title: Emergent Phototactic Responses of Cyanobacteria under Complex Light Regimes.
abstract: Environmental cues can stimulate a variety of single-cell responses, as well as collective behaviors that emerge within a bacterial community. These responses require signal integration and transduction, which can occur on a variety of time scales and often involve feedback between processes, for example, between growth and motility. Here, we investigate the dynamics of responses of the phototactic, unicellular cyanobacteriumSynechocystissp. PCC6803 to complex light inputs that simulate the natural environments that cells typically encounter. We quantified single-cell motility characteristics in response to light of different wavelengths and intensities. We found that red and green light primarily affected motility bias rather than speed, while blue light inhibited motility altogether. When light signals were simultaneously presented from different directions, cells exhibited phototaxis along the vector sum of the light directions, indicating that cells can sense and combine multiple signals into an integrated motility response. Under a combination of antagonistic light signal regimes (phototaxis-promoting green light and phototaxis-inhibiting blue light), the ensuing bias was continuously tuned by competition between the wavelengths, and the community response was dependent on both bias and cell growth. The phototactic dynamics upon a rapid light shift revealed a wavelength dependence on the time scales of photoreceptor activation/deactivation. Thus,Synechocystiscells achieve exquisite integration of light inputs at the cellular scale through continuous tuning of motility, and the pattern of collective behavior depends on single-cell motility and population growth.IMPORTANCEThe photosynthetic cyanobacteriumSynechocystissp. exhibits phototaxis that is dependent on the incident light wavelength through the action of various photoreceptors. In natural environments, cells experience a set of highly dynamic and complex light inputs, yet how cells transduce multiple or dynamic inputs into motion is unknown. In this study, we measured the phototactic behaviors of single cells and communities as a function of light intensity or when illuminated by combinations of lights of different wavelengths or incidence directions. Responses to a spectrum of light regimes revealed thatSynechocystissp. integrates information about the light environment to tune its phototactic response, which is likely generated by competition among photoreceptors and the degree of wavelength-regulated growth to sensitively control the direction and degree of movement.
authors.affiliation: Department of Bioengineering, Stanford University, Stanford, California, USA.
authors.forename: Rosanna Man Wah
authors.initials: RM
authors.lastname: Chau
authors.affiliation: Department of Plant Biology, Carnegie Institution for Science, Stanford, California, USA dbhaya@stanford.edu kchuang@stanford.edu.
authors.forename: Devaki
authors.initials: D
authors.lastname: Bhaya
authors.affiliation: Department of Bioengineering, Stanford University, Stanford, California, USA dbhaya@stanford.edu kchuang@stanford.edu.|Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California, USA.
authors.forename: Kerwyn Casey
authors.initials: KC
authors.lastname: Huang
mesh_terms: D008027:Light; D020283:Photoreceptors, Microbial; D000071444:Phototaxis; D015398:Signal Transduction; D046939:Synechocystis / Q000502:physiology* / Q000528:radiation effects*
chemical_list: D020283:Photoreceptors, Microbial