Nitrogen fixation by the diazotroph Cylindrospermopsis raciborskii (Cyanophyceae)

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 mu mol NO3- center dot L-1) and N-free conditions and then either received a single dose of 941 mu mol NO3- center dot L-1 (N941), 118 mu mol NO3- center dot L-1 (N118) or 0 N. Heterocysts appeared from days 3 to 5 when treatments of high NO3- were transferred to N freemedia (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.