Periodic harvesting of microalgae from calcium alginate hydrogels for sustained high-density production.

High-density biomass production is currently only realized in biofilm-based photobioreactors. Harvest yields of whole biofilms are self-limited by daughter-upon-parent cell growth that hinders light and leads to respiratory biomass losses. In this work, we demonstrate a sustainable multi-harvest approach for prolonged generation of high-density biomass. Calcium-alginate hydrogel cultures loaded with Synechococcus elongatus PCC 7942 achieved production densities comparable to that of biofilms (109 cells/mL) and optimal total productivity in harvest periods of 2 or 3 days that allowed high-density surface growth without self-limiting cell buildup or surface death. Cross-linking calcium concentration had a strong influence on surface growth and harvest yields, especially in the first harvests. Subsequent harvests achieved more uniform biomass yields and distributions, unaffected by bulk respiration or light penetration. Collectively, these results demonstrate the feasibility of sustained, high-density biomass production by periodic harvesting within microalgal hydrogel cultures. Biotechnol. Bioeng. 2017;114: 2023-2031. © 2017 Wiley Periodicals, Inc.

Breathable waveguides for combined light and CO2 delivery to microalgae.

Suboptimal light and chemical distribution (CO2, O2) in photobioreactors hinder phototrophic microalgal productivity and prevent economically scalable production of biofuels and bioproducts. Current strategies that improve illumination in reactors negatively impact chemical distribution, and vice versa. In this work, an integrated illumination and aeration approach is demonstrated using a gas-permeable planar waveguide that enables combined light and chemical distribution. An optically transparent cellulose acetate butyrate (CAB) slab is used to supply both light and CO2 at various source concentrations to cyanobacteria. The breathable waveguide architecture is capable of cultivating microalgae with over double the growth as achieved with impermeable waveguides.

Evanescent photosynthesis: exciting cyanobacteria in a surface-confined light field.

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 (λ = 633 nm) light on a prism surface. Growth onset is consistently observed at intensity levels of 79 ± 10 W m(-2), as measured 1 µm from the surface, and 60 ± 8 W m(-2) as measured by a 5 µm 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.