Evaluation of the interactions between the marine bacterium Pseudomonas fluorescens and the microalga Isochrysis galbana in simulated ballast tank environment

To evaluate the impacts of the interaction between bacteria and microalgae has been the object of study by many research groups around the world. However, little is known about the interference that pigments produced by bacteria, such as the pyoverdine siderophore, can cause to microalgae like Isochrysis galbana. Pyoverdine is a fluorochrome produced by certain Pseudomonas strains, such as P. fluorescens, which plays a role in capturing and transporting iron ions from the environment to the cell. Unlike the oceans where Fe concentrations are extremely low (<10-15 mu M), in a ballast tank it is expected that there is a great supply of iron to the cells and that the absence of light is the main limiting factor until the water is discarded. Interestingly, under certain conditions, bacteria such as P. fluorescens absorb most of the water soluble iron ions and prevent the growth of phytoplankton even if there is sufficient light. Changes in the patterns of light distribution in aquatic environments may affect the physiological characteristics of certain microalgae. This study aimed to evaluate the impacts of the presence of P. fluorescens on the survival and growth of I. galbana inside the tank. For the study, an experiment was carried out to study the interaction between P. fluorescens and I. galbana under simulated conditions of a vessel in the presence/absence of Pseudomonas and light. The results showed that the presence of the bacteria is not the main limiting factor for microalga growth. The effect of the light factor was determinant on the reproduction rate. It is believed that pyoverdine produced by P. fluorescens affected I. galbana stock either by increasing mortality or decreasing growth rate as revealed by laboratory experiments. However, it was not possible to check if the pigment concentration was affected by the growth of microalgae.