Comparative studies on growth and Pb(II) removal from aqueous solution by Nostoc muscorum and Anabaena variabilis.

Industrial wastewater containing heavy metals is a major environmental problem that needs to be treated. This study reported the ability of two fresh water algae cyanobacteria (Nostoc muscorum and Anabaena variabilis) to remove lead from aqueous solutions of four different initial concentrations (0-50 mg/L-1) for 21 days under controlled laboratory conditions. Results obtained in this study showed a maximum removal of Pb(II) (97.8%) by N. muscorum at 15 mg/L-1 initial metal concentration however the maximum removal by A. variabilis at the same concentration was 71.4% after 16 day of incubation. These N. muscorum appeared to be more efficient than A. variabilis for removing Pb(II). Algal growth, pigments in the algae cells were measured during incubation period. Lower concentrations of lead increased biomass, OD, chlorophyll a and carotenoids in both algae. On the other hand, higher concentrations of lead were inhibitory for growth.

A highly asynchronous developmental program triggered during germination of dormant akinetes of filamentous diazotrophic cyanobacteria.

Germination of akinetes of filamentous heterocyst-forming cyanobacteria of the order Nostocales is an essential process that ensures survival and recolonization after long periods of unfavorable conditions, as desiccation, cold and low light. We studied the morphological, physiological and metabolic changes that occur during germination of akinetes in two model species of cell differentiation, Anabaena variabilis ATCC 29413 and Nostoc punctiforme ATCC 29133, which live in different habitats. We characterized the akinete envelopes and showed their similarity to envelopes of N2-fixing heterocysts. Akinete germination started inside the envelopes and was dependent on light intensity but independent of nitrogen supply. During the germination of A. variabilis akinetes, cell division and heterocyst differentiation were highly accelerated. The energy for cell division was initially supplied by respiration of glycogen and subsequently by photosynthesis. By contrast, during germination of N. punctiforme akinetes, cell division and heterocyst differentiation were slow. During the initial 15-20 h, N. punctiforme akinetes increased in volume and some burst. Only then did intact akinetes start to divide and fully germinate, possibly fueled by nutrients released from dead akinetes. The different strategies used by these different cyanobacteria allow successful germination of dormant cells and recolonization under favorable conditions.

Energy transfer in Anabaena variabilis filaments adapted to nitrogen-depleted and nitrogen-enriched conditions studied by time-resolved fluorescence.

Nitrogen is among the most important nutritious elements for photosynthetic organisms such as plants, algae, and cyanobacteria. Therefore, nitrogen depletion severely compromises the growth, development, and photosynthesis of these organisms. To preserve their integrity under nitrogen-depleted conditions, filamentous nitrogen-fixing cyanobacteria reduce atmospheric nitrogen to ammonia, and self-adapt by regulating their light-harvesting and excitation energy-transfer processes. To investigate the changes in the primary processes of photosynthesis, we measured the steady-state absorption and fluorescence spectra and time-resolved fluorescence spectra (TRFS) of whole filaments of the nitrogen-fixing cyanobacterium Anabaena variabilis at 77 K. The filaments were grown in standard and nitrogen-free media for 6 months. The TRFS were measured with a picosecond time-correlated single photon counting system. Despite the phycobilisome degradation, the energy-transfer paths within phycobilisome and from phycobilisome to both photosystems were maintained. However, the energy transfer from photosystem II to photosystem I was suppressed and a specific red chlorophyll band appeared under the nitrogen-depleted condition.

CphA2 is a novel type of cyanophycin synthetase in N2-fixing cyanobacteria.

Most cyanobacteria use a single type of cyanophycin synthetase, CphA1, to synthesize the nitrogen-rich polymer cyanophycin. The genomes of many N2-fixing cyanobacteria contain an additional gene that encodes a second type of cyanophycin synthetase, CphA2. The potential function of this enzyme has been debated due to its reduced size and the lack of one of the two ATP-binding sites that are present in CphA1. Here, we analysed CphA2 from Anabaena variabilis ATCC 29413 and Cyanothece sp. PCC 7425. We found that CphA2 polymerized the dipeptide ß-aspartyl-arginine to form cyanophycin. Thus, CphA2 represents a novel type of cyanophycin synthetase. A cphA2 disruption mutant of A. variabilis was generated. Growth of this mutant was impaired under high-light conditions and nitrogen deprivation, suggesting that CphA2 plays an important role in nitrogen metabolism under N2-fixing conditions. Electron micrographs revealed that the mutant had fewer cyanophycin granules, but no alteration in the distribution of granules in its cells was observed. Localization of CphA2 by immunogold electron microscopy demonstrated that the enzyme is attached to cyanophycin granules. Expression of CphA1 and CphA2 was examined in Anabaena WT and cphA mutant cells. Whilst the CphA1 level increased upon nitrogen deprivation, the CphA2 level remained nearly constant.

Simultaneous gene inactivation and promoter reporting in cyanobacteria.

Determining spatiotemporal gene expression and analyzing knockout mutant phenotypes have become powerful tools in elucidating the function of genes; however, genetic approaches for simultaneously inactivating a gene and monitoring its expression have not been reported in the literature. In this study, we designed a dual-functional gene knockout vector pZR606 that contains a multiple cloning site (MCS) for inserting the internal fragment of a target gene, with a gfp gene as its transcriptional marker located immediately downstream of the MCS. By using this gene knockout system, we inactivated ava_2679 from Anabaena variabilis ATCC 29413, as well as all2508, alr2887, alr3608, and all4388 from Anabaena sp. strain PCC 7120. The ava_2679 knockout mutant fails to grow diazotrophically. Morphological analysis of ava_2679 knockout mutant after nitrogen step-down revealed defective junctions between heterocysts and adjacent vegetative cells, and the heterocyst was 1.53-fold longer compared to wild-type heterocysts. The alr2887, all4388, and alr3608 mutant colonies turned yellow and showed lack of protracted growth when deprived of fixed nitrogen, consistent with the previous reports that alr2887, all4388, and alr3608 are Fox genes. The all2508 encodes a GTP-binding elongation factor (EF4/LepA), and its knockout mutant exhibited reduced diazotrophic growth. The heterocyst development of all2508 knockout was significantly delayed, and only about 4.0 % of vegetative cells differentiated to heterocysts after nitrogen deprivation for 72 h, decreased 49.6 % compared to wild-type. Thus, we discovered that All2508 may regulate heterocyst development spatiotemporally. Concurrently, the GFP reporter revealed that all five target gene expressions were up-regulated in response to nitrogen deprivation. We demonstrated that the pZR606-based specific gene knockout approach worked effectively for the five selected genes, including four previously identified Fox genes or Fox gene homolog, and a previously unknown function of gene all2508. Thus, gene expression and phenotypic analysis of mutants can be achieved simultaneously by targeted gene inactivation using the pZR606-based system. This combined approach for targeted gene inactivation and its promoter reporting with GFP may be broadly applicable to the study of gene function in other prokaryotic organisms.

[Main reason for concentric rings plaque formation of virus infecting cyanobacteria (A-4L) in lawns of Anabaena variabilis].

OBJECTIVE: To clarify an important biological characteristic of virus infecting cyanobacteria (A-4L) and to isolate, identify new bloom-forming cyanobacteria viruses, we studied A-4L concentric rings plaque formation in Anabaena sp. PCC7120. METHODS: One step growth curve was designed to estimate the latent period and burst size of A4L. The initial titer of A-4L was about 2.8 x 10(10) PFU/mL. The appropriate titer suspension of A-4L was inoculated onto the lawns of Anabaena sp. PCC 7120 which have been cultivated at different time. Pathological change of lawns was observed and recorded daily. To investigate the effect of lighting on the concentric rings plaque formation, plates were cultivated and infected under continuous lighting (L: D = 24 h: 0 h), periodic lighting (L:D = 14 h:10 h) or 3 days continuous lighting after periodic lighting for 3 days. The ultra-morphology of purified A-4L was observed by negative staining electron microscopy. RESULTS: The latent period of A-4 (L) was 0.5 h-2 h and the burst size was about 247 infectious units per cell. Under periodic lighting, concentric rings plaques were observed in the plate after infection 3 days to 4 days and the distance between two rings was about 3 mm. Statistic analysis showed that there was a correlation between the number of concentric rings in plaques and infection days, which was "n -1". Compared with the periodic lighting, the plaques without concentric rings were observed under continuous lighting. However, the concentric rings formed under periodic lighting disappeared gradually after turning to continuous lighting, which demonstrated that the formation of concentric rings plaques depended on the periodic lighting. Negative staining electron microscopy showed that the A-4L particle had a spheroidal head with diameter about 50 nm and a tail with length about 10 nm which was similar to the characteristic morphology of cyanobacterial podoviruses. CONCLUSION: A-4L is a virus infecting cyanobacteria which can form concentric rings plaque. And periodic lighting is the key conditions for the concentric rings plaque formation of A4L.

Rapid algal culture diagnostics for open ponds using multispectral image analysis.

This article presents a multispectral image analysis approach for probing the spectral backscattered irradiance from algal cultures. It was demonstrated how this spectral information can be used to measure algal biomass concentration, detect invasive species, and monitor culture health in real time. To accomplish this, a conventional RGB camera was used as a three band photodetector for imaging cultures of the green alga Chlorella sp. and the cyanobacterium Anabaena variabilis. A novel floating reference platform was placed in the culture, which enhanced the sensitivity of image color intensity to biomass concentration. Correlations were generated between the RGB color vector of culture images and the biomass concentrations for monocultures of each strain. These correlations predicted the biomass concentrations of independently prepared cultures with average errors of 22 and 14%, respectively. Moreover, the difference in spectral signatures between the two strains was exploited to detect the invasion of Chlorella sp. cultures by A. variabilis. Invasion was successfully detected for A. variabilis to Chlorella sp. mass ratios as small as 0.08. Finally, a method was presented for using multispectral imaging to detect thermal stress in A. variabilis. These methods can be extended to field applications to provide delay free process control feedback for efficient operation of large scale algae cultivation systems.

Cell-specific gene expression in Anabaena variabilis grown phototrophically, mixotrophically, and heterotrophically.

BACKGROUND: When the filamentous cyanobacterium Anabaena variabilis grows aerobically without combined nitrogen, some vegetative cells differentiate into N2-fixing heterocysts, while the other vegetative cells perform photosynthesis. Microarrays of sequences within protein-encoding genes were probed with RNA purified from extracts of vegetative cells, from isolated heterocysts, and from whole filaments to investigate transcript levels, and carbon and energy metabolism, in vegetative cells and heterocysts in phototrophic, mixotrophic, and heterotrophic cultures. RESULTS: Heterocysts represent only 5% to 10% of cells in the filaments. Accordingly, levels of specific transcripts in vegetative cells were with few exceptions very close to those in whole filaments and, also with few exceptions (e.g., nif1 transcripts), levels of specific transcripts in heterocysts had little effect on the overall level of those transcripts in filaments. In phototrophic, mixotrophic, and heterotrophic growth conditions, respectively, 845, 649, and 846 genes showed more than 2-fold difference (p < 0.01) in transcript levels between vegetative cells and heterocysts. Principal component analysis showed that the culture conditions tested affected transcript patterns strongly in vegetative cells but much less in heterocysts. Transcript levels of the genes involved in phycobilisome assembly, photosynthesis, and CO2 assimilation were high in vegetative cells in phototrophic conditions, and decreased when fructose was provided. Our results suggest that Gln, Glu, Ser, Gly, Cys, Thr, and Pro can be actively produced in heterocysts. Whether other protein amino acids are synthesized in heterocysts is unclear. Two possible components of a sucrose transporter were identified that were upregulated in heterocysts in two growth conditions. We consider it likely that genes with unknown function represent a larger fraction of total transcripts in heterocysts than in vegetative cells across growth conditions. CONCLUSIONS: This study provides the first comparison of transcript levels in heterocysts and vegetative cells from heterocyst-bearing filaments of Anabaena. Although the data presented do not give a complete picture of metabolism in either type of cell, they provide a metabolic scaffold on which to build future analyses of cell-specific processes and of the interactions of the two types of cells.

2-epi-5-epi-Valiolone synthase activity is essential for maintaining phycobilisome composition in the cyanobacterium Anabaena variabilis ATCC 29413 when grown in the presence of a carbon source.

The cyclase 2-epi-5-epi-valiolone synthase (EVS) is reported to be a key enzyme for biosynthesis of the mycosporine-like amino acid shinorine in the cyanobacterium Anabaena variabilis ATCC 29413. Subsequently, we demonstrated that an in-frame complete deletion of the EVS gene had little effect on in vivo production of shinorine. Complete segregation of the EVS gene deletion mutant proved difficult and was achieved only when the mutant was grown in the dark and in a medium supplemented with fructose. The segregated mutant showed a striking colour change from native blue-green to pale yellow-green, corresponding to substantial loss of the photosynthetic pigment phycocyanin, as evinced by combinations of absorbance and emission spectra. Transcriptional analysis of the mutant grown in the presence of fructose under dark or light conditions revealed downregulation of the cpcA gene that encodes the alpha subunit of phycocyanin, whereas the gene encoding nblA, a protease chaperone essential for phycobilisome degradation, was not expressed. We propose that the substrate of EVS (sedoheptulose 7-phosphate) or possibly lack of its EVS-downstream products, represses transcription of cpcA to exert a hitherto unknown control over photosynthesis in this cyanobacterium. The significance of this finding is enhanced by phylogenetic analyses revealing horizontal gene transfer of the EVS gene of cyanobacteria to fungi and dinoflagellates. It is also conceivable that the EVS gene has been transferred from dinoflagellates, as evident in the host genome of symbiotic corals. A role of EVS in regulating sedoheptulose 7-phosphate concentrations in the photophysiology of coral symbiosis is yet to be determined.

The cultivation of Anabaena variabilis in a bubble column operating under bubbly and slug flows.

In a bubble column reactor with an inner diameter of 6cm and a height of 63cm for the culture of cyanobacteria two different shapes of bubbles can be generated, resulting in bubbly flow or slug flow. Growth of Anabaena variabilis under slug flow (1.9g/l/day) was 1.73 times higher than that under bubbly flow (1.1g/l/day) when the specific irradiation rate was maintained above 10µmol/s/g dry cell. Although a stepwise increase in superficial gas velocity enhanced the average cell growth rate under bubbly flow by 1.57 times, the average cell growth rate during the deceleration phase under bubbly flow (1.98g/l/day) was 0.61 times smaller than that under slug flow (3.22g/l/day). These results demonstrate that the bubble shape in the slug flow was advantageous in regards to the radial circulation of cells.

Impact of titanium dioxide nanomaterials on nitrogen fixation rate and intracellular nitrogen storage in Anabaena variabilis.

This study comprehensively investigated the impact of titanium dioxide nanomaterials (nTiO(2)) exposure on cell growth, nitrogen fixation activity, and nitrogen storage dynamics in the primary producer cyanobacteria Anabaena variabilis at various dose concentrations and exposure time lengths. The results indicated that both growth rate (EC(50)-96 h of 0.62 mgTiO(2)/L) and nitrogen fixation activity (EC(50)-96 h of 0.4 mgTiO(2)/L) were inhibited by nTiO(2) exposure. The Hom's law (C(n)T(m)) was used as inactivation model to predict the concentration- and time-dependent inhibition of growth and nitrogen fixation activity. The kinetic parameters determined suggested that the time of exposure has a greater influence than the nTiO(2) concentration in toxicity. We observed, for the first time, that nTiO(2) induced a dose (concentration and time)-dependent increase in both the occurrence and intracellular levels of the nitrogen-rich cyanophycin grana proteins (CGPs). The results implied that CGPs may play an important role in the stress response mechanisms of nTiO(2) exposure and can serve as a toxicity assessment endpoint indicator. This study demonstrated that nitrogen-fixing activity could be hampered by the release of nTiO(2) in aquatic environments; therefore it potentially impacts important biogeochemical processes, such as carbon and nitrogen cycling.

Cyanobactericidal effect of Rhodococcus sp. isolated from eutrophic lake on Microcystis sp.

A bacterium, which was observed in all cultivations of Microcystis sp., was isolated and designated as Rhodococcus sp. KWR2. The growth of bloom-forming cyanobacteria, including four strains of Microcystis aeruginosa and Anabaena variabilis, was suppressed by up to 75-88% by 2% (v/v) culture broth of KWR2 after 5 days. But KWR2 did not inhibit eukaryotic algae, Chlorella vulgaris and Scenedesmus sp. An extracellular algicidal substance produced by KWR2 showed a cyanobactericidal activity of 94% and was water-soluble with a molecular weight of lower than 8 kDa.

Sulfur deficiency changes mycosporine-like amino acid (MAA) composition of Anabaena variabilis PCC 7937: a possible role of sulfur in MAA bioconversion.

In the present investigation we show for the first time that bioconversion of a primary mycosporine-like amino acid (MAA) into a secondary MAA is regulated by sulfur deficiency in the cyanobacterium Anabaena variabilis PCC 7937. This cyanobacterium synthesizes the primary MAA shinorine (RT = 2.2 min, lambda(max) = 334 nm) under normal conditions (PAR + UV-A + UV-B); however, under sulfur deficiency, a secondary MAA palythine-serine (RT = 3.9 min, lambda(max) = 320 nm) appears. Addition of methionine to sulfur-deficient cultures resulted in the disappearance of palythine-serine, suggesting the role of primary MAAs under sulfur deficiency in recycling of methionine by donating the methyl group from the glycine subunit of shinorine to tetrahydrofolate to regenerate the methionine from homocysteine. This is also the first report for the synthesis of palythine-serine by cyanobacteria which has so far been reported only from corals. Addition of methionine also affected the conversion of mycosporine-glycine into shinorine, consequently, resulted in the appearance of mycosporine-glycine (RT = 3.6 min, lambda(max) = 310 nm). Our results also suggest that palythine-serine is synthesized from shinorine. Based on these results we propose that glycine decarboxylase is the potential enzyme that catalyzes the bioconversion of shinorine to palythine-serine by decarboxylation and demethylation of the glycine unit of shinorine.

Effect of mineral phosphates on growth and nitrogen fixation of diazotrophic cyanobacteria Anabaena variabilis and Westiellopsis prolifica.

The nitrogen fixing cyanobacterial strains namely Anabaena variabilis (Nostocales, Nostocaceae) and Westiellopsis prolifica (Nostocales, Hapalosiphonaceae) were evaluated for their nitrogen fixation and growth potential in response to different concentrations (10, 20 and 30 mg P) of the alternate insoluble P-sources Mussorie Rock Phosphate and Tricalcium Phosphate. Distinct and significant intergeneric differences were observed with respect to nitrogen fixation measured as Acetylene Reduction Activity (ARA) and growth potential as soluble proteins, total carbohydrate content, dry weight and total chlorophyll content in response to different concentrations of Mussorie Rock Phosphate and Tricalcium Phosphate. Both the strains showed higher soluble protein content at 20 mg P (Mussorie Rock Phosphate) that increased with time of incubation in A. variabilis. Both cyanobacteria recorded maximum Acetylene Reduction Activity at 20 mg P (Tricalcium Phosphate) followed by activity in presence of soluble phosphate (K2HPO4). The mean activity at all concentrations of insoluble phosphate (Mussorie Rock Phosphate and Tricalcium Phosphate) was more than in the presence of soluble phosphate.

Hydrogen production in nitrogenase mutants in Anabaena variabilis.

Nitrogenase produces hydrogen as a normal byproduct of the reduction of dinitrogen to ammonia. The Nif2 nitrogenase in Anabaena variabilis is an alternative Mo-nitrogenase and is expressed in vegetative cells grown with fructose under strictly anaerobic conditions. We report here that the V75I substitution in the alpha-subunit of Nif2 showed greatly impaired acetylene reduction and reduced levels of (15)N(2) fixation but had similar hydrogen production rates as the wild-type enzyme under argon. Another mutant containing a substitution in the alpha-subunit, V76I, would result in a decrease in the size of the putative gas channel of nitrogenase and, thus, was hypothesized to affect substrate selectivity of nitrogenase. However, this substitution had no effect on the enzyme selectivity, suggesting that access by gases to the active site through this putative gas channel is not limited by the increased size of the amino acid side chain in the alpha-subunit, V76I substitution.

Tolerance to antimicrobial agents and persistence of Escherichia coli and cyanobacteria.

Bacterial persistence is the tolerance of a small part of a cell population to bactericidal agents, which is attained by a suppression of important cell functions and subsequent deceleration or cessation of cell division. The growth rate is the decisive factor in the transition of the cells to the persister state. A comparative study of quickly growing Escherichia coli K-12 strain MC 4100 and cyanobacteria Synechocystis sp. PCC 6803 and Anabaena variabilis ATCC 29413 growing slowly was performed. The cyanobacterial cells, like E. coli cells, differed in sensitivity to antimicrobial substances depending on the growth phase. Carbenicillin inhibiting the synthesis of peptidoglycan, a component of the bacterial cell wall, and lincomycin inhibiting the protein synthesis gave rise to nucleoid decay in cells from exponential cultures of Synechocystis 6803 and did not influence the nucleoids in cells from stationary cultures. Carbenicillin suppressed the growth of exponential cultures and had no effect on cyanobacterial stationary cultures. A suppression of Synechocystis 6803 growth in the exponential phase by lincomycin was stronger than in the stationary phase. Similar data were obtained with cyanobacterial cells under the action of H2O2 or menadione, an inducer of reactive oxygen species production. Slowly growing cyanobacteria were similar to quickly growing E. coli in their characteristics. Persistence is a characteristic feature of cyanobacteria.

High cell density culture of Anabaena variabilis with controlled light intensity and nutrient supply.

Controlling the light energy and major nutrients is important for high cell density culture of cyanobacterial cells. The growth phase of Anabaena variabilis can be divided into an exponential growth phase and a deceleration phase. In this study, the cell growth in the deceleration phase showed a linear growth pattern. Both the period of the exponential growth phase and the average cell growth rate in the deceleration phase increased by controlling the light intensity. To control the light intensity, the specific irradiation rate was maintained above 10 micromol/s/g dry cell by increasing the incident light intensity stepwise. The final cell density increased by controlling the nutrient supply. For the control of the nutrient supply, nitrate, phosphate, and sulfate were intermittently added based on the growth yield, along with the combined control of light intensity and nutrient concentration. Under these control conditions, both final cell concentration and cell productivity increased, to 8.2 g/l and 1.9 g/l/day, respectively.

Growth, CO2 consumption and H2 production of Anabaena variabilis ATCC 29413-U under different irradiances and CO2 concentrations.

AIMS: The objective of this study is to develop kinetic models based on batch experiments describing the growth, CO(2) consumption, and H(2) production of Anabaena variabilis ATCC 29413-U(TM) as functions of irradiance and CO(2) concentration. METHODS AND RESULTS: A parametric experimental study is performed for irradiances from 1120 to 16100 lux and for initial CO(2) mole fractions from 0.03 to 0.20 in argon at pH 7.0 +/- 0.4 with nitrate in the medium. Kinetic models are successfully developed based on the Monod model and on a novel scaling analysis employing the CO(2) consumption half-time as the time scale. CONCLUSIONS: Monod models predict the growth, CO(2) consumption and O(2) production within 30%. Moreover, the CO(2) consumption half-time is an appropriate time scale for analysing all experimental data. In addition, the optimum initial CO(2) mole fraction is 0.05 for maximum growth and CO(2) consumption rates. Finally, the saturation irradiance is determined to be 5170 lux for CO(2) consumption and growth whereas, the maximum H(2) production rate occurs around 10,000 lux. SIGNIFICANCE AND IMPACT OF THE STUDY: The study presents kinetic models predicting the growth, CO(2) consumption and H(2) production of A. variabilis. The experimental and scaling analysis methods can be generalized to other micro-organisms.

Characterization of factors influencing the growth of Anabaena variabilis in a bubble column reactor.

The combined effect of superficial gas velocity, pH, initial phosphate concentration, and light intensity on cell growth was investigated for the mass production of cyanobacterial cells. The light intensity was manipulated to maintain a specific irradiation rate (q(i)) at a constant level for high cell density culture. The optimum condition for the batch culture was achieved at a superficial gas velocity of 2.0 cm/s, pH 7.0, and an initial phosphate concentration of 55 mg/l when the specific irradiation rate was controlled above 11.5 micromol/s/g dry cell. In this condition, the specific growth rate and cell productivity were 1.47 day(-1) and 0.98 g dry cell/l/day, respectively.

Effects of low- or medium-pressure ultraviolet lamp irradiation on Microcystis aeruginosa and Anabaena variabilis.

A pure culture of Microcystis aeruginosa or Anabaena variabilis, the representatives of water blooming algae, was exposed to low-pressure (LP) or medium-pressure (MP) UV lamps. Irradiated pure culture suspension was subsequently incubated for 7d under white light fluorescent lamps. During incubation, profiles of the number of cells, DNA damage and photosynthetic activity were determined. When UV fluence was 600mJ/cm(2), M. aeruginosa cell numbers decreased throughout the 7-d incubation period, to produce 1.5log reduction (LP) or 1.2log reduction (MP) compared with control. The amount of DNA damage was 2.02x10(-4) ESS/base (LP) and 3.42x10(-4) ESS/base (MP) just after UV irradiation, which became 0.05x10(-4) ESS/base and 0.23x10(-4) ESS/base, respectively, after 3d incubation. However, cell number kept decreasing, even after DNA repair. Photosynthetic activity decreased by 1.5log within 1d (LP) or 3d (MP). Thus, reduction in photosynthetic activity could contribute to the reduction in M. aeruginosa cell numbers. A. variabilis cell numbers reduced by 2.3log (LP) or 2.2log (MP) during the 7-d incubation period; however, after DNA damage repair, cell number began to increase. The amount of DNA damage was 6.07x10(-4) ESS/base (LP) and 4.48x10(-4) ESS/base (MP) just after UV irradiation, which became 0.23x10(-4) ESS/base and 0.40x10(-4) ESS/base, respectively, after 3d incubation. No reduction was observed in photosynthetic activity/cell. Therefore, DNA damage is the main contributor of the reduction in cell number of A. variabilis.