Complementary chromatic adaptation as a strategy to increase energy conversion efficiency of microalgae-cyanobacteria consortia in continuous LED photobioreactors

Abstract

Phototrophic cultivation of microalgae represents a potential sustainable process to obtain valuable commercial products, but the industrial cultivation is still hampered by low energy-to-biomass conversion efficiency. However, a number of cyanobacteria species evolved the ability to adapt to the prevailing light spectrum exploiting phycobiliproteins and Complementary Chromatic Adaptation (CCA). Spectral properties of such accessory pigments can be, hence, used to cover the green gap and possibly use light energy more efficiently. In this work, the green microalgae Tetradesmus obliquus and the chromatically adapting cyanobacterium Tolypothrix tenuis were cultivated as an alga-alga consortium in chemostat, aimed at increasing the light utilization efficiency in LEDs illuminated photobioreactors. When grown in co-culture, the advantage of consortium, with respect to monocultures, was maintained under different spectra, achieving an areal productivity up to 38 g m−2 d−1 under low red light. A remarkable photosynthetic conversion efficiency was reached under cocultivation, as a result of a wider absorption of photons with different wavelengths by the two species. Red light also resulted the most suitable one from the overall energy balance when artificial light is used. In this condition, by also considering the lamp efficiency, an overall conversion efficiency equal to 21% was reached (on Photosynthetically Active Radiations of RED light and calculated for the biomass of the two species together). The consortium performances were preliminary evaluated in continuous photobioreactors at pilot scale in 16 L and 160 L reactors, confirming the results obtained at lab scale. Hence, we found that is possible to efficiently convert light energy supplied to photobioreactors through a rationale use of light and widening wavelengths absorption spectrum thanks to co-existing species. Such results indicate that innovative consortia establishment can be advantageous for future research and industrial microalgae cultivation.