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

## TL;DR

Adding kaolin clay to a photosynthetic culture of Rhodopseudomonas palustris improves light distribution and increases acetate production by 45%.

## Contribution

Kaolin clay supplementation is shown to enhance photosynthetic bioprocesses through improved light scattering and cellular aggregation.

## Key findings

- Kaolin clay increases acetate production by 45% compared to controls.
- Kaolin improves light scattering, reducing cellular shading in the culture.
- Rhodopseudomonas palustris preferentially consumes acetate over butyrate at low concentrations.

## Abstract

•Kaolin clay supplementation enhances lateral light distribution in photosynthetic cultures by increasing light scattering, as demonstrated by PPFD measurements taken perpendicular to incident light.•The addition of silica, bentonite, and kaolin increases product concentration by 13 %, 34 %, and 45 % respectively under process conditions.•The presence of kaolin in culture increases cellular aggregation.•Rhodopseudomonas palustris preferentially consumes acetate over butyrate below 1 g/L.

Kaolin clay supplementation enhances lateral light distribution in photosynthetic cultures by increasing light scattering, as demonstrated by PPFD measurements taken perpendicular to incident light.

The addition of silica, bentonite, and kaolin increases product concentration by 13 %, 34 %, and 45 % respectively under process conditions.

The presence of kaolin in culture increases cellular aggregation.

Rhodopseudomonas palustris preferentially consumes acetate over butyrate below 1 g/L.

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 Rhodopseudomonas palustris 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.

Image, graphical abstract