Light management by algal aggregates in living photosynthetic hydrogels.

Rapid progress in algal biotechnology has triggered a growing interest in hydrogel-encapsulated microalgal cultivation, especially for the engineering of functional photosynthetic materials and biomass production. An overlooked characteristic of gel-encapsulated cultures is the emergence of cell aggregates, which are the result of the mechanical confinement of the cells. Such aggregates have a dramatic effect on the light management of gel-encapsulated photobioreactors and hence strongly affect the photosynthetic outcome. In order to evaluate such an effect, we experimentally studied the optical response of hydrogels containing algal aggregates and developed optical simulations to study the resultant light intensity profiles. The simulations are validated experimentally via transmittance measurements using an integrating sphere and aggregate volume analysis with confocal microscopy. Specifically, the heterogeneous distribution of cell aggregates in a gel matrix can increase light penetration while alleviating photoinhibition compared to a flat biofilm. Finally, we demonstrate that light harvesting efficiency can be further enhanced with the introduction of scattering particles within the hydrogel matrix, leading to a four-fold increase in biomass growth. Our study, therefore, highlights a new strategy for the design of spatially efficient photosynthetic living materials that have important implications for the engineering of future algal cultivation systems. Significance StatementThe ability to cultivate microalgae at scale efficiently would allow more sustainable production of food and food additives. However, efficient growth of microalgae requires optimised light conditions, which are usually challenging to obtain using biofilm cultivations mode: as the outer layer of cells are necessarily more exposed to incoming light than the inner layer, posing the problem of photoinhibition on the outer cells receiving too much light, and shading the ones below. Here we study both experimentally and numerically, how microalgae aggregates growing in the confinement of a hydrogel can provide an improved light distribution and therefore biomass growth is maximised. This study proposes new strategies on how to engineer future photobioreactors.