Diazotrophs modulate phycobiliproteins and nitrogen stoichiometry differently than other cyanobacteria in response to light and nitrogen availability

Abstract

AbstractHarmful algal blooms (HABs) frequently occur in eutrophic water bodies, which are often dominated by either non‐diazotrophic or diazotrophic strains of cyanobacteria. The occurrence of HABs is associated with multiple environmental factors that affect cyanobacterial growth, though adaptations of cyanobacteria to respond to diverse growing conditions are largely unknown. One factor affecting HAB persistence is the ability of cyanobacteria to synthesize and metabolize large biomolecules such as the light harvesting pigments phycobiliproteins that can also act as nitrogen (N) storage. Here, we examined how cyanobacteria varied phycobiliprotein content in response to different light intensity and N availability in two HAB forming species, Microcystis aeruginosa (non‐diazotrophic) and Dolichospermum flos‐aquae (diazotrophic). Our results revealed both HAB forming species responded similarly to light intensity, with decreasing light causing an increased production of phycobiliproteins, although at different rates. However, we observed dissimilar phycobiliprotein responses to varied N availability. In low N conditions, M. aeruginosa degraded phycobiliproteins primarily due to the decreased need for photosynthesis, whereas D. flos‐aquae, increased phycobiliproteins production under N‐limited conditions likely caused by the increased energetic requirements for N2‐fixation. Our results indicate that M. aeruginosa stored as much as 30% of cellular N in phycobiliproteins under high N conditions, while D. flos‐aquae only allocated 5% of the cellular N in phycobiliproteins. This study reveals a fundamental physiological difference between non‐diazotrophic and diazotrophic cyanobacteria, highlighting the critical role of phycobiliproteins on the persistence of cyanobacteria blooms under scenarios with varied light intensity and N availability.