Anabaena azollae - The cyanobacterial partner of Azolla filiculoides reciprocates variably to dose- and duration-dependent Bisphenol-A exposure

Bisphenol A (BPA) is one of the most studied endocrine-disrupting chemicals (EDC), for its large-scale production and disposal, and ubiquitous presence in the environment. It is also known to impart significant phytotoxic effects on almost all aquatic flora including cyanobacteria. Yet, there are limited studies on the responses of symbiotic cyanobacteria i.e. Anabaena azollae - residing inside Azolla filiculoides. In this context, this study aimed to investigate the response of A. azollae upon exposure to different doses and durations of BPA, which marks this as one of the first such studies. Morphological study reaffirmed the infiltration of BPA through the host's physical barrier and induced various anomalies like filament fragmentation, and cellular disruption especially heterocysts in the case of the higher doses of BPA (20, and 30 mg L^-1). Additionally, exposure to higher doses further stimulated the antioxidative enzymes, secondary metabolites and stress/defence markers. However, the exaggerated ROS production (nearly 190%-230% O₂^.- and 557%-783% H₂O₂) at 30 mg L^-1 severely disrupted the membrane integrity, osmotic balance, and reduced essential biomolecules like sugars, proteins and lipid accumulation. Moreover, higher doses of BPA treatment compromised photosynthetic activity by reducing the photosynthetic pigments and phycobiliproteins (PBPs). Conversely, up to the dose of 10 mg L^-1, better cellular integrity, improved accumulation of biomolecules, pigments, and ROS detoxification denoted the neutral/positive effect on the symbionts. The lower doses (≥10 mg L^-1) also showed positive indications like higher accumulation of biomolecules proteins (16%-30%), lipids (9%-49%), sugar (18%-52%), and pigments like phycobiliproteins (6%-97%), phycocyanins (6%-134%), which seemed to support the biomass of cyanobionts. Moreover, the said doses stimulated the accumulation of phenolics (98%-117%), flavonoids (159%-224%), and released polysaccharides (60%-183%) alongside stress markers like ascorbate and proline indicating the evocation of defense strategy against BPA stress.