Differential inter-and intra-specific responses of Aphanizomenon strains to nutrient limitation and algal growth inhibition

Cyanobacterial blooms of Aphanizomenon spp. occur worldwide, with deleterious effects on aquatic communities. The present work aimed to investigate the effects of different levels of phosphorus and nitrogen on the growth of Aphanizomenon bloom-forming strains, belonging to the species A. issatschenkoi, A. aphanizomenoides and A. gracile. Algal inhibition tests were also performed to assess the potential allelopathic effects of the strains’ filtrates on the growth of four green algae (Pseudokirchneriella subcapitata, Chlorella vulgaris, Pandorina morum and Coelastrum astroideum). Phosphorus had a significant effect on the growth of all the strains tested; orthophosphate concentrations 0.3 mg PO4 32 L21 led to a decrease of more than 50% in the cyanobacterial growth of most strains. The growth of A. gracile strains was unaffected by variation in nitrate levels, but strains of A. issatschenkoi (UADFA1) and A. aphanizomenoides (UADFA6, UADFA7 and UADFA13) were moderately to extremely sensitive to nitrate depletion in the medium. These results strengthen the idea that intra-specific variation may play an important role in bloom dynamics; however, culture conditions and mutations may explain the morphological and physiological changes observed. Allelopathic assays revealed significant inhibition of microalgal growth by filtrates of strains from the three species, but also indicated intra-specific differences in allelopathic activity.The research has been supported by FCT (Fundação para a Ciência e a Tecnologia) in the form of a PhD grant to D.R.F. (SFRH/BD/23864/2005). B.B.C. is hired under the programme Ciência 2008 (FCT, Portugal), co-funded by the Human Potential Operational Programme (National Strategic Reference Framework 2007-2013) and European Social Fund (EU).publishe