# Differences in photosystem II activity and carbon allocation during photomixotrophic growth in distinct wild‐type strains of Synechocystis sp. PCC 6803

## TL;DR

This study shows how different strains of a cyanobacterium adjust their photosynthesis and carbon storage when growing with both light and organic carbon.

## Contribution

The study reveals strain-specific metabolic strategies in Synechocystis sp. PCC 6803 under photomixotrophic conditions, highlighting physiological diversity.

## Key findings

- WT 1 and WT 2 strains showed near-complete inhibition of PSII electron transfer after glucose supplementation.
- WT 1 and WT 2 accumulated more glycogen despite slower glucose uptake compared to WT 3 and WT 4.
- Structural integrity of photosystems was preserved despite electron transport blockage in some strains.

## Abstract

The regulation of photosynthetic electron transport during photomixotrophic growth in cyanobacteria remains incompletely understood. In this study, we characterized four wild‐type strains (WT 1–4) of Synechocystis sp. PCC 6803 and observed distinct strain‐specific differences in photosystem II (PSII) function under photomixotrophic conditions. Specifically, WT 1 and WT 2 exhibited near‐complete inhibition of electron transfer from QA
− to QB following approximately 3 days of glucose supplementation, possibly mediated by binding of the small PSII‐associated protein, Psb28‐2, and resulting in a metabolic shift toward photoheterotrophy. Observed electron transport blockage was associated with changes in the abundances of various photosynthetic proteins. However, the structural integrity of both Photosystems appeared to be largely preserved. Such stabilization may be driven by a transient downregulation of linear electron transport to prevent overreduction of the electron transport chain under photomixotrophy. In contrast, WT 3 and WT 4 maintained photomixotrophic growth throughout the experiment but exhibited slower growth rates than WT 1 and WT 2. Although glucose uptake was slower in WT 1 and WT 2, both strains accumulated more glycogen than WT 3 and WT 4, suggesting divergent regulation of carbon allocation and storage metabolism. Together, these findings highlight the capacity of cyanobacterial strains to deploy distinct metabolic strategies to optimize photosynthetic function, carbon assimilation, and energy storage under photomixotrophic conditions.

Some cyanobacteria can use both light and organic carbon to support growth, but how they regulate photosynthesis under these conditions remains insufficiently understood. In this study, we show that wild‐type strains of Synechocystis sp. PCC 6803 can adopt distinct physiological strategies affecting PSII performance and carbon storage. This work reveals hidden diversity in how cyanobacteria adapt to combined carbon sources changing energy input, with implications for improving photosynthetic efficiency and designing robust strains for biotechnological applications.