AME 82:265-274 (2019)  -  DOI: https://doi.org/10.3354/ame01896

ABSTRACT: The toxic cyanobacterium Nodularia spumigena regularly forms large surface blooms in the central Baltic Sea during the summer period. Bloom formation occurs when the water body is virtually free of the nutrients nitrate and phosphate. The capacity of Nodularia spp. for nitrogen fixation makes it independent from combined nitrogen sources; however, its adaptation to low phosphate levels is poorly investigated. We compared the behaviour of N. spumigena strain CCY9414 under high- and low-phosphate conditions in the laboratory. Cells cultivated under P-limited conditions showed no growth but higher expression of pho regulon genes, such as pstS, which are known to respond to P-starvation. However, electron microscopy showed a massive accumulation of polyphosphate granules in long-term P-starved cells and not in those grown in P-containing medium. The polyphosphate nature of the black granules was verified by electron energy-loss spectroscopy and subsequently by polyphosphate quantification in Nodularia cells. Polyphosphate accumulation was also observed in cells of Aphanizomenon flos-aquae obtained from a summer bloom, where only traces of phosphate were available in the surrounding water body. These findings indicate that bloom-forming cyanobacteria prefer to store the traces of available phosphate in the form of polyphosphates, instead of using it for further growth. This strategy could help in competition with other microorganisms for phosphate during bloom conditions, whilst allowing rapid cell division upon more favourable phosphate concentrations.

KEY WORDS: Cyanobacterium · Electron microscopy · Gene expression · Polyphosphate