Chlorophyta growth promotion by the plant symbiotic cyanobactericidal bacterium Pseudomonas sp. and its active substances

Pseudomonas sp. strain Go58 isolated from the biofilm on a water plant possesses cyanobactericidal activity through its production of cyanobactericidal molecules, yet strain Go58 interestingly promotes the growth of the chlorophyte (Chlorophyta; green algae) Desmodesmus subspicatus. An in-depth chemical analysis of the culture broth revealed the production of indole-3-acetic acid, a well-known plant hormone, together with its atypical structural isomer, 3-(hydroxyacetyl)indole. Both these compounds similarly enhanced the growth of D. subspicatus but did not affect other microalgae, including cyanobacteria and a diatom. No synergistic activity was observed between the two compounds, indicating that they likely target the same molecular pathway. In the co-cultivation condition, production of these compounds by strain Go58 was not boosted but rather reduced. Application of these compounds or strain Go58 also fostered the growth of the water plant Lemna minor. These findings suggest that Pseudomonas sp. Go58 is able to establish a mutualistic relationship with host water plants by producing both plant hormones and cyanobactericidal molecules in return for a suitable habitat and nutrients. Although chlorophytes are not expected to be a primary target of the strain Go58, promoting the growth of non-harmful Chlorophyta could help mitigate harmful cyanobacterial bloom.