Improving a photosynthetic bioprocess with a ubiquitous additive: Using clay powder in the cultivation of Rhodopseudomonas palustris

A key challenge in process optimization is reactor performance, particularly in light-dependent bioprocesses. While novel photobioreactor designs exist, adapting established bioreactors with simple media supplementation may provide a more practical and modular alternative to complex mechanical modifications. In this study, we evaluated the effect of supplementing 0.2 % (w/v) clay powder on acetate production by <i>Rhodopseudomonas palustris</i> grown on butyrate. Among the clay types tested, kaolin showed the most pronounced benefits, with acetate accumulation increasing by 45 % relative to controls. These improvements are attributed to a combination of enhanced light distribution and increased cellular aggregation. Photosynthetic photon flux density measurements confirmed that kaolin reduced the extent of cellular shading by increasing light scattering within the culture. Taken together, these findings demonstrate that supplementation with a widely available clay can improve light penetration in suspended phototrophic cultures, offering a novel andaccessible strategy for optimizing photosynthetic bioprocesses.