Bacterial Delight: Deciphering how visible light affects bacterial biological control agents in the phyllosphere of greenhouse crops

Light can be an enabler of life but also a stressor depending on the organism and the environment. When introducing biological control agents (BCAs) to the phyllosphere of greenhouse crops, several parameters need to be met to reach high efficacy. The newly applied invaders in the harsh phyllosphere environment need to successfully attach, compete and reproduce whilst also acting against a pathogen to be regarded as effective. Light has been found to induce biosurfactant and biofilm formation in non-phototrophic bacterial BCAs. These induced mechanisms could be key in aiding the introduction of non-phototrophic BCAs. In this thesis, the effects of placement in the canopy, leaf type, visible light treatment, exposure dose and sole carbon utilisation were investigated with respect to three nonphototrophic bacterial BCAs (Bacillus amyloliquefaciens, Pseudomonas chlororaphis, Streptomyces griseoviridis). Both greenhouse and laboratory experiments were used to decipher how visible light affects these BCAs. Exposure dose, light distribution in combination with a wavelength were found to affect the BCAs when introduced in the phyllosphere of greenhouse crops. Based on viable counts S. griseoviridis was recovered at high rates irrespective of the light treatment, whereas P. chlororaphis and B. amyloliquefaciens preferred the green and white light treatments. It was found that different wavelengths influence all three BCAs’ sole carbon utilisation, where two BCAs, B. amyloliquefaciens and P. chlororaphis, had higher respiration rates under the blue spectrum, and S. griseoviridis preferred the red spectrum. In the case of P. chlororaphis, this was further translated when cascade effects were noted in the citrate cycle as an effect of the increased source utilisation under blue light treatment.