Evanescent photosynthesis: exciting cyanobacteria in a surface-confined light fieldElectronic supplementary information (ESI) available. See DOI: 10.1039/c2cp40271h

The conversion of solar energy to chemical energy useful for maintaining cellular function in photosynthetic algae and cyanobacteria relies critically on light delivery to the microorganisms. Conventional direct irradiation of a bulk suspension leads to non-uniform light distribution within a strongly absorbing culture, and related inefficiencies. The study of small colonies of cells in controlled microenvironments would benefit from control over wavelength, intensity, and location of light energy on the scale of the microorganism. Here we demonstrate that the evanescent light field, confined near the surface of a waveguide, can be used to direct light into cyanobacteria and successfully drive photosynthesis. The method is enabled by the synergy between the penetration depth of the evanescent field and the size of the photosynthetic bacterium, both on the order of micrometres. Wild type Synechococcus elongatus (ATCC 33912) cells are exposed to evanescent light generated through total internal reflection of red ( = 633 nm) light on a prism surface. Growth onset is consistently observed at intensity levels of 79 10 W m 2 , as measured 1 m from the surface, and 60 8 W m 2 as measured by a 5 m depthwise average. These threshold values agree well with control experiments and literature values based on direct irradiation with daylight. In contrast, negligible growth is observed with evanescent light penetration depths less than the minor dimension of the rod-like bacterium (achieved at larger light incident angles). Collectively these results indicate that evanescent light waves can be used to tailor and direct light into cyanobacteria, driving photosynthesis. Exciting photosynthetic cyanobacteria using near-field evanescent waves provides a new approach to cell illumination at micron scales. This tool provides a means of controlled light delivery to cells that can advance our understanding of cyanobacteria performance.