazil, N. spumigena has developed large blooms in experimental shrimp production farms. In this study, we implemented an experiment to evaluate the influence of salinity and temperature on several physiological attributes of a Brazilian strain of N. spumigena: cell growth, nodularin production, the ability to form akinetes and germination potential. A factorial experiment (3x4) was conducted to test the effects of temperature (15, 23, and 30 °C) and salinity (1, 7, 15, and 30 ppm) on the growth, production of nodularin and the differentiation of akinetes. The germination potential of the akinetes was tested after incubation for 30, 60, 90, 120, 180 and 360 days at 4 °C in the dark. N. spumigena grew under a wide range of salinity and temperatures, but salinity had a greater influence. The highest cell densities at the temperatures tested were observed at salinity 7 ppm and the lowest at salinity 1 ppm. The toxin nodularin was produced in all treatments, but there was an inverse relationship between the content of nodularin per cell and cell density. The akinetes were also differentiated in all the treatments, with the exception of S1T30. However, the largest proportion of akinetes (65% of the cells) was observed in the treatment with low cell growth at salinity 15 and 15 °C, possibly indicating an effort of the population to survive over the long term. The akinetes germinated in all treatments, but those cultivated at salinity 7 and 15 ppm (except S15T30) showed a higher percentage of germination and, in contrast, the lowest germination rate was observed in the treatments with salinity 1 ppm. Thus, the low salinity (1) at all the temperatures tested has the potential to inhibit N. spumigena blooms, since it hampers cell growth and the formation of akinetes.

Taxonomic transfer of Gongrosira fluminensis Fritsch (Chaetophorales, Chlorophyceae) to Lithotrichon Darienko et Pröschold (Ulvales, Ulvophyceae) based on morphological observation and phylogenetic analyses

One green algal specimen from China was identified as Gongrosira fluminensis Fritsch, due to its unique morphology that pseudoparenchymal basal stratum with rounded or polygonal cells grew into dense little-branched upright threads that reached approximately the same height with specific akinete formation and germination and formed strong cushions without calcification. Examination of the ultrastructural characteristics of plasmodesmata and pyrenoid confirmed that Gongrosira fluminensis Fritsch should be excluded from the order Chaetophorales. The phylogenetic evidence based on DNA sequence data from the nucleus (18S rDNA, ITS rDNA) and chloroplast (tufA) sequences clearly revealed that the Gongrosira fluminensis Fritsch should be classified in the Ulvales (Ulvophyceae) as the new combination species of the genus Lithotrichon Darienko et Proschold, instead of the Chaetophorales (Chlorophyceae). More specimens in conjunction with natural morphological investigation and molecular analyses are required to reevaluate the microfilamentous genus Gongrosira Kutzing and reveal hidden diversities among the Ulvophyceae.

Phylogenetic connection among close genera of Aphanizomenonaceae (Cyanobacteria): Amphiheterocytum gen. nov., Cylindrospermopsis and Sphaerospermopsis

Aphanizomenonaceae is a recently proposed family of Cyanobacteria encompassing 12 planktic genera. The formation and distribution of heterocytes and akinetes along the trichome are the main morphological characteristics of this family. In this context, Sphaerospermopsis and Cylindrospermopsis are close related genera both molecularly and morphologically differing by 1) rounded akinetes adjacent to the heterocytes disposed along the trichome in Sphaerospermopsis; 2) apical heterocytes and akinetes separated by a few cells (2–3) in Cylindrospermopsis. Recently, during a survey on cyanobacteria from tropical Brazilian eutrophic lakes, strains with similarities to Sphaerospermopsis and Cylindrospermopsis were isolated. Aiming to elucidate the phylogenetic placement of these strains, we performed morphological, physiological and molecular studies. The Brazilian strains were confirmed to be closely related to Sphaerospermopsis and Cylindrospermopsis, but the phylogeny, based on 16S rRNA gene and 16S–23S rRNA intergenic region, clustered them in a well-defined clade separate from these genera. According to these results, we detailed the morphology, physiology, and phylogeny of the proposed new genus Amphiheterocytum, which showed potential to create blooms in tropical shallow lakes.

Growth and special structures production of Nostoc paludosum (Nostocaceae, Cyanobacteria) under nutrient starvation and different light intensities

The effects of the absence of nitrogen and phosphorus on AA media and three different light intensities (100% Light, 60.0± 2.7 μmol m-2 s-1, 50% Light, 30.0± 2.7 μmol m-2 s-1, and 13.5% Light, 8.1± 2.7 μmol m-2 s-1) on cell production and synthesis of heterocytes and akinetes were determined in a strain of Nostoc paludosum. In the experiment concerning the absence of nutrients, significant variations were observed between the control group and the groups with absence of nutrients, especially in cell numbers and in synthesis of heterocytes and akinetes. The absence of nitrogen boosted the formation of heterocytes and the absence of phosphorus produced the most akinetes. As for the different light conditions, the growth curves determined for each treatment showed that cell synthesis is slightly affected by the reduction of illuminance. The different light intensities are capable of reducing the maximum growth rates of Nostoc paludosum, with 13.5% light restriction being the most effective on limiting the cell growth rate and inducing the formation of akinetes. The synthesis of heterocyte does not seem directly correlated to light intensity, being better explained by nutritional factors. The data found contributes to the understanding of some of the factors involving growth and synthesis of special structures in Nostoc paludosum.

Influence of Silver, Zinc Oxide and Copper Oxide Nanoparticles on the Cyanobacterium Calothrix elenkinii

An investigation was undertaken to compare the effect of different concentrations of three metal nanoparticles i.e. silver (Ag), zinc oxide (ZnO) and copper oxide (CuO), on a cyanobacterium Calothrix elenkinii. Chlorophyll accumulation in C. elenkinii acclimatised to Ag NPs after 1 week at concentrations of 0.1 and 0.5 mg L−1, while it reduced at 1.0 mg L−1. A similar trend was recorded with CuO NPs, in terms of protein content with incubation time. Nitrogen-fixing potential also showed a gradual increase with Ag NPs, up to concentrations of 0.2 mg L−1 during incubation of 1 week, but sharply decreased at 0.5 mg L−1 after 48 h. Both the nitrogen-fixing potential and protein content were stimulated in the presence of CuO NPs. ZnO NPs, in general, were not compatible with the growth and metabolic activities of this cyanobacterium. Microscopic studies revealed the aggregation of NPs on the surface of C. elenkinii filaments, resulting in distinct morphological changes, particularly enhanced hormogonia and akinete formation. It is envisaged that with the increase in incubation period, C. elenkinii acclimatised to counteract the effect of Ag and CuO NPs and utilise them for its growth. The adaptive strategies of this cyanobacterium make it an ideal model system for future nanotoxicological studies.

8. 黄緑色藻類Tribonema bombycinumのアキネート形成による耐凍性の上昇(平成11年度第45回低温生物工学会研究報告)

8. 黄緑色藻類Tribonema bombycinumのアキネート形成による耐凍性の上昇(平成11年度第45回低温生物工学会研究報告)

Lack of Methylated Hopanoids Renders the Cyanobacterium Nostoc punctiforme Sensitive to Osmotic and pH Stress.

To investigate the function of 2-methylhopanoids in modern cyanobacteria, the hpnP gene coding for the radical S-adenosyl methionine (SAM) methylase protein that acts on the C-2 position of hopanoids was deleted from the filamentous cyanobacterium Nostoc punctiforme ATCC 29133S. The resulting ΔhpnP mutant lacked all 2-methylhopanoids but was found to produce much higher levels of two bacteriohopanepentol isomers than the wild type. Growth rates of the ΔhpnP mutant cultures were not significantly different from those of the wild type under standard growth conditions. Akinete formation was also not impeded by the absence of 2-methylhopanoids. The relative abundances of the different hopanoid structures in akinete-dominated cultures of the wild-type and ΔhpnP mutant strains were similar to those of vegetative cell-dominated cultures. However, the ΔhpnP mutant was found to have decreased growth rates under both pH and osmotic stress, confirming a role for 2-methylhopanoids in stress tolerance. Evidence of elevated photosystem II yield and NAD(P)H-dependent oxidoreductase activity in the ΔhpnP mutant under stress conditions, compared to the wild type, suggested that the absence of 2-methylhopanoids increases cellular metabolic rates under stress conditions.IMPORTANCE As the first group of organisms to develop oxygenic photosynthesis, Cyanobacteria are central to the evolutionary history of life on Earth and the subsequent oxygenation of the atmosphere. To investigate the origin of cyanobacteria and the emergence of oxygenic photosynthesis, geobiologists use biomarkers, the remnants of lipids produced by different organisms that are found in geologic sediments. 2-Methylhopanes have been considered indicative of cyanobacteria in some environmental settings, with the parent lipids 2-methylhopanoids being present in many contemporary cyanobacteria. We have created a Nostoc punctiforme ΔhpnP mutant strain that does not produce 2-methylhopanoids to assess the influence of 2-methylhopanoids on stress tolerance. Increased metabolic activity in the mutant under stress indicates compensatory alterations in metabolism in the absence of 2-methylhopanoids.

Carbon assimilation and accumulation of cyanophycin during the development of dormant cells (akinetes) in the cyanobacterium Aphanizomenon ovalisporum.

Akinetes are spore-like non-motile cells that differentiate from vegetative cells of filamentous cyanobacteria from the order Nostocales. They play a key role in the survival and distribution of these species and contribute to their perennial blooms. Here, we demonstrate variations in cellular ultrastructure during akinete formation concomitant with accumulation of cyanophycin; a copolymer of aspartate and arginine that forms storage granules. Cyanophycin accumulation is initiated in vegetative cells few days post-exposure to akinete inducing conditions. This early accumulated cyanophycin pool in vegetative cells disappears as a nearby cell differentiates to an akinete and stores large pool of cyanophycin. During the akinete maturation, the cyanophycin pool is further increased and comprise up to 2% of the akinete volume. The cellular pattern of photosynthetic activity during akinete formation was studied by a nano-metric scale secondary ion mass spectrometry (NanoSIMS) analysis in 13C-enriched cultures. Quantitative estimation of carbon assimilation in vegetative cells and akinetes (filament-attached and -free) indicates that vegetative cells maintain their basal activity while differentiating akinetes gradually reduce their activity. Mature-free akinetes practically lost their photosynthetic activity although small fraction of free akinetes were still photosynthetically active. Additional 13C pulse-chase experiments indicated rapid carbon turnover during akinete formation and de novo synthesis of cyanophycin in vegetative cells 4 days post-induction of akinete differentiation.

Deciphering the mechanisms against oxidative stress in developing and mature akinetes of the cyanobacterium Aphanizomenon ovalisporum

Cells of filamentous cyanobacteria of the orders Nostocales and Stigonematales can differentiate into dormant forms called akinetes. Akinetes play a key role in the survival, abundance and distribution of the species, contributing an inoculum for their perennial blooms. In the cyanobacterium Aphanizomenon ovalisporum, potassium deficiency triggers the formation of akinetes. Here we present experimental evidence for the production of reactive oxygen species (ROS) during akinete development in response to potassium deficiency. The function of ROS as a primer signal for akinete differentiation was negated. Nevertheless, akinetes acquired protective mechanisms against oxidative damage during their differentiation and maintained them as they matured, giving akinetes advantages enabling survival in harsh conditions.

Biosynthesis of monodisperse gold nanoparticles by green alga Rhizoclonium and associated biochemical changes

In search for a suitable algal bioreagent for monodisperse (homogenous mixture of same size and shape) gold nanoparticle production, a eukaryotic green alga Rhizoclonium fontinale was found to produce spherical (~16 nm size) nanoparticles, when exposed to 15 mg L−1 auric chloride solution at pH 9 for 72 h. In this connection, the effects of different concentrations of gold ions, amount of biomass, and pH of the exposure medium on nanoparticle synthesis were evaluated. Upregulation of stress-related compounds like, carotenoids and stress enzymes—catalase, ascorbate peroxidase, and super oxide dismutase of gold-exposed biomass, were recorded up to 24 h at the onset of gold nanoparticles synthesis, afterwards enzyme activity ceased but the nanoparticle production continued up to 72 h of exposure. Cell wall thickening, rapid akinete formation, pigment loss, giant cell formation, pyknosis, and purple coloration of the filaments during algae–gold interaction were also evident. The nanoparticles were characterized by UV–Vis spectroscopy, dynamic light scattering, high-resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. Environmental scanning electron microscopy indicated cell wall distortion of the filaments in Au3+ exposure as well as only intracellular production of gold nanoparticles by Rhizoclonium as no nanoparticles were found on the surface.

Population dynamics and akinete formation of an invasive and a native cyanobacterium in temperate lakes

Most cyanobacteria of the order Nostocales in the temperate zone survive winters in the sediment as akinetes. The present study compared seasonal population dynamics and the formation of akinetes of the invasive cyanobacterium Cylindrospermopsis raciborskii with the native species Aphanizomenon gracile in three German lakes. The effect of light, temperature and growth rate on akinete formation was investigated in the field over a 3-year period and additionally in a culture study with nine different C. raciborskii and A. gracile strains. The results of the field study showed, for both species, a strong negative correlation between akinete formation and light and temperature, and a weaker negative correlation between akinete formation and the net growth rate of the populations. Akinete formation of the isolates was mainly affected by temperature and not by light intensity. The investigation of the seasonal population dynamics showed that the growth period of C. raciborskii in the pelagic zone was on average 64 days shorter compared with A. gracile. However, the amount of akinetes produced by C. raciborskii during 1 year was, in more than half of the cases, similar or higher than that produced by native A. gracile. The ratio of akinetes to total cell number was significantly higher for C. raciborskii compared with A. gracile, 9 and 2%, respectively. Therefore, we conclude that C. raciborskii populations in temperate lakes have a similar potential for an akinete-based life cycle performance as do the native A. gracile populations.

Potassium deficiency triggers the development of dormant cells (akinetes) in Aphanizomenon ovalisporum (Nostocales, Cyanoprokaryota)(1).

Akinetes are spore-like nonmotile cells that differentiate from vegetative cells of filamentous cyanobacteria from the order Nostocales. They play a key role in the survival and distribution of these species and contribute to their perennial blooms. Various environmental factors were reported to trigger the differentiation of akinetes including light intensity and quality, temperature, and nutrient deficiency. Here, we report that deprivation of potassium ion (K(+) ) triggers akinete development in the cyanobacterium Aphanizomenon ovalisporum. Akinetes formation is initiated 3 d-7d after an induction by K(+) depletion, followed by 2-3weeks of a maturation process. Akinete formation occurs within a restricted matrix of environmental conditions such as temperature, light intensity or photon flux. Phosphate is essential for akinete maturation and P-limitation restricts the number of mature akinetes. DNA replication is essential for akinete maturation and akinete development is limited in the presence of Nalidixic acid. While our results unequivocally demonstrated the effect of K(+) deficiency on akinete formation in laboratory cultures of A.ovalisporum, this trigger did not cause Cylindrospermopsis raciborskii to produce akinetes. Anabaena crassa however, produced akinetes upon potassium deficiency, but the highest akinete concentration was achieved at conditions that supported vegetative growth. It is speculated that an unknown internal signal is associated with the cellular response to K(+) deficiency to induce the differentiation of a certain vegetative cell in a trichome into an akinete. A universal stress protein that functions as mediator in K(+) deficiency signal transduction cascade, may communicate between the lack of K(+) and akinete induction.

A polyketide interferes with cellular differentiation in the symbiotic cyanobacterium Nostoc punctiforme

Nostoc punctiforme is a filamentous cyanobacterium capable of forming symbiotic associations with a wide range of plants. The strain exhibits extensive phenotypic characteristics and can differentiate three mutually exclusive cell types: nitrogen-fixing heterocysts, motile hormogonia and spore-like akinetes. Here, we provide evidence for a crucial role of an extracellular metabolite in balancing cellular differentiation. Insertional mutagenesis of a gene of the polyketide synthase gene cluster pks2 led to the accumulation of short filaments carrying mostly terminal heterocysts under diazotrophic conditions. The mutant has a strong tendency to form biofilms on solid surfaces as well as in liquid culture. The pks2(-) strain keeps forming hormogonia over the entire growth curve and shows an early onset of akinete formation. We could isolate two fractions of the wild-type supernatant that could restore the capability to form long filaments with intercalary heterocysts. Growth of the mutant cells in the neighbourhood of wild-type cells on plates led to a reciprocal influence and a partial reconstruction of wild-type and mutant phenotype respectively. We postulate that extracellular metabolites of Nostoc punctiforme act as life cycle governing factors (LCGFs) and that the ratio between distinct factors may guide the differentiation into different life stages.