Nitrate transport and not photoinhibition limits growth of the freshwater cyanobacterium Synechococcus species PCC 6301 at low temperature

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20 degrees C and 38 degrees C, the growth rate decreased with decreasing temperature, and growth ceased at 15 degrees C. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38 degrees C remained at 15 degrees C. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m(-2) s(-1)) at 15 degrees C. Little or no loss of oxygen evolution was observed under the normal light intensity (250 mu E m(-2) s(-1)) for growth at 15 degrees C. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15 degrees C, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.