CO₂ Fixation Rate and RuBisCO Content Increase in the Halotolerant Cyanobacterium, Aphanothece halophytica, Grown in High Salinities

Abstract

The growth of the halotolerant cyanobacterium Aphanothece halophytica, previously adapted to 0.5 molar NaCl, was optimal when NaCI concentration in culture medium was in the range 0.5 to 1.0 molar. The growth was delayed at either too low or too high salinities with lag time of ca. 0.5 day in 0.25 molar NaCl and ca. 2 days in 2 molar NaCl under the experimental conditions. However, the growth rates at the logarithmic phase were similar in the culture media containing NaCl in the range 0.25 to 2.0 molar. The capacity of photosynthetic CO2 fixation increased 3.7-fold in the cells at the logarithmic phase as NaCl concentration in the culture medium increased from 0.25 to 2.0 molar. The protein level of ribulose 1,5-bisphosphate carboxylase/oxygenase was also found to increase with increasing salinity using both an immunoblotting method and protein A-gold immunoelectron microscopy. These results indicate that high photosynthetic capacity and high ribulose 1,5-bisphosphate carboxylase/oxygenase content may entail an important role in betaine synthesis and adaptation of the A. halophytica cells to high NaCl level.spinach chloroplasts to provide osmotic adjustment during salt stress. It is also known that highly halotolerant cyanobacteria accumulate betaine as a major osmoticum, whereas less tolerant cyanobacteria accumulate either sucrose or glucosylglycerol (19). The unicellular cyanobacterium Aphanothece halophytica is a highly halotolerant organism that can grow at high external NaCl concentrations up to 3 M. It was recently demonstrated that betaine is accumulated as the major osmoticum inside A. halophytica cells in response to changes in external salinity (20).We have previously reported (13, 15) that betaine masks inhibitory effect ofCl-on the enzyme activity of RuBisCO3 from A. halophytica and prevents the enzyme dissociation into constituent subunits. Therefore, betaine is involved in not only osmoregulation but also stabilization of enzymes in the cells grown in high salinities.As a step to understanding the molecular basis of salt tolerance and, in particular, its relation to cellular metabolisms in photosynthetic cells, we studied salinity effects on growth, photosynthetic CO2 fixation, and RuBisCO content of the A. halophytica cells and found that both the rate of photosynthetic CO2 fixation and RuBisCO content in the cells increase with increasing external salinity, whereas growth rates are not changed.Salinity is one ofthe important elements to limit crop productivity. Since the synthesis and accumulation of low mol wt organic "compatible" solutes such as sugars and quaternary ammonium compounds are known to be essential for adaptability ofplant cells to high salinity (10,11,22,28,29), it can readily be surmised that changes in carbon, nitrogen, and/or energy metabolisms are tightly connected to the overall process. The molecular basis of adaptation and tolerance to salinity in plants is not well understood.Betaine (glycinebetaine) has previously been demonstrated to be a major osmoticum in a nu...

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Effects of salt stress on basic processes of photosynthesis

“…Salt stress enhances the oxygenase activity of RuBPCO while it curtails its carboxylase activity (Sivakumar et al 2000). In the halotolerant cyanobacterium A. halophytica, the content of RuBPCO and the rate of CO 2 fixation are increased in response to high salt stress (Takabe et al 1988). Echevarría et al (2001) and García-Mauriño et al (2003) reveal that PEPC activity is enhanced by salt stress.…”

Halotolerant Cyanobacterium Aphanothece halophyticaContains an Na+/H+ Antiporter, Homologous to Eukaryotic Ones, with Novel Ion Specificity Affected by C-terminal Tail

“…halophytica Culture Conditions-A. halophytica cells were grown photoautotrophically in BG11 medium plus 18 mM NaNO 3 and Turk Island salt solution as described previously (15)(16)(17) except that the NaCl concentration of the culture medium was adjusted to a range from 0.25 to 2.5 M as desired. Cotton-plugged 500-ml conical flasks containing 200 ml of medium each were used and shaken on a reciprocal shaker without supplementation of condensed CO 2 gas.…”

“…Aphanothece halophytica is a halotolerant cyanobacterium that can grow in a wide range of salinity conditions from 0.25 to 3.0 M NaCl (15,16). A. halophytica accumulates an osmoprotectant glycine betaine at high salinity (15).…”