Phosphorus (P) is an essential macronutrient central to the exchange and storage of energy and information in the cell. Due to its limited bioavailability, P often becomes a limiting nutrient in aquatic and terrestrial ecosystems hence the studies of responses to stress caused by P starvation cyanobacteria, the primary producers, are of considerable interest. Indeed, the availability of P is among main factors limiting diazotrophy in cyanobacteria. To gain a deeper understanding of the effect of P starvation on cyanobacteria in non-diazotrophic conditions, we studied a model system of two near—isogenic strains, Nostoc sp. PCC 7120 and Nostoc sp. PCC 7118 differ in their ability to form heterocysts. Specifically, we investigated the differences in the responses of these strains to P starvation by comparing their growth kinetics, photosynthetic pigment content, ultrastructural rearrangements of vegetative cells, and the expression profile of key genes of phosphorus metabolism. The tolerance of PCC 7120 to P starvation was higher than that of PCC 7118, which manifested itself in a higher growth rate, less profound ultrastructural changes (in particular, phycobilisomes as well as polyphosphate reserves were retained in the cells of PCC 7120). At the same time, the accumulation of cyanophycin, a depot of nitrogen and energy, increased several-fold in the cells of both strains during P starvation, but this increase was larger in PCC 7118 cells. Assumably, the increased resilience of the PCC 7120 to P starvation stems from its higher ability to accumulate intracellular reserves of P in the form of polyphosphates. Our findings suggest that the phenotypic differences between the strains Nostoc sp. PCC 7118 and Nostoc sp. PCC 7120 are not limited to the different ability to form heterocysts. A deeper understanding of the drivers of stress response phenotypic diversity in near-isogenic strains would require a comparative analysis of their whole-genome sequences.

Apart from their functions in the nervous system of animals, neurotransmitters operate as regulatory agents and signals in diverse kingdoms of life. Some neurotransmitters have recently been revealed to exert specific effects on microalgae, predominantly functioning as algal growth stimulators. This article presents new data on the effects of such neurotransmitters as serotonin, norepinephrine, dopamine, histamine, and acetylcholine on the fatty acid and pigment composition of the green microalga Scenedesmus quadricauda (Turp.) Breb. K-1149. It was established that acetylcholine and, to a lesser extent, histamine increased the total fatty acid content of S. quadricauda cells, whereas serotonin and dopamine decreased the fatty acid content. Acetylcholine, histamine, and norepinephrine elevated the percentage of polyunsaturated fatty acids; in contrast, serotonin and dopamine increased the share of saturated fatty acids. Acetylcholine and, to a lesser extent, norepinephrine increased the total chlorophyll content per gram of dry weight in S. quadricauda, while histamine decreased the chlorophyll content. Histamine also increased the chlorophyll a/chlorophyll b and carotenoid/chlorophyll ratios, which were decreased by dopamine. The data obtained are of biotechnological and ecological interest. The stimulation of fatty acid accumulation and the increase in the percentage of polyunsaturated species was caused by the neurotransmitters acetylcholine and histamine at low (1–10 μM) concentrations, which potentially enables facilitating the biotechnological production of health-promoting preparations for therapeutic and cosmetic purposes. However, other neurotransmitters (dopamine and serotonin) tested increased the relative content of saturated fatty acids; therefore, they apparently can be used to stimulate biofuel production, since saturated fatty acid-rich lipids are advantageous raw materials for biodiesel production. The impact of neurotransmitters on microalgal fatty acid composition and photosystem components may be considered in terms of ongoing chemical interaction between microalgae and other aquatic ecosystem components that are known to produce neurotransmitters.

The effect of nitrogen starvation and, for the first time, low temperature, as well as their simultaneous effect, on the physiology and ultrastructure of cells of microalgae of the genus Lobosphaera (Chlorophyta, Trebouxiophyceae) was studied. Nitrogen deficiency in both strains led to a decrease in the content of chlorophyll by three times and an increase in the proportion of carotenoids by two times. A decrease in the content of both chlorophyll and carotenoids was observed at +10°C. The simultaneous effect of two factors resulted in a threefold decrease in the chlorophyll content in NAMSU 924/2 and a sixfold decrease in NAMSU (CALU) 1497; the proportion of carotenoids in both strains decreased by 1.5–2 times. Data on ultrastructural changes in cells of microalgae of the genus Lobosphaera under the influence of stress factors have been obtained. A similar nature of the response in both strains to stress conditions was noted. Nitrogen deficiency led to the accumulation of numerous lipid droplets in the cytoplasm of cells along the cell wall. Long-term incubation on a nitrogen-free medium led to the filling of the entire volume of cells with lipid droplets, disassembly of the membrane system of chloroplasts, that reduction in sizeand being located between densely lying lipid droplets. At low temperatures, the number of thylakoids decreased, while the interthylakoid space and the size of chloroplasts increased. With simultaneous exposure to nitrogen starvation and low temperature, numerous lipid droplets accumulated, the number of thylakoids decreased, the interthylakoid space and the size of the chloroplast increased, which was noted under separate exposure to stress factors. The pyrenoid in both strains did not undergo significant changes in all cases.

In the present study, complex patterns of algal response to chromium (K2Cr2O7, Cr) and cadmium (CdSO4, Cd) toxicity were examined. Chlorophyll and starch content, photosynthetic activity and respiration rate, fatty acid content and composition, cell density (growth rate), malondialdehyde content (oxidative stress), ferric reducing antioxidant power and phenolic content (antioxidant activity) were measured in the model green algae species Scenedesmus quadricauda and Chlorella sorokiniana during exposure to 20 or 50 μM of Cr or Cd. Cell response to Cd showed similar patterns in both algae, whereas Cr effects differed, especially regarding the antioxidant activity. Malondialdehyde content, starch content, and respiration rate were the parameters most affected by both metals in both species. Only respiration rate increased dramatically under all treatments studied. In general, the Cd or Cr poisoning induced the transition of cell metabolism from “assimilatory” mode with active photosynthesis to “catabolic” mode characterized by elevated respiration and increased content of starch, a substrate for respiration. Chlorella sorokiniana cells showed a prominent response to Cr indicating suitability for bioassay of Cr contamination in the aquatic environment. A method for comprehensive screening of algal species on their heavy metal tolerance is proposed.

The immobilization of cells of a mixed culture of the microalgae (MA) Micractinium sp. NAMSU A-19 and cyanobacteria (CB) Synechococcus sp. 1Dp66E-1 on a chitosan-based polymer was studied. A polycationic sorbent based on a natural chitosan polymer with a molecular weight of 600 kDa was obtained from the crosslinking of chitosan with glutaraldehyde via cryopolymerization. It has a high affinity for the surface structures of oxygenic phototrophic microorganisms (OPMs) and enables strong cellular attachment to the sorbent surface. The study of the kinetics and evaluation of the effectiveness of mixed-culture immobilization showed a high sorption capacity of the chitosan sorbent. During the first hour of cultivation, the immobilization efficiency was on average 40–52%, and, almost all cells were immobilized after 48 h. The highly porous, nontoxic, and biodegradable sorbent provided steady cellular attachment for 7 days of cultivation and did not affect the growth of immobilized mixed culture, either on the surface or in the inner layers of the polymer. The study of mixed-culture immobilization via scanning electron microscopy showed that CB and MA cells are tightly attached to the surface of the chitosan sorbent. This is followed by the formation of strands of the extracellular polymer matrix and a biofilm consisting of cells of the mixed culture of MA and CB and heterotrophic bacteria associated with Micractinium sp. Immobilization on a chitosan sorbent contributes to an increase in the bioremoval of nitrates and phosphates by the tested mixed culture.

A huge interest in CO2-tolerant microalgae is fueled by development of CO2-biomitigation methods based on intensive cultivation of microalgae. Still, the mechanisms of CO2-tolerance are scarcely investigated. Previously, we described a symbiotic Desmodesmus sp. IPPAS S-2014 from a White Sea hydroid tolerant to extremely high (20–100%) CO2 levels. In the present work, we compared its ultrastructural and physiological responses to those of a novel free-living White Sea strain of Tetradesmus obliquus IPPAS S-2023 characterized in the companion paper. The strain S-2023 is closely related to Desmodesmus sp. IPPAS S-2014 but lacks its tolerance to extremely high CO2 (it is unable to survive at 100% CO2 and exhibits a reduced-growth phenotype when sparged with 20% CO2: air mixture). We compared the responses of the cell organization and photosynthetic activity to 20% CO2 in the tolerant and the intolerant White Sea chlorophytes using chlorophyll fluorescence measurements and ultrastructural analysis (transmission electron microscopy). The features peculiar to the CO2-intolerant chlorophyte include (i) inability to maintain pH homeostasis, (ii) a steady decline in the photosynthetic activity of the cells, (iii) a reduction of the photosynthetic membranes, and (iv) delayed accumulation of starch (starch grains) and its subsequent conversion to reserve lipids (oil bodies). Nitrogen starvation enhances the effects of high-CO2 stress in the CO2-intolerant microalga. The results of this work are discussed in the context of selection of tolerant algal strains for CO2 biomitigation applications.

Morphological, biochemical, and molecular genetic studies of green microalgae from the collection of cyanobacteria, algae, and algal parasites of St. Petersburg State University (CALU) (presumably belonging to the Parietochloris genus) were conducted in order to estimate biotechnological potential and clarify the phylogenetic position. It was determined that the studied strains have a close relationship to two genera from different classes (Lobosphaera (Trebouxiaceae) and Deasonia (Actinochloridaceae)) and can be of biotechnological interest as producers of valuable polyunsaturated fatty acids (especially arachidonic, linoleic, and α-linolenic).

Parietochloris incisa is a unicellular, fresh water green alga, capable of accumulating high amounts of the valuable very long chain polyunsaturated arachidonic acid (AA) in triacylglycerols (TAG) of cytoplasmic oil-bodies. To find out the cultivation conditions providing maximum AA yield, the effects of light irradiance and N- availability on the dry weight (DW), chlorophyll, ca-rotenoid and AA content have been studied. Under nitrogen starvation, TAG accounted for over 30% of dry weight (DW) and AA content became as high as ~ 55% of total fatty acids. From the standpoint of biomass accumulation, light intensity of ca. 400 (cid:7) E m –2 · s –1 was found to be optimal for growing P. incisa on complete medium. Lower light intensities (or higher cell density of inoculum) resulted in higher AA yield when the alga was cultivated on nitrogen-free media. In the absence of nitrogen, algal cells were unable to cope with high light and suffered from pho-tooxidative damage, whereas the nitrogen-sufficient culture survived under such illumination conditions probably due to accumulation of carotenoids. Nitrogen-deprived P. incisa cells displayed elevated sensitivity to light.

We studied effects of рН and СО2 enrichment on the physiological condition and biochemical composition of a carotenogenic microalga Coelastrella (Scotiellopsis Vinatzer) rubescens Kaufnerová et Eliás (Scenedesmaceae, Sphaeropleales, Chlorophyceae), a promising source of natural astaxanthin. The microalga was grown at a constant pH (5, 6, 7 or 8) maintained by direct СО2 injection. The air-sparged culture served as the control. Cell division rate and size, dry biomass productivity, the rates of nitrogen and phosphorus uptake as well as photosynthetic pigment and total lipid content and fatty acid composition were followed. С. rubescens possessed a narrow-range рН tolerance (the optimum рН 6–7). Under these conditions, the highest values of the maximum (1.0–1.1 1/day) and average (0.3–0.35 1/day) specific growth rate, chlorophyll а (4.8–4.9%) and total carotenoid dry weight percentages (1.7–1.8%) were recorded. Cell lipid fatty acid unsaturation index (1.851) and polyunsaturated fatty acid percentage (36–39%) and С18:3 ω3/С18:1 ω9 ratio (3.8–4.5) were also the highest under these conditions. A decline of рН to 5 brought about severe stress manifesting itself as a cell division cessation, photosynthetic apparatus reduction, two-fold increase in cell volume, accumulation of dry weight and lipids and a considerable decline in fatty acid unsaturation. Cultivation of С. rubescens without СО2 enrichment resulted in a rapid alkalization of the medium to рН 9.5–10.5 impairing the physiological condition of the cells. Reasons of the deteriorative effects of suboptimal pH values on the physiological condition of C. rubescens are discussed.

The growth and biomass accumulation of three microalgal strains of Desmodesmus (Scenedesmaceae, Chlorophyceae), 1Рm66В, 2Cl66E, 3Dp86Е-1, isolated from White Sea benthic invertebrates were studied under conditions of batch culture in different standard (BG-11, Prat, Goldberg, Gromov, Tamiya, artificial sea water) and modified media. Culture condition and biomass accumulation were recorded as well as the uptake of nitrate and phosphate. Vigorous growth of the microalgae brought about a significant alkalization of the culture medium to pH 10. The most significant biomass accumulation was recorded in BG-11 (the complete medium and one with addition of artificial sea water), Tamiya and Prat media. Addition of the sea water did not affect the growth of Desmodesmus sp. in the nitrate-containing media although that maintained the growth of the microalgae in the nitrogen-lacking media without cell aggregation. The obtained results suggest the suitability of BG-11 medium for isolation and cultivation of both symbiotic and free-living microalgae. The Prat medium is more suitable for maintaining the microalgal strains in collection.

The growth and biomass accumulation of three microalgal strains of Desmodesmus (Scenedesmaceae, Chlorophyceae), 1Рm66В, 2Cl66E, and 3Dp86Е-1, isolated from the White Sea benthic invertebrates were studied under conditions of batch culture in different standard media (BG-11, Prat, Goldberg, Gromov, Tamiya, artificial seawater) and modified media. The culture condition, biomass accumulation, and uptake of nitrate and phosphate were recorded. A significant alkalization of the culture medium up to pH 10 has been observed during a vigorous growth of the microalgae. The most significant biomass accumulation has been recorded in BG-11 (in complete or modified medium with addition of artificial seawater), Tamiya, and Prat media. Addition of seawater did not affect the growth of Desmodesmus sp. in the nitrate-containing media, although that maintained growth of the microalgae in the nitrogen-lacking media without cell aggregation. The BG-11 medium appears suitable for isolation and cultivation of both symbiotic and free-living microalgae by all the tested features. The Prat medium is the best for maintaining the microalgal strains in living collection.

The potential of the use of a new microalga strain Chlorella vulgaris IPPAS C-2015 (Chlorophyta, Trebouxiophyceae) for poultry wastewater treatment has been studied. The artificial wastewater (AWW) from chicken litter that mimicked real poultry wastewater was prepared, and the efficiency of the bioremoval of the inorganic anion from it and destruction of the organic contaminant in it during the new strain of C. vulgaris semi-continuous cultivation was assessed. After three days of C. vulgaris culturing, the initial nitrate and orthophosphate levels in AWW decreased by more than 90% and more than 48%, respectively, and organic compounds were degraded by 80% on average (judging by chemical oxygen consumption). During the cultivation of the microalgae in AWW, the bacteria associated with the C. vulgaris pre-culture gradually replaced the bacteria characteristic of the AWW. The microalga biomass grown in AWW contained a great amount of polyunsaturated long-chain fatty acids from the С 18 family. The capacities of the new C. vulgaris strain of being used in the combined poultry wastewater treatment and utilization of the resulting biomass are discussed together with the potential advantages of the microalgae-based over conventional biological wastewater treatment technologies.

Впервые изучена физиологическая гетерогенность близкородственных симбиотических водорослей из таксономически удаленных животных-хозяев на примере трех штаммов одноклеточных водорослей из рода Desmodesmus (Chlorophyceae), выделенных из донных беспозвоночных Белого моря. Охарактеризовано влияние азотного голодания и света высокой интенсивности на рост, динамику содержания хлорофиллов (Хл), суммарных каротиноидов (Кар) и жирных кислот (ЖК) липидов клеток. У всех изученных штаммов азотное голодание вызывало снижение скорости накопления биомассы, а также содержания Хл и Кар в клетках на фоне накопления суммы ЖК липидов. Ультраструктурное исследование выявило редукцию фотосинтетического аппарата и увеличение доли объема клетки, занятого олеосомами и крахмальными зернами, а также утолщение клеточной стенки. Снижение эффективной освещенности клеток в более плотных культурах, как правило, замедляло изменения пигментного состава и профиля ЖК, вызванные азотным голоданием. В большинстве случаев содержание Хл снижалось быстрее, чем содержание Кар. У двух из трех изученных штаммов этот процесс протекал синхронно со снижением ненасыщенности ЖК липидов. Обсуждаются возможности биотехнологического применения симбиотических микроводорослей с учетом особенностей их физиологии в условиях стресса.

The physiological heterogeneity of closely related symbiotic microalgae from taxonomically distant animal hosts was studied for the first time. Three strains of unicellular algae from the genus Desmodesmus (Chlorophyceae) isolated from White Sea benthic invertebrates were used as the object in this study. The effects of nitrogen starvation and high light intensity on the growth, changes in chlorophyll and total carotenoid contents and fatty acid content and composition of the microalgal cell lipids were followed. Nitrogen starvation declined the biomass accumulation rate as well as chlorophyll and carotenoid contents on the background of the enhanced fatty acid accumulation in all strains studied. The ultrastructural study revealed the reduction of photosynthetic apparatus and an increase in the proportion of cell volume occupied by oil bodies and starch grains as well as an increase in the cell wall thickness. A decline in the effective per cell irradiance in the cultures of higher cell density, as a rule, slowed down the changes in pigment and fatty acids composition characteristic of nitrogen starvation. The rates of biomass accumulation and cell biochemical composition under the nitrogen-starvation conditions were strain-specific. In most cases, the decline in chlorophylls proceeded at a higher rate in comparison with that of carotenoid decline. In two of the three strains studied, both these process occurred synchronously with the decline in the unsaturation of the cell lipid fatty acids. The possibilities of biotechnological application of the symbiotic microalgae are discussed with the peculiarities of their stress physiology in mind.

The potential use of a new microalgal strain Chlorella vulgaris IPPAS C-2015 (Chlorophyta, Trebouxiophyceae) for poultry wastewater treatment has been studied. The efficiency of inorganic anion bioremoval from and organic contaminant destruction in the artificial wastewater (AWW) prepared from chicken litter mimicking real poultry wastewater were estimated during the new strain of C. vulgaris semi-continuous cultivation. After three days of cultivation, the nitrate and orthophosphate ions levels were decreased by more than 90% and more than 48%, respectively, and 80% of the organic compounds on average (judging from chemical oxygen minimum index) were degraded in the AWW. During the cultivation of the microalgae, the bacteria associated with the C. vulgaris pre-culture gradually replaced the bacteria characteristic of the AWW. The microalgal biomass grown in AWW possessed a high content of polyunsaturated long-chain fatty acids from the С i8 family. The possibilities of the new C. vulgaris strain application to the combined poultry wastewater treatment and utilization of the resulting microalga biomass are discussed together with the potential advantages of the micro- algae-based over the conventional biological wastewater treatment technologies.