Unicellular Cyanobacterium Synechococcus Research Articles

Equal-quantum Action Spectra Indicate Fluence-rate-selective Action of Multiple Photoreceptors for Photomovement of the Thermophilic Cyanobacterium Synechococcus elongatus¶

Unicellular thermophilic cyanobacterium Synechococcus elongatus displayed phototaxis on agar plate at 55°C. Equal-quantum action spectra for phototactic migration were determined at various fluence rates using the Okazaki Large Spectrograph as the light source. The shapes of the action spectra drastically changed depending on the fluence rate of the unilateral monochromatic irradiation: at a low fluence rate (3 μmol/m2/s), only lights in the red region had significant effect; at a medium fluence rate (10 μmol/m2/s), four major action peaks were observed at 530 nm (green), 570 nm (yellow), 640 nm (red) and 680 nm (red). At high fluence rates (30–90 μmol/m2/s), the former two peaks remained, while red peaks at 640 nm and 680 nm disappeared and, interestingly, an action peak around 700–740 nm (far-red) newly appeared. These results indicate that two or more distinct photoreceptors are involved in the phototaxis and that suitable photoreceptors are selectively active in response to the stimulus of light fluence rates. Far-red or red background lights irradiated vertically from above drastically inhibited phototaxis toward red light or far-red light, respectively. These results indicate involvement of some phytochrome(s).

Distinctive Types of ATP-dependent Clp Proteases in Cyanobacteria

Cyanobacteria are the only prokaryotes that perform oxygenic photosynthesis and are thought to be ancestors to plant chloroplasts. Like chloroplasts, cyanobacteria possess a diverse array of proteolytic enzymes, with one of the most prominent being the ATP-dependent Ser-type Clp protease. The model Clp protease in Escherichia coli consists of a single ClpP proteolytic core flanked on one or both ends by a HSP100 chaperone partner. In comparison, cyanobacteria have multiple ClpP paralogs plus a ClpP variant (ClpR), which lacks the catalytic triad typical of Ser-type proteases. In this study, we reveal that two distinct soluble Clp proteases exist in the unicellular cyanobacterium Synechococcus elongatus. Each protease consists of a unique proteolytic core comprised of two separate Clp subunits, one with ClpP1 and ClpP2, the other with ClpP3 and ClpR. Each core also associates with a particular HSP100 chaperone partner, ClpC in the case of the ClpP3/R core, and ClpX for the ClpP1/P2 core. The two adaptor proteins, ClpS1 and ClpS2 also interact with the ClpC chaperone protein, likely increasing the range of protein substrates targeted by the Clp protease in cyanobacteria. We also reveal the possible existence of a third Clp protease in Synechococcus, one which associates with the internal membrane network. Altogether, we show that presence of several distinctive Clp proteases in cyanobacteria, a feature which contrasts from that in most other organisms.

Complete nucleotide sequence of the freshwater unicellular cyanobacterium Synechococcus elongatus PCC 6301 chromosome: gene content and organization

The entire genome of the unicellular cyanobacterium Synechococcus elongatus PCC 6301 (formerly Anacystis nidulans Berkeley strain 6301) was sequenced. The genome consisted of a circular chromosome 2,696,255 bp long. A total of 2,525 potential protein-coding genes, two sets of rRNA genes, 45 tRNA genes representing 42 tRNA species, and several genes for small stable RNAs were assigned to the chromosome by similarity searches and computer predictions. The translated products of 56% of the potential protein-coding genes showed sequence similarities to experimentally identified and predicted proteins of known function, and the products of 35% of the genes showed sequence similarities to the translated products of hypothetical genes. The remaining 9% of genes lacked significant similarities to genes for predicted proteins in the public DNA databases. Some 139 genes coding for photosynthesis-related components were identified. Thirty-seven genes for two-component signal transduction systems were also identified. This is the smallest number of such genes identified in cyanobacteria, except for marine cyanobacteria, suggesting that only simple signal transduction systems are found in this strain. The gene arrangement and nucleotide sequence of Synechococcus elongatus PCC 6301 were nearly identical to those of a closely related strain Synechococcus elongatus PCC 7942, except for the presence of a 188.6 kb inversion. The sequences as well as the gene information shown in this paper are available in the Web database, CYORF (http://www.cyano.genome.jp/).

Integrating the Circadian Oscillator into the Life of the Cyanobacterial Cell

In two decades, the study of circadian rhythms in cyanobacteria has gone from observations of phenomena in intractable species to the development of a model organism for mechanistic study, atomic-resolution structures of components, and reconstitution of a circadian biochemical oscillation in vitro. With sophisticated biochemical, biophysical, genetic, and genomic tools in place, the circadian clock of the unicellular cyanobacterium Synechococcus elongatus is poised to be the first for which a systems-level understanding can be achieved.

Expression of the Circadian Clock-Related Gene pex in Cyanobacteria Increases in Darkness and Is Required to Delay the Clock

The time measurement system of the unicellular cyanobacterium Synechococcus elongatus PCC 7942 is analogous to the circadian clock of eukaryotic cells. Circadian clock-related genes have been identified in this strain. The clock-related gene pex is thought to maintain the normal clock period because constitutive transcription or deficiency of this gene causes respectively longer (approximately 28 h) or shorter (approximately 24 h) circadian periods than that of the wild type (approximately 25 h). Here, the authors report other properties of pex in the circadian system. Levels of pex mRNA increased significantly in a 12-h exposure to darkness. Western blotting with a GST-Pex antibody revealed a 13.5-kDa protein band in wild-type cells that were incubated in the dark, while this protein was not detected in pex-deficient mutant cells. Therefore, the molecular weight of the Pex protein appears to be 13.5 kDa in vivo. The PadR domain, which is conserved among DNA-binding transcription factors in lactobacilli, was found in Pex. In the pex mutant, several 12-h light/12-h dark cycles reset the phase of the clock by 3 h earlier (phase advance) compared to wild-type cells. The degree of the advance in the pex mutant was proportional to the number of exposed light-dark cycles. In addition, ectopic induction of pex with an inducible Escherichia coli promoter, Ptrc, delayed the phase in the examined recombinant cells by 2.5 h (phase delay) compared to control cells. These results suggest that the dark-responsive gene expression of pex delays the circadian clock under daily light-dark cycles.

Physiological responses of Aureoumbra lagunensis and Synechococcus sp. to nitrogen addition in a mesocosm experiment

Aureoumbra lagunensis is the causative organism of the Texas brown tide and is notable because it dominated the Laguna Madre ecosystem from 1990 to 1997. This species is unusual because it has the highest known critical nitrogen to phosphorus ratio (N:P) for any microalgae ranging from 115 to 260, far higher than the 16N:1P Redfield ratio. Because of its high N:P ratio, Aureoumbra should be expected to respond to N additions that would not stimulate the growth of competitors having the Redfield ratio. To evaluate this prediction, a mesocosm experiment was performed in the Laguna Madre, a South Texas coastal lagoon, in which a mixed Aureoumbra– Synechococcus (a cyanobacterium) community was enclosed in 12 mesocosms and subjected to nitrogen addition (6 controls, 6 added ammonium) for 16 days. After day 4, added nitrogen did not significantly increase Aureoumbra specific growth rate but the alga retained dominance throughout the experiment (64–75% of total cell biovolume). In control mesocosms, Aureoumbra became less abundant during the first 4 days of the experiment but rebounded by the end of the experiment and was dominant over Synechococcus. Despite the lack of a strong positive growth response, Aureoumbra did respond physiologically to N addition. By the end of the experiment, the average N:P ratio of the Aureoumbra-dominated community was 86 in the N+ treatment and 41 in the control, indicating that the alga became less N-limited in the N+ treatment. The average C:N ratio was 6.6 in the N+ treatment (8.6 in the control) and suggests that the alga was not N-limited, however, C:N ratio may not be a good indicator of nitrogen limitation since this alga can produce significant quantities of carbon-containing extracellular polysaccharides, depending on growth conditions. Both Aureoumbra cellular chlorophyll fluorescence and cell size increased in response to added N, indicating a reduction in N limitation. It appeared that the N additions were not large and/or frequent enough to stimulate Aureoumbra growth. The main competitor, the unicellular cyanobacterium Synechococcus, responded positively to the nitrogen addition by increased specific growth rate. Unlike Aureoumbra, no significant effect on Synechococcus cellular pigment fluorescence or cell size was noted. Literature data suggest that Synechococcus, like Aureoumbra, may have a critical N:P ratio much higher than 16:1, which could explain its response.

Phosphate and ATP uptake and growth kinetics in axenic cultures of the cyanobacterium Synechococcus CCMP 1334

Phosphorus potentially limits the growth and productivity of the unicellular cyanobacterium Synechococcus in many oligotrophic oceans. Both dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) could be important P sources in these regimes. We compared DIP and adenosine tri-phosphate (ATP) uptake and growth kinetics using axenic semi-continuous cultures of Synechococcus CCMP 1334 grown on both P substrates, under both P-limited and P-replete conditions. Molar C:total P ratios and N:total P ratios were 417–431 and 65–68, respectively, under severely P-limited growth conditions for both P sources, but were near Redfield values at >1.0 µM P as either DIP or ATP, and were inversely proportional to growth rates. Synechococcus grown on ATP as the sole P-source have the same maximum growth rates, lower half-saturation constants for growth, and similar cellular P quotas compared with cultures grown on DIP. DI33P and 33P-ATP uptake experiments confirmed that both can be used by Synechococcus, with relative uptake rates depending on prior growth history. DIP-limited cultures assimilated ATP faster than DIP, while DIP-replete cultures took up DIP more rapidly. In contrast, for Synechococcus grown under ATP-limited conditions, there is no priority for the utilization of ATP or DIP, but ATP-replete cultures have a greater ability to use DIP. Like other phytoplankton, kinetic constants for Synechococcus DIP uptake and cellular P quotas, C:P and N:P ratios of DIP-grown cells differ when total cellular and intracellular P pools are compared, suggesting a need to re-evaluate these parameters to account for abiotic phosphate scavenging onto cell surfaces. DIP half-saturation constants for growth of this Synechococcus strain would not support observed natural growth rates at the nanomolar DIP concentrations found in most oligotrophic regimes, and hence populations in these areas probably rely primarily on DOP.

Effect of temperature and light on growth of and photosynthesis by Synechococcus isolates typical of those predominating in the octopus spring microbial mat community of Yellowstone National Park.

Previous molecular analysis of the Octopus Spring cyanobacterial mat revealed numerous genetically distinct 16S rRNA sequences from predominant Synechococcus populations distantly related to the readily cultivated unicellular cyanobacterium Synechococcus lividus. Patterns in genotype distribution relative to temperature and light conditions suggested that the organisms contributing these 16S rRNA sequences may fill distinct ecological niches. To test this hypothesis, Synechococcus isolates were cultivated using a dilution and filtration approach and then shown to be genetically relevant to natural mat populations by comparisons of similarities of 16S rRNA genes and 16S-23S internal transcribed spacer (ITS) regions. Most isolates were identical or nearly identical at both loci to predominant mat genotypes; others showed 1- to 2-nucleotide differences at the 16S rRNA locus and even greater difference in ITS sequences. Isolates with predominant mat genotypes had distinct temperature ranges and optima for growth that were consistent with their distributions in the mat. Isolates with genotypes not previously detected or known to be predominant in the mat exhibited temperature ranges and optima that were not representative of predominant mat populations and also grew more slowly. Temperature effects on photosynthesis did not reflect temperature relations for growth. However, the isolate with the highest temperature optimum and upper limit was capable of performing photosynthesis at a higher temperature than other isolates. Growth rate and photosynthetic responses provided evidence for light acclimation but evidence of, at best, only subtle light adaptation.

Impacts of Shading on Sponge-Cyanobacteria Symbioses: A Comparison between Host-Specific and Generalist Associations

The marine sponge Lamellodysidea chlorea contains large populations of the host-specific, filamentous cyanobacterium Oscillatoria spongeliae. Other marine sponges, including Xestospongia exigua, contain the generalist, unicellular cyanobacterium Synechococcus spongiarum. The impact of cyanobacterial photosynthesis on host sponges was manipulated by shading these sponge-cyanobacteria associations. If cyanobacteria benefit their hosts, shading should reduce this benefit. Chlorophyll a concentrations were measured as an index of cyanobacterial abundance. After two weeks, shaded L. chlorea lost more mass than controls, while shaded and control X. exigua did not lose a significant amount of mass. Chlorophyll a concentrations in shaded X. exigua were lower than in controls, but were not significantly different between shaded and control L. chlorea. In addition, L. chlorea shaded in situ lost over 40% of their initial area, but did not differ in chlorophyll a concentrations from controls. These results suggest that Oscillatoria symbionts benefit their host sponges in a mutualistic association. Synechococcus symbionts may be commensals that exploit the resources provided by their sponge hosts without significantly affecting sponge mass. When shaded, Synechococcus symbionts may be consumed by their hosts or may be able to disperse from this unfavorable environment. These data support the hypothesis that more specialized symbionts provide a greater benefit to their hosts, but hypotheses concerning the dispersal abilities of these symbionts remain to be explored. Sponge-cyanobacteria symbioses provide model systems for investigating the costs and benefits of symbiosis and the roles of dispersal, environmental conditions, and phylogenetic history in determining the specificity of endosymbionts for their hosts.

Monomeric NarB is a dual-affinity nitrate reductase, and its activity is regulated differently from that of nitrate uptake in the unicellular diazotrophic cyanobacterium Synechococcus sp. strain RF-1.

Synechococcus sp. strain RF-1 is a unicellular freshwater cyanobacterium that fixes N(2) aerobically and exhibits a circadian rhythm for nitrogenase activity under a light-dark regimen. Synechococcus sp. strain RF-1 also utilizes nitrate, nitrite, or ammonium for growth. Under the diazotrophic growth, the nitrate uptake in Synechococcus sp. strain RF-1 was induced by nitrate or nitrite but repressed by ammonium. In contrast, a prominent nitrate reductase (NR) activity was detected in diazotrophically grown cells using the reduced methyl viologen assay. The NR activity was not inhibited by ammonium and only slightly enhanced by nitrate. The different expression patterns of nitrate uptake and NR in Synechococcus sp. strain RF-1 were reflected in general at the transcript level determined by reverse transcriptase PCR. Under both nitrate-induced and uninduced conditions, the in situ NR activity exhibited similar biphasic kinetics for nitrate. The recombinant NR encoded by the narB gene of Synechococcus sp. strain RF-1, expressed in E. coli, also showed the biphasic kinetics with similar pH and temperature profiles. By in-gel NR activity assay, the recombinant NarB was found to exist as a single form. Both the high- and low-affinity NR activities of the recombinant NarB showed the same thermostability. When modified at the N terminus by a polyhistidine tag, the recombinant NR activity was shifted from biphasic to hyperbolic kinetics and showed only a single K(m) for nitrate, indicating the functional importance of the NarB N-terminal structure in NR kinetics.

Transcriptional effects of the signal transduction protein P II ( glnB gene product) on NtcA-dependent genes in Synechococcus sp. PCC 7942

P II proteins signal the cellular nitrogen status in numerous bacteria, and in cyanobacteria P II is subjected to serine phosphorylation when the cells experience a high C to N balance. In the unicellular cyanobacterium Synechococcus sp. PCC 7942, the P II protein ( glnB gene product) is known to mediate the ammonium-dependent inhibition of nitrate and nitrite uptake. The analysis of gene expression through RNA/DNA hybridization indicated that a P II-null mutant was also impaired in the induction of NtcA-dependent, nitrogen assimilation genes amt1 (ammonium permease), glnA (glutamine synthetase) and nir (nitrite reductase), as well as of the N-control gene ntcA, mainly under nitrogen deprivation. This gene expression phenotype of the glnB mutant could be complemented by wild-type P II protein or by modified P II proteins that cannot be phosphorylated and mimic either the phosphorylated (GlnB S49D and GlnB S49E) or unphosphorylated (GlnB S49A) form of P II. However, strains carrying the GlnB S49D and GlnB S49E mutant proteins exhibited higher levels of expression of nitrogen-regulated genes than the strains carrying the wild-type P II or the GlnB S49A protein.

Carbon supply and 2-oxoglutarate effects on expression of nitrate reductase and nitrogen-regulated genes in Synechococcus sp. strain PCC 7942

Synthesis of nitrate reductase in the unicellular cyanobacterium Synechococcus sp. strain PCC 7942 took place at a slow rate when the cells were incubated without a supply of inorganic carbon, but addition to these cells of CO 2/bicarbonate or, in a Synechococcus strain transformed with a gene encoding a 2-oxoglutarate permease, 2-oxoglutarate stimulated expression of the enzyme. Induction by 2-oxoglutarate was also observed at the mRNA level for two nitrogen-regulated genes, nir and amt1, but not for the photosystem II D1 protein-encoding gene psbA1. Our results are consistent with a role of 2-oxoglutarate in nitrogen control in cyanobacteria.

Extracellular degradation of phenol by the cyanobacterium Synechococcus PCC 7002

Investigations of the unicellular marine cyanobacterium Synechococcus PCC 7002 revealed its ability to metabolize phenol under non-photosynthetic conditions up to 100 mg L−1. Under continuous light, photoautotrophic growth was reduced only slightly in the presence of this phenol concentration, but no transformation was observed. However neither under photoautotrophic nor heterotrophic conditions were the cells able to use phenol for growth. During the degradation of phenol in the dark cis,cis-muconic acid was produced as the major product, which was identified by gas chromatography/mass spectrometry. This result was confirmed by an identical absorption spectrum and an identical retention time in high performance liquid chromatographic analysis with authentic muconic acid as standard. This provides the first record for an ortho-fission of a phenolic substance by cyanobacteria. Further investigations of the breakdown mechanism of phenol have shown that the transformation is an extracellular process inhibited by heat, proteases and metal ions and is probably catalyzed by a protein.

Mutations in KaiA, a clock protein, extend the period of circadian rhythm in the cyanobacterium Synechococcus elongatus PCC 7942.

KaiA KaiB and KaiC are essential circadian clock proteins in the unicellular cyanobacterium Synechococcus elongatus PCC 7942. KaiA protein activates transcription of the kaiBC operon, which is believed to be a crucial step in the oscillating feedback loop of cyanobacteria. In this study, approximately approximately 400 mutations were introduced into kaiA by PCR-based mutagenesis, and rhythmic phenotypes of these mutants were studied by a bioluminescence reporter. In contrast to mutations in KaiB or KaiC, the vast majority of KaiA mutations extended the period and only rarely shortened it. The period could be extended to 35 h without lowering the mean or peak levels of kaiBC expression. However, several mutations resulted in low-amplitude oscillations or arrhythmia, which were accompanied by lowered kaiBC transcription. These results imply that the KaiA protein can change the period length of the circadian rhythm directly (through an unknown biochemical mechanism) or indirectly (by lowering kaiBC expression). Specific mutations of KaiA were identified in 34 mutants. While mutations mapped to various locations of the KaiA sequence, two clusters of period-altering mutations were found. This suggested that these regions are important domains of the KaiA protein for defining the period length. On the other hand, different sequences within KaiA to which arrhythmic mutations were mapped are important to enhance kaiBC expression.

Roles for sigma factors in global circadian regulation of the cyanobacterial genome.

The circadian clock of the unicellular cyanobacterium Synechococcus elongatus PCC 7942 imposes a global rhythm of transcription on promoters throughout the genome. Inactivation of any of the four known group 2 sigma factor genes (rpoD2, rpoD3, rpoD4, and sigC), singly or pairwise, altered circadian expression from the psbAI promoter, changing amplitude, phase angle, waveform, or period. However, only the rpoD2 mutation and the rpoD3 rpoD4 and rpoD2 rpoD3 double mutations affected expression from the kaiB promoter. A striking differential effect was a 2-h lengthening of the circadian period of expression from the promoter of psbAI, but not of those of kaiB or purF, when sigC was inactivated. The data show that separate timing circuits with different periods can coexist in a cell. Overexpression of rpoD2, rpoD3, rpoD4, or sigC also changed the period or abolished the rhythmicity of PpsbAI expression, consistent with a model in which sigma factors work as a consortium to convey circadian information to downstream genes.

The NtcA-activated amt1 gene encodes a permease required for uptake of low concentrations of ammonium in the cyanobacterium Synechococcus sp. PCC 7942.

In the unicellular cyanobacterium Synechococcus sp. PCC 7942, ammonium/methylammonium transport activity has been characterized but ammonium transport genes have not been described. The amt1 gene encoding a permease responsible for high-affinity [14C]methylammonium transport in Synechococcus sp. PCC 7942 was cloned and inactivated. The Amt1 permease appeared essential to take up ammonium when it was present at low concentrations in the external medium and might also be involved in recapture of ammonium leaked out from the cells. Expression of amt1, which was induced in the absence of ammonium and also influenced by the inorganic carbon supply, was dependent on the NtcA transcriptional regulator. The promoter of amt1 was found to exhibit the structure of NtcA-activated promoters, and specific binding of purified NtcA to amt1 promoter sequences was observed. The results of this study indicate that amt1 belongs to the NtcA regulon and that NtcA may respond to both nitrogen and carbon availability.

Diffusion of Phycobilisomes on the Thylakoid Membranes of the Cyanobacterium Synechococcus 7942: EFFECTS OF PHYCOBILISOME SIZE, TEMPERATURE, AND MEMBRANE LIPID COMPOSITION

A variant of fluorescence recovery after photobleaching allows us to observe the diffusion of photosynthetic complexes in cyanobacterial thylakoid membranes in vivo. The unicellular cyanobacterium Synechococcus sp. PCC7942 is a wonderful model organism for fluorescence recovery after photobleaching, because it has a favorable membrane geometry and is well characterized and transformable. In Synechococcus 7942 (as in other cyanobacteria) we find that photosystem II is immobile, but phycobilisomes diffuse rapidly on the membrane surface. The diffusion coefficient is 3 x 10(-10) cm(2) s(-1) at 30 degrees C. This shows that the association of phycobilisomes with reaction centers is dynamic; there are no stable phycobilisome-reaction center complexes in vivo. We report the effects of mutations that change the phycobilisome size and membrane lipid composition. 1) In a mutant with no phycobilisome rods, the phycobilisomes remain mobile with a slightly faster diffusion coefficient. This confirms that the diffusion we observe is of intact phycobilisomes rather than detached rod elements. The faster diffusion coefficient in the mutant indicates that the rate of diffusion is partly determined by the phycobilisome size. 2) The temperature dependence of the phycobilisome diffusion coefficient indicates that the phycobilisomes have no integral membrane domain. It is likely that association with the membrane is mediated by multiple weak interactions with lipid head groups. 3) Changing the lipid composition of the thylakoid membrane has a dramatic effect on phycobilisome mobility. The results cannot be explained in terms of changes in the fluidity of the membrane; they suggest that lipids play a role in controlling phycobilisome-reaction center interaction.

Two KaiA-binding domains of cyanobacterial circadian clock protein KaiC

kaiABC, a gene cluster, encodes KaiA, KaiB and KaiC proteins that are essential to circadian rhythms in the unicellular cyanobacterium Synechococcus sp. strain PCC 7942. Kai proteins can interact with each other in all possible combinations. This study identified two KaiA-binding domains (C KABD1 and C KABD2) in KaiC at corresponding regions of its duplicated structure. Clock mutations on the two domains and kaiA altered the strength of C KABD–KaiA interactions assayed by the yeast two-hybrid system. Thus, interaction between KaiA and KaiC through C KABD1 and C KABD2 is likely important for circadian timing in the cyanobacterium.

Genome analysis of Synechococcus sp. strain PCC6301

The genome of the unicellular cyanobacterium Synechococcus sp. strain PCC6301 (Anacystis nidulans 6301) is 2.7 Mb in size. To understand the genome structure and function we constructed a set of ordered lambda clones that covered the entire region of the Synechococcus PCC6301 genome. The insert DNA of lambda phage clones was amplified by LA PCR and sheared to short DNA fragments. The resultant DNA fragments were subcloned into a SmaI site of pUC18, and then shotgun sequenced. We present here the gene organization of Synechococcus PCC6301 genome.

Equal-quantum action spectra indicate fluence-rate-selective action of multiple photoreceptors for photomovement of the thermophilic cyanobacterium Synechococcus elongatus.

Unicellular thermophilic cyanobacterium Synechococcus elongatus displayed phototaxis on agar plate at 55 degrees C. Equal-quantum action spectra for phototactic migration were determined at various fluence rates using the Okazaki Large Spectrograph as the light source. The shapes of the action spectra drastically changed depending on the fluence rate of the unilateral monochromatic irradiation: at a low fluence rate (3 mumol/m2/s), only lights in the red region had significant effect; at a medium fluence rate (10 mumol/m2/s), four major action peaks were observed at 530 nm (green), 570 nm (yellow), 640 nm (red) and 680 nm (red). At high fluence rates (30-90 mumol/m2/s), the former two peaks remained, while red peaks at 640 nm and 680 nm disappeared and, interestingly, an action peak around 700-740 nm (far-red) newly appeared. These results indicate that two or more distinct photoreceptors are involved in the phototaxis and that suitable photoreceptors are selectively active in response to the stimulus of light fluence rates. Far-red or red background lights irradiated vertically from above drastically inhibited phototaxis toward red light or far-red light, respectively. These results indicate involvement of some phytochrome(s).