General distribution of the nitrogen control gene ntcA in cyanobacteria.

The ntcA gene from Synechococcus sp. strain PCC 7942 encodes a regulatory protein which is required for the expression of all of the genes known to be subject to repression by ammonium in that cyanobacterium. Homologs to ntcA have now been cloned by hybridization from the cyanobacteria Synechocystis sp. strain PCC 6803 and Anabaena sp. strain PCC 7120. Sequence analysis has shown that these ntcA genes would encode polypeptides strongly similar (77 to 79% identity) to the Synechococcus NtcA protein. Sequences hybridizing to ntcA have been detected in the genomes of nine other cyanobacteria that were tested, including strains of the genera Anabaena, Calothrix, Fischerella, Nostoc, Pseudoanabaena, Synechococcus, and Synechocystis.     

Antibacterial, antifungal and cytotoxic activity of terrestrial cyanobacterial strains from Serbia

Cyanobacteria are known to be a rich source of biologically active compounds some of which can have pharmaceutical importance. In this work we present the screening results of cyanobacterial strains for their antibacterial, antifungal, and cytotoxic activity. Cyanobacterial strains were isolated from various soil types in province of Vojvodina and Central Serbia, Republic of Serbia. The screening included 9 strains of Anabaena and 9 strains of Nostoc. Both, extracellular products (from the culture liquid) and cellular crude lipophilic extracts were tested against 13 bacterial strains and 8 fungal strains. Cytotoxic activity was tested against three human cell lines. Methanol extracts were prepared according to ?stensvik. Antibacterial and antifungal activities were determined measuring inhibition zone, 48 h after inoculation. The cytotoxic activity was determined by sulforhodamine B (SRB) colorimetric assay. Of all cyanobacterial strains tested, 52% showed some antifungal and 41% antibacterial activity. Two out of six tested strains possessed cytotoxic activity. The cytotoxic activity of Anabaena strain S12 was found both in culture liquid and crude cell extract. It occurred specifically between the 21st and 42nd day of cultivation against HeLa and MCF7 cells, but had no activity against cell line derived from a healthy tissue. A high percentage of the active strains among the tested strains justify the effort of screening cyanobacteria that are isolated from terrestrial environments. The most promising strains for the fur- ther study are Anabaena strain S12 which showed strong cytotoxic and antibacterial activity and Ana- baena strain S20 which produces a potent antifungal compound. The future work, besides further screening and chemical identification of the active compounds, should also include the development of culture techniques that would lead to more efficient production of biologically active compounds.     

Amino acid transport in taxonomically diverse cyanobacteria and identification of two genes encoding elements of a neutral amino acid permease putatively involved in recapture of leaked hydrophobic amino acids.

The activities of uptake of thirteen 14C-labeled amino acids were determined in nine cyanobacteria, including the unicellular strains Synechococcus sp. strain PCC 7942 and Synechocystis sp. strain PCC 6803; the filamentous strain Pseudanabaena sp. strain PCC 6903, and the filamentous, heterocyst-forming strains Anabaena sp. strains PCC 7120 and PCC 7937; Nostoc sp. strains PCC 7413 and PCC 7107; Calothrix sp. strain PCC 7601 (which is a mutant unable to develop heterocysts); and Fischerella muscicola UTEX 1829. Amino acid transport mutants, selected as mutants resistant to some amino acid analogs, were isolated from the Anabaena, Nostoc, Calothrix, and Pseudanabaena strains. All of the tested cyanobacteria bear at least a neutral amino acid transport system, and some strains also bear transport systems specific for basic or acidic amino acids. Two genes, natA and natB, encoding elements (conserved component, NatA, and periplasmic binding protein, NatB) of an ABC-type permease for neutral amino acids were identified by insertional mutagenesis of strain PCC 6803 open reading frames from the recently published genomic DNA sequence of this cyanobacterium. DNA sequences homologous to natA and natB from strain PCC 6803 were detected by hybridization in eight cyanobacterial strains tested. Mutants unable to transport neutral amino acids, including natA and natB insertional mutants, accumulated in the extracellular medium a set of amino acids that always included Ala, Val, Phe, Ile, and Leu. A general role for a cyanobacterial neutral amino acid permease in recapture of hydrophobic amino acids leaked from the cells is suggested.     

Hydrogen metabolism by filamentous cyanobacteria

Apparent discrepancies in the literature concerning the amounts of H₂ produced by strains of Anabaena cylindrica are explained. These are not due to differences in strains used by different workers nor to differences in growth conditions, but rather appear to be due to the fact that cultures show an increasing dependence with age on CO₂ for sustained H₂ production. Two distinct hydrogenase activities were measured and characterized, both in vivo and in vitro in A. cylindrica B629; these were H₂ uptake activity and H₂ evolution from reduced methyl viologen. Gentle cell disruption techniques were used to gain further evidence that the latter activity was soluble. H₂ uptake was strongly inhibited by acetylene in vivo in the light or in the dark with phenazine methosulfate added, but only after a prolonged lag period. In extracts this lag did not occur. A detailed study of the nitrogenase and hydrogen uptake activities and their interrelationship both in the light and in the dark in A. cylindrica B629 showed that only in the dark in the presence of O₂ did H₂ uptake support C₂H₂ reduction significantly. Under several conditions in which nitrogenase activity was inhibited H₂ uptake was unaffected. H₂ metabolism was tested in three nonheterocystous filamentous cyanobacteria under different growth and incubation conditions. These were Plectonema boryanum, Schizothrix calcicola, and Oscillatoria brevis. Myxosarcina chroococcoides and Fischerella muscicola were also investigated. Cyanobacterial species vary markedly in their hydrogen metabolism and in the composition of the three H₂ metabolizing enzymes.     

Cyanobacterial diversity shifts induced by butachlor in selected Indian rice fields in Eastern Uttar Pradesh and Western Bihar analyzed with PCR and DGGE

The present study examines the effects of 30 mg/kg butachlor on the cyanobacterial diversity of rice fields in Eastern Uttar Pradesh and Western Bihar in India. A total of 40 samples were grouped into three classes [(i) acidic, (ii) neutral, and (iii) alkaline soils], based on physicochemical and principle component analyses. Acidic soils mainly harbored Westillopsis, Trichormus, Anabaenopsis, and unicellular cyanobacteria; whereas Nostoc, Anabaena, Calothrix, Tolypothrix, and Aulosira were found in neutral and alkaline soils. Molecular characterization using 16S rRNA PCR and DGGE revealed the presence of 13 different phylotypes of cyanobacteria in these samples. Butachlor treatment of the soil samples led to the disappearance of 5 and the emergence of 2 additional phylotypes. A total of 40 DGGE bands showed significant reproducible changes upon treatment with butachlor. Phylogenetic analyses divided the phylotypes into five major clusters exhibiting interesting links with soil pH. Aulosira, Anabaena, Trichormus, and Anabaenopsis were sensitive to butachlor treatment, whereas uncultured cyanobacteria, a chroococcalean member, Westillopsis, Nostoc, Calothrix, Tolypothrix, Rivularia, Gloeotrichia, Fischerella, Leptolyngbya, and Cylindrospermum, appeared to be tolerant against butachlor at their native soil pH. Butachlor-induced inhibition of nitrogen fixation was found to be 65% (maximum) and 33% (minimum) in the soil samples of pH 9.23 and 5.20, respectively. In conclusion, low butachlor doses may prove beneficial in paddy fields having a neutral to alkaline soil pH.     

Allelopathic activity among Cyanobacteria and microalgae isolated from Florida freshwater habitats

We evaluated allelopathic interactions between strains of Cyanobacteria and green algae isolated from south and central Florida. Allelopathy, including inhibition or stimulation of growth, was assessed by cocultivation of each of the isolated strains, as well as by evaluation of extracts prepared from the isolates. All of the strains of Cyanobacteria, and four of the six isolates of green algae, showed some allelopathic activity (i.e. inhibition or stimulation of the growth of other strains). Of these, the most pronounced activity was observed for the cyanobacterial isolate Fischerella sp. strain 52-1. In the cocultivation experiments this cyanobacterium inhibited the growth of all tested green algae and Cyanobacteria. The crude lipophilic extracts from Fischerella sp. strain 52-1 isolated from both the biomass and the culture liquid inhibited photosynthesis of the green alga Chlamydomonas sp. in a concentration- and time-dependent manner and caused extensive loss of ultrastructural cell organization. Preliminary chemical characterization of compounds extracted from Fischerella sp. strain 52-1 indicated the presence of indole alkaloids, and further characterization has confirmed that these compounds belong to the hapalindoles previously isolated from other species of Fischerella and related genera. Further chemical characterization of these compounds, and further investigation of their apparent role in allelopathy is ongoing.     

Inhibition of common fouling organisms by marine bacterial isolates ith special reference to the role of pigmented bacteria

Two questions of relevance to the establishment of marine biofouling communities were addressed, viz (1) what is the frequency with which bacterial strains isolated from living and inanimate surfaces in the marine environment show inhibitory activity against the settlement of common fouling organisms, and (2) is the antifouling bacterium, D2, an inhabitant of different marine waters, and how unique is this bacterium, in its mode of action against different target organisms? With respect to the first question, ninety three marine bacteria isolated from various rock surfaces from the marine environment were tested against larvae of Balanus amphitrite and spores of Ulva lactuca. Settlement assays against the diatom Amphora sp. were also performed on 10 of these strains. Nine bacterial isolates were shown to be inhibitory against larval settlement and eight of these strains were also inhibitory against algal spores. Altogether 16 strains were inhibitory against the settlement of algal spores while none of the bacterial strains inhibited diatom settlement. With respect to the second question, D2, a dark green pigmented bacterium, isolated from an adult tunicate off the Swedish west coast, has been found to be a very effective inhibitor against common fouling organisms. In order to see if this bacterium can be found in other marine waters, bacteria from living surfaces of marine plants and animals from waters around Sydney, Australia, were isolated and screened for inhibitory activity against barnacle larvae. Seventy four percent of the 23 plant isolates were shown to be inhibitory against larval settlement while only 30% of the 23 isolates from marine animals reduced settlement. Twenty two of the isolates from different seaweeds were dark pigmented and 20 of these strains inhibited settlement of barnacle larvae and algal spores. Three of the strains showed the same phenotypic expression as D2, and the results indicate that these strains may be D2 or closely related strains, suggesting that D2 may be a common inhabitant in the marine environment.     

Purification and characterization of extracellular beta-glucosidase from Sinorhizobium kostiense AFK-13 and its algal lytic effect on Anabaena flos-aquae

A beta-glucosidase from the algal lytic bacterium Sinorhizobium kostiense AFK-13, grown in complex media containing cellobiose, was purified to homogeneity by successive ammonium sulfate precipitation, and anion-exchange and gel-filtration chromatographies. The enzyme was shown to be a monomeric protein with an apparent molecular mass of 52 kDa and isoelectric point of approximately 5.4. It was optimally active at pH 6.0 and 40'C and possessed a specific activity of 260.4 U/mg of protein against 4-nitrophenyl-beta-D-glucopyranoside (pNPG). A temperature-stability analysis demonstrated that the enzyme was unstable at 50 degrees C and above. The enzyme did not require divalent cations for activity, and its activity was significantly suppressed by Hg+2 and Ag+, whereas sodium dodecyl sulfate (SDS) and Triton X-100 moderately inhibited the enzyme to under 70% of its initial activity. In an algal lytic activity analysis, the growth of cyanobacteria, such as Anabaena flos-aquae, A. cylindrica, A. macrospora, Oscillatoria sancta, and Microcystis aeruginosa, was strongly inhibited by a treatment of 20 ppm/disc or 30 ppm/disc concentration of the enzyme.     

Modelling the distribution of Anabaena and Melosira in a stratified river weir pool

We have developed a model to predict the stratification dynamics and its influence on the distribution of algal species in a river with strong diurnal changes in temperaturestratification. While the phytoplankton concentrations vary inresponse to buoyancy and light limited growth, the populationsare also redistributed vertically due to diurnal mixing causedby surface evaporation and night time cooling. Theparameterization of the vertical eddy diffusivity coefficientwithin the model takes into account the generation of mixing byheat losses at the surface and the suppression of mixing bythermal stratification. The model is successfully tested against extensive temperatureand algal concentration data in Maude weir pool on theMurrumbidgee River, Australia. Given literature values offloating/sinking rates and maximum growth rates for thecyanobacterium Anabaena and the diatom Melosira, wewere able to reproduce extremely well the time evolution ofconcentrations of each species over long periods of low flow. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cyanobacteria as agents for the control of pollution by pesticides and chlorinated organic compounds

Cyanobacteria are phototrophic aquatic micro-organisms that are found in a variety of environments, including polluted ones. Fifteen strains of cyanobacteria that belong to three taxonomic groups are able to degrade lindane (λ-hexachlorocyclohexane, a recalcitrant pesticide). The initial degradation pathway has been studied in two filamentous nitrogen-fixing strains of Anabaena sp. PCC 7120 and Nostoc ellipsosporum. These cyanobacteria dechlorinated loindane first to pentachlorocyclohexene and then to a mixture of trichlorobenzenes, and possibly further. Lindane dechlorination by these organisms occurred only in the presence of nitrate in the medium. Both ammonium and darkness inhibited the process. This combination of observations led us to the hypothesis that the nitrate-reduction system of cyanobacteria may be involved in dechlorination. The hypothesis was proven by the analysis of Anabaena sp. transpositional mutants in four genes of the nir operon. The mutants were unable to dechlorinate lindane. However, there was no correlation between lindane dechlorination and activities of individual proteins encoded by this operon. Genetic engineering of Anabaena sp. and N. ellipsosporum that introduced linA lindane dechlorination operon from Psuedomonas paucimobilis allowed us to uncouple dechlorination from nitrate requirement. Introduction, by genetic engineering, of fcABC (the 4-chlorobenzoate dechlorination system from Arthrobacter globiformis) to Anabaena sp and N. ellipsosporum rendered these strains newly capable of 4-chlorobenzoate dechlorination both constitutively and inducible by an environmental factor.     

Toxic cyanobacteria (blue-green algae) in Finnish fresh and coastal waters

A survey of the occurrence of toxic blooms of cyanobacteria in Finnish fresh and coastal waters was made during 1985 and 1986. Toxicity of the freeze-dried water bloom samples was tested by mouse-bioassay (i.p.). Forty-four per cent (83/188) of the bloom samples were found to be lethally toxic. Hepatotoxic blooms (54) were almost twice as common as neurotoxic ones (29). Anabaena was the most frequently found genus in toxic and non-toxic blooms and it was present in all neurotoxic samples. Statistical associations were found between hepatotoxicity and incidence of Microcystis aeruginosa, M. viridis, M. wesenbergii, Anabaena flos-aquae and Anabaena spiroides. Neurotoxicity was statistically associated with Anabaena lemmermannii, Anabaena flos-aquae and Gomphosphaeria naegeliana. Isolation of strains of cyanobacteria confirmed the occurrence of hepatotoxic and neurotoxic strains of Anabaena, as well as hepatotoxic strains of Microcystis and Oscillatoria species.Toxic blooms caused cattle poisonings at three different lakes during the study period. Toxic blooms also occurred in drinking water sources. Our study shows that toxic cyanobacteria are more common in Finnish lakes than would be expected on the basis of animal poisonings. The results of this study show the existence of toxic cyanobacteria in Finnish water supplies and the need for their continued study as agents of water based disease.     

Selective inhibition of toxic cyanobacteria by β-carboline-containing bacterium Bacillus flexus isolated from Saudi freshwaters

A bacterial strain SSZ01 isolated from a eutrophic lake in Saudi Arabia dominated by cyanobacterial blooms, showed an antialgal activity against cyanobacteria species. Based on the analysis of the 16S rDNA gene sequence, the isolated strain (SSZ01) most likely belonged to the genus Bacillus with a 99% similarity to Bacillus flexus strain EMGA5. The thin layer chromatography (TLC) analysis of the ethyl acetate extract of this bacterium revealed that this strain can produce harmine and norharmane compared to different β-carboline analog standards. Harmine and norharmane were also detected in considerable amounts in bacterial growth medium, indicating a potential excretion of these compounds into the aquatic environment. The crude extract of Bacillus flexus as well as pure materials of harmine and norharmane inhibited the growth of tested species of cyanobacteria. However, the bacterial crude extract has a higher toxicity against tested species of cyanobacteria than harmine and norharmane. In addition, harmine was more toxic to cyanobacteria than norharmane. On the other hand, neither pure compounds of harmine and norharmane nor crude bacterial extract showed any antialgal activity against tested species of green algae. The results of the present study suggest that B. flexus SSZ01 or its crude extract containing harmine and norharmane could be a candidate for the selective control of cyanobacterial blooms without affecting other algal species.     

97 Sensitivity of cyanobacteria to a potential biological control agent, bacterium SG-3

Cyanobacteria cause many problems in freshwater ecosystems. For example, the production of off-flavor compounds by cyanobacteria causes serious problems in catfish aquaculture. Control of cyanobacteria is generally limited to treatment with copper compounds, which are non-selective and sometimes ineffective at controlling certain species of cyanobacteria. Biological control could provide selective management by removing unwanted species while leaving desirable algae species. A bacterium (SG-3) (NRRL B-30043) lyses a number of planktonic species of cyanobacteria including bloom-forming species of Anabaena and Oscillatoria . We tested SG-3 for activity against 10 isolates, representing seven species, of mat-forming cyanobacteria within the genera Oscillatoria, Lyngbya , and Phormidium . Plugs (0.5 cm diameter) were cut from mats of the cyanobacterium, inoculated with liquid cultures of SG-3, and incubated as static cultures. The reduction in dry weights ranged from –0.5% to 90% compared to the untreated controls and appeared to be species specific. For example, dry weight reductions of Oscillatoria deflexoides and O. amoena ranged from 80 to 90% whereas the reduction of O. limosa tended to be lower at 36 to 72%. Although results varied among and within species, they indicate that this bacterium could have potential for use as a biological control for mat-forming cyanobacteria. Light microscopic observations indicate the bacteria do not penetrate the cyanobacteria cells. Currently, we are studying the possible causes of the observed cell lysis.     

Characterization of true-branching cyanobacteria from geothermal sites and hot springs of Costa Rica

Costa Rica is at the centre of the Mesoamerican biodiversity hotspot. Little is known about cyanobacteria from this region so far. Here, four isolates of the order Stigonematales (section V) were characterized in a polyphasic approach. All strains were isolated from geothermal sites and hot springs of Costa Rica. However, one of them, identified as Westiellopsis sp. Ar73, did not grow at more than 40°C. Based on its identical 16S rRNA to several previously isolated Westiellopsis sp. and Fischerella muscicola strains, a ubiquitous distribution throughout tropical and subtropical regions can be implied. In contrast, the isolates MV9, MV11 and RV14 grew well up to 50–55°C. Based on morphologic, ultrastructural, molecular and physiologic data, MV9, MV11 and RV14 were identified to belong to the genus Fischerella . Two distinct intergenic transcribed spacer (ITS) types, with or without tRNA genes, were detected for all Stigonematales analysed here, indicating ITS polymorphism as a characteristic feature of heterocystous cyanobacteria. In phylogenetic trees, these Fischerella spp. formed a new and distinct clade within the wider lineage of thermophilic Fischerella ( Mastigocladus cf. laminosus ), which might represent a geographic lineage. Thus, geographic isolation may be an underestimated aspect of microbial evolution. The strains presented here are suitable as new models to study this group of cyanobacteria.     

Isolation and identification of antialgal substances produced byPseudomonas aeruginosa

Pseudomonas aeruginosa strongly inhibited the growth of green microalgae and cyanobacteria by the release of low molecular weight, thermoresistant factors. The antialgal substances were resistant to various enzymes and remained active in agar after a 3-months storage period at 4 °C in the absence of light. The results indicate that inhibition of algal growth was mediated by the phenazine pigments released by the bacterium. Pyocyanine and an unidentified pale blue pigment had no effect on algal growth, whereas 1-hydroxyphenazine and oxychlororaphine showed strong antialgal activity.Groupe de Recherche en Recyclage Biologique et AquicultureDépartement des Sciences et Technologie des Aliments     

Acclimation of chlorophyll a and carotenoid levels to different irradiances in four freshwater cyanobacteria

This study investigated carotenoid and chlorophyll a (Chl-a) contents under two different growth irradiances in four freshwater cyanobacterial strains. We found an increased weight ratio of zeaxanthin to Chl-a after exposure to high irradiances over several days. Two out of four strains showed higher zeaxanthin amounts on a biomass basis as well. It appears that cyanobacteria enhance their carotenoid pool in response to high light conditions, as increased production of other carotenoids with photoprotective abilities has also been observed under high irradiance levels. Cyanobacteria do not possess the violaxanthin cycle, which enables a rapid reversible conversion from violaxanthin into zeaxanthin and functioning as a quencher of excessive energy, and elevated zeaxanthin concentrations could therefore be seen as an adaptive strategy against excess light energy. Some differences in the acclimation pattern were revealed between different cyanobacteria. Anabaena torulosa contained higher amounts of every carotenoid, while Nostoc sp. mainly increased zeaxanthin, and myxoxanthophyll. Anabaenopsis elenkinii produced exceptionally high amounts of myxoxanthophyll and beta-carotene under higher irradiances. Anabaena cylindrica generally showed less variation of carotenoids under different irradiances.     

Glycolate metabolism in cyanobacteria

A comparative analysis of glycolate excretion in 11 cyanobacteria showed that 8 strains, although grown and assayed in air, excreted glycolate. The largest quantities were excreted by the filamentous strains Plectonema boryanum 73110 and Anabaena cylindrica (Lemm). The carbon lost by excretion was at most 9% of the net fixed carbon in air for heterocystous cyanobacteria but increased (up to 60%) in some strains under a high pO₂ (0.03 kPa CO₂ in pure O₂). A. cylindrica excreted glycolate at a maximum level of 2 and 10 μmol (mg chl a )⁻¹ h⁻¹ in air and at high pO₂, respectively. The excretion continued for several hours. Increases in light intensity and pO₂ and a shift in pH from 7 to 9 increased the amount of glycolate excreted. A. cylindrica also showed the most O₂-sensitive fixation of CO₂. In vitro activity of phosphoglycolate phosphatase (EC 3.1.3.18) was found in all strains tested, with the highest activities noted for Gloeobacter violaceus 7.82 and Gloeothece 6909 and for young cultures of A. cylindrica . The lowest activities were found in Anabaena 7120 and Anacystis nidulans 625, strains excreting no or only minor quantities of glycolate.     

Screening microalgae for some potentially useful agricultural and pharmaceutical secondary metabolites

Nearly two hundred microalgal strains (174 Chlorophyta and 23 Cyanobacteria) were screened against some bacteria, filamentous fungi and yeasts using a disc-diffusion type bioassay. From this initial screening, 10 Chlorophyta strains from three genera (Desmococcus, Chlorella and Scenedesmus) were selected because of their high antimicrobial activity. These 10 strains were partially purified and tested using MIC antimicrobial and microtiter IC₅₀ anticancer assays. These preselected algal strains showed a high incidence of antibacterial activity against both Gram-positive (9 out of 10 species) and Gram-negative (7 out of 10 species) bacteria. The extracts were also effective against some tumour cell lines.     

Surviving Chytridiomycosis: Differential Anti-Batrachochytrium dendrobatidis Activity in Bacterial Isolates from Three Lowland Species of Atelopus

In the Neotropics, almost every species of the stream-dwelling harlequin toads (genus Atelopus) have experienced catastrophic declines. The persistence of lowland species of Atelopus could be explained by the lower growth rate of Batrachochytrium dendrobatidis (Bd) at temperatures above 25°C. We tested the complementary hypothesis that the toads' skin bacterial microbiota acts as a protective barrier against the pathogen, perhaps delaying or impeding the symptomatic phase of chytridiomycosis. We isolated 148 cultivable bacterial strains from three lowland Atelopus species and quantified the anti-Bd activity through antagonism assays. Twenty-six percent (38 strains representing 12 species) of the bacteria inhibited Bd growth and just two of them were shared among the toad species sampled in different localities. Interestingly, the strongest anti-Bd activity was measured in bacteria isolated from A. elegans, the only species that tested positive for the pathogen. The cutaneous bacterial microbiota is thus likely a fitness-enhancing trait that may (adaptation) or not (exaptation) have appeared because of natural selection mediated by chytridiomycosis. Our findings reveal bacterial strains for development of local probiotic treatments against chytridiomycosis and also shed light on the mechanisms behind the frog-bacteria-pathogen interaction.     

Analysis of the hli gene family in marine and freshwater cyanobacteria

Certain cyanobacteria thrive in natural habitats in which light intensities can reach 2000 micromol photon m(-2) s(-1) and nutrient levels are extremely low. Recently, a family of genes designated hli was demonstrated to be important for survival of cyanobacteria during exposure to high light. In this study we have identified members of the hli gene family in seven cyanobacterial genomes, including those of a marine cyanobacterium adapted to high-light growth in surface waters of the open ocean (Prochlorococcus sp. strain Med4), three marine cyanobacteria adapted to growth in moderate- or low-light (Prochlorococcus sp. strain MIT9313, Prochlorococcus marinus SS120, and Synechococcus WH8102), and three freshwater strains (the unicellular Synechocystis sp. strain PCC6803 and the filamentous species Nostoc punctiforme strain ATCC29133 and Anabaena sp. [Nostoc] strain PCC7120). The high-light-adapted Prochlorococcus Med4 has the smallest genome (1.7 Mb), yet it has more than twice as many hli genes as any of the other six cyanobacterial species, some of which appear to have arisen from recent duplication events. Based on cluster analysis, some groups of hli genes appear to be specific to either marine or freshwater cyanobacteria. This information is discussed with respect to the role of hli genes in the acclimation of cyanobacteria to high light, and the possible relationships among members of this diverse gene family.