Nitrogen fixation by the diazotroph <i>Cylindrospermopsis raciborskii</i> (Cyanophyceae)

Abstract

Nitrogen fixation has been proposed as a mechanism that allows the diazotrophic cyanobacterium, Cylindrospermopsis raciborskii , to bloom in nitrogen‐limited freshwater systems. However, it is unclear whether dinitrogen fixation (N 2 fixation) can supplement available dissolved inorganic nitrogen ( DIN ) for growth, or only provides minimum nitrogen (N) for cell maintenance under DIN deplete conditions. Additionally, the rate at which cells can switch between DIN use and N 2 fixation is unknown. This study investigated N 2 fixation under a range of nitrate concentrations. Cultures were grown with pretreatments of nitrate replete (single dose 941 μmol · L −1 ) and N‐free conditions and then either received a single dose of 941 μmol · L −1 (N941), 118 μmol · L −1 (N118) or 0 N. Heterocysts appeared from days 3 to 5 when treatments of high were transferred to N free media (N941:N0), and from day 5 in N941 transferred to N118 treatments. Conversely, transferring cells from N0 to N941 resulted in heterocysts being discarded from day 3 and day 5 for N0:N118. Heterocyst appearance correlated with a detectable rate of N 2 fixation and up‐regulation of nifH gene expression, the discard of heterocysts occurred after sequential reduction of nifH expression and N 2 fixation. Nitrate uptake rates were not affected by pretreatment, suggesting no regulation or saturation of this uptake pathway. These data demonstrate that for C. raciborskii , N 2 fixation is regulated by the production or discard of heterocysts. In conclusion, this study has shown that N 2 fixation only provides enough N to support relatively low growth under N‐limited conditions, and does not supplement available nitrate to increase growth rates.