Akinete Formation Research Articles

Extract of Microcystis water bloom affects cellular differentiation in filamentous cyanobacterium Trichormus variabilis (Nostocales, Cyanobacteria)

Cyanobacteria produce a wide spectrum of biologically active compounds such as microcystins, whose effects on photoautotrophic organisms and role in the aquatic ecosystems have been most intensively discussed and studied. In our study, we examined effects of semipurified Microcystis extract containing microcystins (0.2–20 nM corresponding to 0.2–20 μg L−1) on age-induced cell differentiation of filamentous cyanobacterium Trichormus variabilis. The heterocyst and akinete formation was significantly decreased after exposure to extract containing 2 or 20 nM of microcystins within 10 days of exposure. To our knowledge, this is the first information about effects of metabolites produced by planktonic cyanobacteria on cell differentiation in filamentous cyanobacteria. Since the effects were induced by environmentally relevant concentrations of microcystins, our observations may indicate not only that microcystins or other bioactive peptides could affect conservation, overwintering, and nitrogen-fixation in filamentous cyanobacteria under environmental conditions but also contribute to the understanding of possible signaling function of cyanobacterial metabolites.

Ultrastructure of cyanobacterium Nostoc sp. f. Blasia cell forms in persisting populations

Cell clusters formed in persistent populations of Nostoc sp. f. Blasia, a cyanobacterium capable of cell differentiation, under prolonged storage in the dark at low temperatures were studied for the first time. Cell reorganization was observed, including changes in the ultrastructure of thylakoids, the cell wall peptidoglycan layer, and carboxysomes. Subcellular structures involved in intercellular communication within the clusters were revealed (structures similar to microplasmodesms and contact pores, secretory vesicles, etc.) Persistence of cyanobacterial populations was concluded to result from formation not only of specialized dormant cells (akinetes), but also L-forms, as well as from the modification changes of the clustered vegetative cells. A cluster containing the vegetative cells and L-like forms within a common intercellular matrix is considered a structural unit at the supracellular level, which is responsible for survival of cyanobacterial populations when mass akinete formation does not occur.

Life cycle strategies of bloom-forming, filamentous cyanobacteria in the Baltic Sea

Abstract Late-summer blooms of the Nostocalean cyanobacteria Nodularia spumigena , Aphanizomenon flos-aquae and Anabaena spp., occur annually over vast areas of the Baltic Sea. Apart from the planktonic bloom period, the annual cycle of these cyanobacteria is poorly known. With a combination of pelagial monitoring, sedimentation traps and germination experiments, we studied the annual cycle of formation, sedimentation and germination of cyanobacterial akinetes (resting stages) in the northern Baltic Sea during 2005–2006. Akinetes were only rarely encountered in the fall plankton, but they were abundant in the near-bottom sedimentation trap from the middle of July to August. All three taxa formed akinetes, but the numbers of A. flos-aquae akinetes were generally low. Of the three taxa, only Anabaena spp. were regularly found to germinate from the sediment samples. Estimates of Anabaena viable seed bank size ranged from 205 to 1913 akinetes g −1 (ww) of sediment. Aphanizomenon flos-aquae never germinated, and N. spumigena did so only occasionally. It is concluded that the three co-occurring taxa probably have different life cycle strategies, A. flos-aquae being holoplanktonic and Anabaena spp. planktonic during summer, but overwintering as akinetes in the bottom. Summer populations of N. spumigena may originate from both sedimented akinetes and trichomes that overwinter in the water column.

Impact of the inoculum size on the population of Nostocales cyanobacteria in a temperate lake

Akinete formation enables cyanobacteria of the order Nostocales to survive adverse conditions (e.g winters in temperate zones) and is assumed to be a prerequisite for the invasion of tropical species like Cylindrospermopsis raciborskii into the temperate zones. We chose C. raciborskii for a 4-year study of the seasonal patterns of all life cycle stages in the pelagic and benthic zones of a shallow eutrophic lake in NE Germany. Akinete production started when C. raciborskii populations reached maximum cell abundance and water temperatures started to decrease, resulting in an increase in the number of akinetes at the sediment surface. The number of akinetes on the sediment (inoculum for the next year) was population size dependent. It decreased slightly in winter but dropped sharply from end of April to June/July, reflective of germination. Thus, germination started at temperatures of about 13°C Typically, first filaments appeared in June and populations peaked in August. However, the size of the population in summer was not determined by the size of the inoculum. Consequently, it is assumed that population development is determined by growth conditions and loss processes after germination.

Fluorescence microscopy reveals variations in cellular composition during formation of akinetes in the cyanobacterium Aphanizomenon ovalisporum

Akinetes are differentiated, thick-walled, resting cells produced by many species of the filamentous heterocystous cyanobacteria belonging to the Nostocales and Stigonematales. They are commonly developed after the end of exponential growth and are normally resistant to low temperatures and to desiccation for extended periods. Akinetes maintain a low level of metabolic activities such as photosynthesis, respiration and de novo synthesis of proteins and lipids. In a previous study we reported that mature akinetes of Aphanizomenon ovalisporum maintained only residual photosynthetic activity. However the cellular abundance of PSI and PSII reaction centres increased in akinetes relative to exponentially grown vegetative cells while the ratio of PSI to PSII remained relatively unchanged. In the present study we provide spectral and biochemical evidence that akinetes lose their phycobilisomes during their development. DAPI stained trichomes of exponentially grown cultures emitted fluorescence signals attributed to nucleic acids and polyphosphate granules. Akinetes that developed in akinete-induced cultures were preferentially enriched with nucleic acids and deprived of polyphosphate bodies, whereas adjacent vegetative cells were loaded with polyphosphate bodies. Examination of akinetes from akinete-induced cultures of A. ovalisporum by spectral confocal laser scanning microscopy indicated reduction of the phycobilisome fluorescence signal. Mature isolated akinetes could be divided into two distinct populations: (i) a majority of akinetes that lost their phycobilisome fluorescence signal, but maintain chlorophyll fluorescence attributed to the reaction centres, and (ii) a minor group of mature akinetes that maintained a high phycobilisome fluorescence signal.

The influence of light quality on akinete formation and germination in the toxic cyanobacterium Anabaena circinalis

Physiological control of akinete formation and subsequent germination is likely to be important in understanding and predicting how natural populations of cyanobacteria respond to their environment. While previous research has indicated nutrient limitation may be important in akinete formation new results presented here indicate that in the toxic and bloom-forming species Anabaena circinalis there was a profound effect of spectral quality. Under 40μmolphotonsm−2s−1 photosynthetically active irradiance (PAR) of predominately red irradiance akinete production was maximal at 2.1×10−4akinetesvegetativecell−1d−1, some 3000 times greater than the 6.5×10−8akinetesvegetativecell−1d−1 observed under equivalent PAR but predominately blue light. For cells grown under a range of predominantly red, white and green irradiance even short exposures to blue light reduced akinete formation rates by a factor of ten relative to controls, indicating that exposure to blue light inhibits akinete formation. Germination of akinetes was not influenced by the irradiance under which akinetes were formed: 88±4.1% (mean±1 S.D.) of akinetes germinated with no evidence of an effect on germination success due to their production under predominately red, white or green irradiance (germination of akinetes produced under blue light was not tested). Spectral quality had a significant impact on both vegetative cell and germling growth rates. The results indicate a significant reduction in the cellular differentiation of A. circinalis vegetative cells into akinetes that is mediated by blue light. In an ecological context the production of akinetes will be greater in environments with less blue light; potentially including those with slower flow, more stratification, less vertical mixing and more turbidity. The resulting spatial pattern of akinete production is likely to influence the location of akinetes in sediments and the development of subsequent blooms from excysting germlings.

Agent‐based modeling of the complex life cycle of a cyanobacterium (Anabaena) in a shallow reservoir

The cyanobacterium Anabaena flos‐aquae and many other phytoplankton species have a complex life cycle that includes a resting stage (akinete). We present a new agent‐based (also known as individual‐based) model of Anabaena that includes the formation and behavior of akinetes. The model is part of a coupled Eulerian‐Lagrangian model and can reproduce the main features of the observed seasonal and interannual population dynamics in Bugach Reservoir (Siberia), including an unexpectedly large bloom in a year with low nutrient concentrations. Model analysis shows that the internal loading of phosphorus (P) due to germination from the sediment bed is ~10% of the total input. However, most of the long‐term nutrient uptake for Anabaena occurs in the sediment bed, which suggests that the sediment bed is not just a convenient overwintering location but may also be the primary source of nutrients. An in silico tracing experiment showed that most water column cells (~90%) originated from cells located in the sediment bed during the preceding winter. An in silico gene knockout experiment (akinete formation is prohibited) showed that the formation of resting stages is of critical importance to the survival of the population on an annual basis. A nutrient‐reduction management scenario indicates that Anabaena densities increase because they are less sensitive to water column nutrient levels (because of the sediment bed source) than other species.

The cyanobacterial genus Macrospermum.

This small tropical cyanobacterial group, containing Anabaena volzii LEMMERMANN and a few related species (A. fuellebornii SCHMIDLE, A. unispora GARDNER, A. mysorensis GONZALVES & KAMAT), differs substantially phenotypically from all other planktic or benthic Anabaena types, mainly by the subsymmetric structure of the trichomes, type of akinete formation and restricted ecology. The taxonomic uniformity of all other Anabaenalike clusters (typical benthic Anabaena, planktic Anabaena subg. Dolichospermum, Trichormus, Aphanizomenon, Cuspidothrix) was already supported by molecular analyses. All of them also have their typical morphological markers, which are clearly different from the "Anabaena volzii - cluster". Therefore, this group can not be classified in any of the mentioned revised genera and must be described as a separate generic entity of heterocytous cyanobacteria (although they have not been sequenced to date). The new genus Macrospermum is therefore defined in my article with 4 related, morphologically distinguishable species. The generic name is selected according to the unusually large akinetes.

Planktic morphospecies of the cyanobacterial genus Anabaena = subg. Dolichospermum - 2. part: straight types.

This small tropical cyanobacterial group, containing Anabaena volzii Lemmermann and a few related species (A. fuellebornii SCHMIDLE, A. unispora GARDNER, A. mysorensis GONZALVES & KAMAT), differs substantially phenotypically from all other planktic or benthic Anabaena types, mainly by the subsymmetric structure of the trichomes, type of akinete formation and restricted ecology. The taxonomic uniformity of all other Anabaenalike clusters (typical benthic Anabaena, planktic Anabaena subg. Dolichospermum, Trichormus, Aphanizomenon, Cuspidothrix) was already supported by molecular analyses. All of them also have their typical morphological markers, which are clearly different from the "Anabaena volzii - cluster". Therefore, this group can not be classified in any of the mentioned revised genera and must be described as a separate generic entity of heterocytous cyanobacteria (although they have not been sequenced to date). The new genus Macrospermum is therefore defined in my article with 4 related, morphologically distinguishable species. The generic name is selected according to the unusually large akinetes.

Survival and reproduction in some algae under stress conditions

Pithophora oedogonia and Cladophora glomerata survived lowest 60 and 58%, respectively, in June when the pond diurnal water temperature (PDWT) increased to a maximum of 28 degrees C. The lowering of PDWT only by 1 degrees C in July improved survivability of both algae to their almost maximum level of 100 and 96%, respectively. Further lowering of PDWT to 17-22 degrees C in November initiated akinete formation in P. oedogonia. The process of akinete initiation, maturation and germination continued till April when PDWT increased to 20-24 degrees C, but not beyond that in May when PDWT was 21-26 degrees C. By this time, probably all akinetes have germinated in situ, and the alga was entirely vegetative. P. oedogonia population is not synchronous in nature, since during the 5-6-month reproductive season, some filaments were in active vegetative stage, some had akinete initiation, some had completed akinete formation, and some had akinetes germinating. C. glomerata grew dense vegetative in November and initiated (zoo)sporangial primordia formation (to some extent) in February (when PDWT was lowest, viz. 10-14 degrees C) till April. Meanwhile, no (zoo)-sporangial primordia either produced any zoospore or germinated into a germ tube; and all released their cytoplasmic content and died (along with some vegetative cells) with an increase in PDWT to 21-26 degrees C in May. Vaucheria geminata vegetative patches appeared on the soil surface, 2nd week of January by lowering of atmospheric diurnal temperature (ADT) to 9-16 degrees C in the 1st week. The alga started sexual reproduction by the 2nd week of March (when ADT increased to 20-23 degrees C) and completed the process of reproduction by the 1st week of April (when ADT increased to 24-26 degrees C) and died thereafter. P. oedogonia, C. glomerata and V. geminata survived better and longer in submerged conditions than air-exposed (which was true for P. oedogonia and C. glomerata aquatic habitat and also indicated that the soil alga V. geminata could survive to some extent if submerged in rain water). P. oedogonia formed akinetes and C. glomerata (zoo)sporangial primordia only in submerged condition and not when air-exposed on moist soil surface. V. geminata did not complete the life cycle both under submerged and air-exposed conditions. Vegetative survival in P. oedogonia, C. glomerata, V. geminata, Aphanothece pallida, Gloeocapsa atrata, Scytonema millei, Myxosarcina burmensis, Phormidium bohneri, Oscillatoria animalis, O. subbrevis, Lyngbya birgei, L. major, Microcoleus chthonoplastes and Rhizoclonium crassipellitum, reproduction in P. oedogonia, C. glomerata and V. geminata, cell division in A. pallida and G. atrata, heterocyst and false branch formation in S. millei, all, were adversely affected at approximately 28.5 degrees C for t12 h at light intensity of approximately 160 micromol m(-2) s(-1); high intensity does not ameliorate high temperature damage to any algae. The presence of liquid water, than its absence, outside the different algae moderated the severity of heat to some extent but not when the heat was severe.

Planktic morphospecies of the cyanobacterial genus Anabaena = subg. Dolichospermum - 1. part: coiled types.

This small tropical cyanobacterial group, containing volzii Lemmermann and a few related species (A. fuellebornii SCHMIDLE, A. unispora GARDNER, A. mysorensis GONZALVES & KAMAT), differs substantially phenotypically from all other planktic or benthic types, mainly by the subsymmetric structure of the trichomes, type of akinete formation and restricted ecology. The taxonomic uniformity of all other Anabaenalike clusters (typical benthic Anabaena, planktic subg. Dolichospermum, Trichormus, Aphanizomenon, Cuspidothrix) was already supported by molecular analyses. All of them also have their typical morphological markers, which are clearly different from the Anabaena volzii - cluster. Therefore, this group can not be classified in any of the mentioned revised genera and must be described as a separate generic entity of heterocytous cyanobacteria (although they have not been sequenced to date). The new genus Macrospermum is therefore defined in my article with 4 related, morphologically distinguishable species. The generic name is selected according to the unusually large akinetes.

Seasonal dynamics of akinetes of Anabaena flos-aquae in bottom sediments and water column of small Siberian reservoir

Seasonal dynamics of Anabaena flos-aquae (Lyngb.) Breb., including vegetative cells, akinetes and akinete envelopes, in bottom sediments and water column at both littoral and deeper central stations of a small Siberian reservoir was studied. Two types of akinetes were observed: in the first half of summer Anabaena formed akinetes, which served for vegetative reproduction and germinated in water column soon after differentiation, while in the second half of summer the akinetes produced served as a resting stages, which were deposited to bottom sediments. Canonical correlation analyses revealed that decrease of water temperature was the main environmental factor that stimulated the akinete formation. In contrast to the general opinion, concentration of inorganic phosphorus slightly, but positively influenced the akinete formation. Thus, akinetes formed in response to the temperature decrease, needs a certain level of this nutrient. At littoral and open-water stations abundance and seasonal dynamics of akinetes in water column and their sinking pattern were very similar. However, seasonal dynamics of abundance of akinetes in sediments in these two reservoir locations differed: whereas the abundance of akinetes in open water increased permanently during the summer, that in the littoral decreased soon after their sedimentation. The cause for decrease in abundance of akinetes in bottom sediments in winter is unknown.

Characterization of a model system for the study of Nostoc punctiforme akinetes

Nostoc punctiforme is a filamentous cyanobacterium that is capable of dark heterotrophy and cellular differentiation into nitrogen-fixing heterocysts, motile hormogonia, or spore-like akinetes. The study of akinete differentiation at the molecular level has been limited by the asynchronous development and limited number of akinetes formed within a filament. A system in which to study the development and genetic regulation of akinetes was investigated using a zwf mutant lacking glucose-6-phosphate dehydrogenase, the initial enzyme of the oxidative pentose phosphate pathway. Upon dark incubation in the presence of fructose, the zwf(-) strain ceased growth and differentiated into akinete-like cells, whereas the wild-type strain exhibited heterotrophic growth. Dark-induced zwf akinetes exhibited periodic acid-Schiff staining characteristics identical to that observed for wild-type akinetes, and synchronous induction of akinetes occurred in treated cultures. Dark-induced zwf akinetes exhibited increased resistance to the environmental stresses of desiccation, cold, or treatment with lysozyme relative to vegetative cells of both strains. Transcription of the avaK akinete marker gene was strongly induced in developing zwf akinetes as shown by Northern blotting and green fluorescent protein transcriptional reporter fusions. ATP levels did not vary significantly between dark incubated strains, indicating that a signal other than energy level may trigger akinete formation. This phenotypic and genetic evidence showing near-synchronous induction of dark-induced zwf akinetes indicates that this system will provide a valuable tool for the molecular genetic study of akinete development in N. punctiforme.

Some nostocalean Cyanoprokaryotes from lentic habitats of Eastern and Southern Africa

The article comprises the second part (Nostocales) of our study of little known planktic and metaphytic cyanoprokaryotes (cyanophytes, cyanobacteria) from ponds, lakes and water reservoirs in East and Southern Africa. The morphology and variation of 17 species was studied, and comments about their ecology and taxonomy are included. Seven new species are defined: Anabaena austro-africana, A. carmichaelii, A. bituri, A. maxima, A. nygaardii, Aphanizomenon capricorni and Cylindrospermopsis helicoidea. In one tropical species, Cylindrospermopsis africana, akinete formation was found and described for the first time. These investigations of cyanoprokaryotic algae from Africa indicate the existence of a specific and unique tropical microflora. Based on our findings we predict that many more taxa will be found in the future. (Less)

Toxicity of parathion‐methyl to cells, akinetes and heterocysts of the cyanobacterium Cylindrospermum, sp. and the probit analysis of toxicity

SummaryThe toxicity of a commercial formulation of the insecticide parathion‐methyl to the N2‐fixing filamentous cyanobacterium (blue‐green alga) Cylindrospermum, sp. was studied. A concentration of parathion‐methyl of 0.5 ppm caused growth increase in liquid growth media. The minimum inhibitory concentration of parathion‐methyl for both types (N2, fixing and nitrate supplemented) of liquid and solid media was 1.0 ppm. LC50 values were: 4.4 ppm (liquid, N2, fixing), 5.5 ppm (liquid, nitrate supplemented), 3.3 ppm (agar, N2‐fixing) and 4.0 ppm (agar, nitrate supplemented). LC100 values for N2‐fixing liquid and both types of agar media were 10.0 ppm, while for the liquid nitrate supplemented medium the LC100 was 12.0 ppm. Both akinete (spore) formation and germination were inhibited below the highest permissive concentration of 8.0 ppm, with the insecticide incorporated in the agar media. In soil, the LC50 and LC100 values for parathion‐methyl were 13.6 and 30 ppm, respectively. Both the dehydrogenase activity of heterocysts (monitored by 2,3,5‐triphenyl tetrazolium chloride reduction) and the nitrogen concentration of cultures (estimated by the micro‐Kjeldahl method) were affected by the insecticide, but the latter (N2‐fixation) was more sensitive. The Kruskal‐Wallis H test on the numbers of vegetative cells in the filaments revealed that the insecticide significantly affected the division of vegetative cells. The cyanobacterium could detoxify the growth medium containing high levels (30 and 40 ppm) of the insecticide in short‐term exposures at the expense of cell viability.

Potential triggers for akinete differentiation in an Australian strain of the cyanobacterium Cylindrospermopsis raciborskii (AWT 205/1)

Understanding the triggers for some cyanobacteria of the Nostocales and Stigonematales orders to produce specialised reproductive cells termed akinetes, is very important to gain further insights into their ecology. By improving our understanding of their life cycle, appropriate management options may be devised to control the formation of these cells, and therefore the potential bloom inoculum which they are thought to provide, may be reduced. This study investigated the effect of chemical (phosphorus limitation), and environmental variables (temperature shock) on akinete differentiation in the freshwater cyanobacterium Cylindrospermopsis raciborskii (AWT 205/1). From the preliminary results, it is suggested that the availability of phosphorus and changes in temperature were a necessary requirement for the formation of akinetes in this particular strain of C. raciborskii. In the four phosphorus treatments investigated (0, 3, 38 and 75 μg l−1 P), only the two higher treatments produced akinetes (approximately 220 ml−1). When the first akinetes were observed in the 38 and 75 μg l−1 P treatments, filterable reactive phosphorus (FRP) concentrations in the medium were approximately 22 and 52 μg l−1 P, respectively, indicating that there was no phosphorus limitation. In the temperature shock experiment, akinetes were observed in the 15 and 20 °C treatments. However, akinetes were degraded (pale yellow colour, limited swelling and shrivelled edges) and in much lower concentrations, which was thought to be a result of the daily temperature shock. We suggest that the formation of akinetes in C. raciborskii (AWT 205/1) can be triggered by an initial temperature shock and that phosphorus is a necessary requirement to allow further growth and full development of akinetes.

Formation, sedimentation and germination properties of Anabaena akinetes

The formation and sedimentation of akinetes of Anabaena cylindrica were analyzed quantitatively to identify practical strategies for removing or harvesting akinetes. The critical iron concentration for development of vegetative cells of this cyanobacterium into akinetes was between 0.060 and 0.240 mg/l. Before germination, approximately 10% of the total akinetes that were formed in trichomes settled down, being separated from the floating algal mat on the water surface. The effects of drying processes, which are inevitably involved in the production of biofertilizers, on the germination ability of the akinetes were also examined. Akinetes demonstrated tolerance against severe drying processes, i.e., heat-drying in an oven at 60 °C for 50 h and drying under sunlight for 10 h, suggesting that they can be used as a biofertilizer after drying processes. It was also shown that dried akinetes can be stored in a dried state for at least several weeks.

Vegetative survival, akinete and zoosporangium formation and germination in some selected algae as affected by nutrients, pH, metals, and pesticides.

Lack of nitrogen, phosphorus, or all nutrients, extremes of pH (< 4, > 11), presence of 'heavy' metals (Co, Cu, Zn, Hg, Pb; 0.5-10 ppm) or pesticides (carbofuran, 2,4-D, dithane, phorate, or bavistin; 1-50 ppm) decreased to various extent or completely inhibited the survival or vegetative cells in all studied algae. The formation of akinetes, their viability and germination in Anabaena iyengarii, Westiellopsis prolifica, Nostochopsis lobatus and Pithophora oedogonia and the formation of zoosporangia, their viability, and the germination of zoospores in Cladophora glomerata and Rhizoclonium hieroglyphicum was affected. The formation of viable akinetes or zoosporangia was shown to be directly linked with vegetative cell survival and growth; it could not be induced by any chemical stress imposed.

A light and electron microscopy study on the formation, structure and germination of akinetes of Stigeoclonium tenue (Chaetophorales, Chlorophyceae)

Akinete formation in Stigeoclonium tenue (AG) KUTZ began under unfavourable conditions (exhaustion of nutrients and/or evaporation of the culture medium). Cells from both prostrate and erect portions produced akinetes. A gradual change in the shape of vegetative cells was observed. They lost their typical straight edges and acquired an oval contour with constrictions at the level of the transverse cell walls. At the ultrastructural level, the changes were: 1) accumulation of starch and lipids as reserve substances; 2) disorganization of the chloroplast; 3) reduction of the vacuole until it disappeared in mature akinetes; 4) thickening of the wall resulting from the synthesis of both an additional new own wall next to the parental wall and a mucilage external layer; 5) disorganization of the plasmodesmata in the transverse walls of vegetative cells. All of these drastic cytological changes are essential to the development of akinetes which provide S. tenue with an effective survival mechanism under unfavourable conditions.

Viability of dried filaments, survivability and reproduction under water stress, and survivability following heat and UV exposure in Lyngbya martensiana, Oscillatoria agardhii, Nostoc calcicola, Hormidium fluitans, Spirogyra sp. and Vaucheria geminata.

Dried vegetative filaments of Spirogyra sp., Vaucheria geminata and Nostoc calcicola died within 1/2, 1 and 4 h, respectively; those of Hormidium fluitans, Oscillatoria agardhii and Lyngbya martensiana retained under similar storage conditions viability for 3, 5 and 10 d, respectively. The viability of dried vegetative filaments of L. martensiana, O. agardhii and H. fluitans decreased on storage at 20 degrees C in the dark. L. martensiana and O. agardhii tolerated 0.8 mol/L NaCl. The resistance to desiccation in L. martensiana and O. agardhii exhibited similar dependence as that to frost, to heat and UV light. O. agardhii filaments became slightly broader and their cells developed large number of gas vacuoles when grown in 0.8 mol/L NaCl-containing medium. The water stress imposed on growing algae either on high-agar solid media or in NaCl-containing liquid media reduced hormogonium formation in L. martensiana and O. agardhii, heterocyst and akinete formation in N. calcicola and fragmentation in H. fluitans; it did not induce conjugation in Spirogyra sp. and formation of reproductive organs in V. geminata. In all studied algae the stress reduced at various levels the survival of vegetative parts. Generally, algal body form and composition rather than habitats seem to decide primarily the level of resistance against various stress conditions.