Two cyanobacterial species exhibit stress responses when grown together in visible light or far-red light.

Although most cyanobacteria grow in visible light (VL; λ = 400-700 nm), some cyanobacteria can also use far-red light (FRL; λ = 700-800 nm) for oxygenic photosynthesis by performing far-red light photoacclimation. These two types of cyanobacteria can be found in the same environment. However, how they respond to each other remains unknown. Here, we reveal that coculture stresses FRL-using Chlorogloeopsis fritschii PCC 9212 and VL-using Synechocystis sp. PCC 6803. No significant growth difference was found in Synechocystis sp. PCC 6803 between the coculture and the monoculture. Conversely, the growth of Chlorogloeopsis fritschii PCC 9212 was suppressed in VL under coculture. According to transcriptomic analysis, Chlorogloeopsis fritschii PCC 9212 in coculture shows low transcript levels of metabolic activities and high transcript levels of ion transporters, with the differences being more noticeable in VL than in FRL. The transcript levels of stress responses in coculture were likewise higher than in monoculture in Synechocystis sp. PCC 6803 under FRL. The low transcript level of metabolic activities in coculture or the inhibition of cyanobacterial growth indicates a possible negative interaction between these two cyanobacterial strains.IMPORTANCEThe interaction between two cyanobacterial species is the primary focus of this study. One species harvests visible light, while the other can harvest far-red and visible light. Prior research on cyanobacteria interaction concentrated on its interactions with algal, coral, and fungal species. Interactions between cyanobacterial species were, nevertheless, rarely discussed. Thus, we characterized the interaction between two cyanobacterial species, one capable of photosynthesis using far-red light and the other not. Through experimental and bioinformatic approaches, we demonstrate that when one cyanobacterium thrives under optimal light conditions, it stresses the remaining cyanobacterial species. We contribute to an ecological understanding of these two kinds of cyanobacteria distribution patterns. Cyanobacteria that utilize far-red light probably disperse in environments with limited visible light to avoid competition with other cyanobacteria. From a biotechnological standpoint, this study suggests that the simultaneous cultivation of two cyanobacterial species in large-scale cultivation facilities may reduce the overall biomass yield.

Identification of a Far-Red Light-Inducible Promoter that Exhibits Light Intensity Dependency and Reversibility in a Cyanobacterium.

As the demand for sustainable energy has increased, photoautotrophic cyanobacteria have become a popular platform for developing tools in synthetic biology. Although genetic tools are generally available for several model cyanobacteria, such tools have not yet been developed for many other strains potentially suitable for industrial applications. Additionally, most inducible promoters in cyanobacteria are controlled by chemical compounds, but adding chemicals into growth media on an industrial scale is neither cost-effective nor environmentally friendly. Although using light-controlled promoters is an alternative approach, only a cyanobacterial expression system inducible by green light has so far been described and employed for such applications. In this study, we have established a conjugation-based technique to express a reporter gene (eyfp) in the nonmodel cyanobacterium, Chlorogloeopsis fritschii PCC 9212. We also identified a promoter specifically activated by far-red light from the Far-Red Light Photoacclimation gene cluster of Leptolyngbya sp. JSC-1. This promoter, PchlFJSC1, was successfully used to drive eyfp expression. PchlFJSC1 is tightly regulated by light quality (i.e., wavelength) and leads to an approximately 30-fold increase in EYFP production when cells were exposed to far-red light. The induction level was controlled by the far-red light intensity, and induction stopped when cells were returned to visible light. This system has the potential for further applications in cyanobacteria by providing an additional choice of light wavelength to control gene expression. Collectively, this study developed a functional gene-expression system for C. fritschii PCC 9212 that can be regulated by exposing cells to far-red light.

Use of Quartz Sand Columns to Study Far-Red Light Photoacclimation (FaRLiP) in Cyanobacteria.

Some cyanobacteria can perform far-red light photoacclimation (FaRLiP), which allows them to use far-red light (FRL) for oxygenic photosynthesis. Most of the cyanobacteria able to use FRL were discovered in low visible-light (VL; λ = 400-700 nm) environments that are also enriched in FRL (λ = 700-800 nm). However, these cyanobacteria grow faster in VL than in FRL in laboratory conditions, indicating that FRL is not their preferred light source when VL is available. Therefore, it is interesting to understand why such strains were primarily found in FRL-enriched but not VL-enriched environments. To this aim, we established a terrestrial model system with quartz sand to study the distribution and photoacclimation of cyanobacterial strains. A FaRLiP-performing cyanobacterium, Leptolyngbya sp. JSC-1, and a VL-utilizing model cyanobacterium, Synechocystis sp. PCC 6803, were compared in this study. We found that, although Leptolyngbya sp. JSC-1 can grow well in both VL and FRL, Synechocystis sp. PCC 6803 grows much faster than Leptolyngbya sp. JSC-1 in VL. In addition, the growth was higher in liquid cocultures than in monocultures of Leptolyngbya sp. JSC-1 or Synechocystis sp. PCC 6803. In an artificial terrestrial model system, Leptolyngbya sp. JSC-1 has an advantage when growing in coculture at greater depths by performing FaRLiP. Therefore, strong competition for VL and slower growth rate are possible reasons why FRL-utilizing cyanobacteria are found in environments with low VL intensities. This model system provides a valuable tool for future studies of cyanobacterial ecological niches and interactions in a terrestrial environment. IMPORTANCE This study uses sand columns to establish a terrestrial model system for the investigation of the distribution and acclimation of cyanobacteria to far-red light. Previous studies of this group of cyanobacteria required direct in situ samplings. The variability of conditions and abundances of the cyanobacteria in natural settings impeded detailed analyses and comparisons. Therefore, we established this model system under controlled conditions in the laboratory. In this system, the distribution and acclimation of two cyanobacteria were similar to the situation observed in natural environments, which validates that it can be used to study fundamental questions. Using this approach, we made the unanticipated observation that two cyanobacteria grow faster in coculture than in axenic cultures. This laboratory-based model system can provide a valuable new tool for comparing cyanobacterial strains (e.g., mutants and wild type), exploring interactions between cyanobacterial strains and interactions with other bacteria, and characterizing ecological niches of cyanobacteria.