Nitrogen fixation by the diazotroph Cylindrospermopsis raciborskii (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 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 fixation is unknown. This study investigated N fixation under a range of nitrate concentrations. Cultures were grown with pretreatments of nitrate replete (single dose 941 μmol NO3- · L ) and N-free conditions and then either received a single dose of 941 μmol NO3- · L (N941), 118 μmol NO3- · L (N118) or 0 N. Heterocysts appeared from days 3 to 5 when treatments of high NO3- 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 fixation and up-regulation of nifH gene expression, the discard of heterocysts occurred after sequential reduction of nifH expression and N 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 fixation is regulated by the production or discard of heterocysts. In conclusion, this study has shown that N fixation only provides enough N to support relatively low growth under N-limited conditions, and does not supplement available nitrate to increase growth rates.

Physiological responses of the freshwater N2‐fixing cyanobacterium Raphidiopsis raciborskii to Fe and N availabilities

“…Similar temporal trends of overall C 2 H 4 yields increasing gradually with incubation time are evident for CS-506 and CS-509 strains with significantly (p < 0.05) higher cumulative C 2 H 4 production over 24 h for CS-506 cells than those for CS-509 cultures in the JM*(−N, 1000) and JM*(−N, 200) media (Supporting Information Fig. These observations are consistent with the fact that nitrogenase activity by R. raciborskii in N-replete media only ceases once the heterocysts are no longer present with their disappearance occurring after 10 days or more (Willis et al, 2016a). Interestingly, the C 2 H 4 yields (15.4-51.6 nmol at the termination of the experiment) for CS-506 and CS-509 strains treated with JM*(+N, 1000) revealed that both strains retained constitutive nitrogenase activity if heterocysts are present, even in the presence of nitrate throughout the incubation period (24 h), with this result mirroring previous studies in which nitrogenase activity was observed to be present in R. raciborskii incubated with nitrate or ammonium for 24-120 h (Sprober et al, 2003;Stucken, 2010).…”

“…S4) and normalized nitrogenase activity (Table 1 and Supporting Information Fig. Moreover, these results also suggest that, in response to depletion of combined nitrogen, N 2 -fixation occurs simultaneously with photosynthesis during daytime (Willis et al, 2016a), and also occurs, in the absence of photosynthesis, during night. Moreover, these results also suggest that, in response to depletion of combined nitrogen, N 2 -fixation occurs simultaneously with photosynthesis during daytime (Willis et al, 2016a), and also occurs, in the absence of photosynthesis, during night.…”

“…In recent decades, this diazotrophic cyanobacterium has spread from its native tropical region to subtropical and temperate zones (Burford et al, 2016;Burford et al, 2018). The rapid expansion of R. raciborskii may be due to several possibly synergistic factors including global climate change (Botana, 2016), strong physiological adaptability of this microorganism to low light and temperature (Antunes et al, 2015;Kehoe et al, 2015) and flexible acquisition of nutrients, particularly nitrogen (N) (Willis et al, 2016a). Although the N 2 -fixation rate of R. raciborskii is lower than ammonium and nitrate uptake rates and accounts for less than 10% of total N assimilation by this organism, N 2 -fixation by R. raciborskii plays a vital role in regulating the physiological adaptation of this organism in N-fluctuating environments.…”

“…The rapid expansion of R. raciborskii may be due to several possibly synergistic factors including global climate change (Botana, 2016), strong physiological adaptability of this microorganism to low light and temperature (Antunes et al, 2015;Kehoe et al, 2015) and flexible acquisition of nutrients, particularly nitrogen (N) (Willis et al, 2016a). For example, N 2 -fixation provides an essential N source to support the relatively low growth and cell homeostasis of R. raciborskii under prolonged N-limited conditions (Willis et al, 2016a). For example, N 2 -fixation provides an essential N source to support the relatively low growth and cell homeostasis of R. raciborskii under prolonged N-limited conditions (Willis et al, 2016a).…”

“…For example, N 2 -fixation rates of the marine diazotrophic cyanobacterium Trichodesmium increased from 0.07 to 0.5 mmol-NÁmol-C −1 Áh −1 on Fe supplementation from 4 nM to 4 μM (Berman-Frank et al, 2007). Furthermore, in contrast to the very few studies regarding the physiological response of diazotrophic cyanobacteria to varying levels of Fe availability (Berman-Frank et al, 2007;Jacq et al, 2014), numerous studies of the impact of N and P availabilities on the growth of diazotrophic cyanobacteria including R. raciborskii have been described (Kenesi et al, 2009;Muhid et al, 2013;Amaral et al, 2014;Chislock et al, 2014;Willis et al, 2016a;Yang et al, 2018). Furthermore, in contrast to the very few studies regarding the physiological response of diazotrophic cyanobacteria to varying levels of Fe availability (Berman-Frank et al, 2007;Jacq et al, 2014), numerous studies of the impact of N and P availabilities on the growth of diazotrophic cyanobacteria including R. raciborskii have been described (Kenesi et al, 2009;Muhid et al, 2013;Amaral et al, 2014;Chislock et al, 2014;Willis et al, 2016a;Yang et al, 2018).…”