An investigation into the effects of light intensity on nitrate and nitrite excretion and uptake by Chaetoceros curvisetus (STX-200)
Creators: name: Anderson, Steven Milton
Publisher: name: The University of Texas at Austin
Year: 1980
Subjects: subject: Chaetoceros curvisetus
subject: Nitrite uptake
subject: Nitrite excretion
subject: Nitrate uptake
subject: Nitrate excretion
subject: Light intensity
subject: Diatoms--Effect of light on
subject: Nitrogen--Fixation
subject: Nitrogen excretion
subject: Nitrates
subject: Nitrites

description: The nitrate and nitrite uptake and excretion characteristics of Chaetoceros curvisetus Cleve, STX-200, when grown in batch culture under simulated natural environmental conditions and then subjected to constant illumination or abrupt variations in illumination, were investigated. The cultures were grown in a medium containing 30 μM nitrite as the only nitrogen source and with the other essential nutrients in excess. Light intensities of 0, 25, 50, 100, 200, 500, 1000, 2000, 3000, and 5000 μein/m²/sec were generated with a 1000-watt tungsten-halogen lamp in combination with copper sulfate and heat filters. The cultures were spiked with nitrate and the nitrate and nitrite concentrations were monitored simultaneously and continuously with a Technicon Autoanalyzer II interfaced with a Hewlett-Packard 9825A computer programmed to take voltage readings from the Autoanalyzer colorimeters at 30 second intervals. Michaelis-Menten uptake kinetics were observed which allowed the use of linearized Michaelis-Menten equations to generate V [subscript max] , K [subscript s], and K [subscript I] values for nitrate uptake and light intensity in nitrate uptake. The uptake of nitrate by Chaetoceros curvisetus, STX-200, when grown in batch culture, is light-limited over a relatively small range of light intensities. At constant light intensities of between 200 and 2000 μein/m²/sec , the maximum rate of nitrate uptake was approximately 0.30 μg-at NO₃-N/μg-at Protein-N/hr. Below 200 μein/m²/sec, the maximum rate of nitrate uptake declined to a minimum of 0.16 μg-at NO₃-N/μg-at Protein-N/hr in the dark. Above 2000 μein/m²/sec, nitrate uptake fell significantly to a rate of 0.22 μg-at NO₃-N/μg-at Protein-N/hr. The V [subscript max] and K [subscript I] for light intensity in nitrate uptake for the natural light intensity range of 0-2000 μein/m²/sec were 0.343 μg-at NO₃-N/μg-at Protein-N/hr and 26 μein/m²/sec with a correlation coefficient of 1.000. The K [subscript s] value for nitrate uptake did not fluctuate as a function of light intensity remaining relatively constant at 0.62 μg-at NO₃-N/1. Intermittent periods of light and dark produced rapid fluctuations in nitrate uptake rates with reduced uptake occurring in the dark. The calculated V [subscript max] for light intensity in nitrate uptake of 0.341 μg-at NO₃-N/μg-at Protein-N/hr was almost identical to that calculated for the constant light studies. In contrast with the constant light studies, no reduction in nitrate uptake rate was observed at light intensities greater than 2000 μein/m²/sec with the intermittent light regime. The K [subscript I] for light intensity in nitrate uptake was 187 μein/m²/sec in the variable light studies. Nitrite was excreted at almost all light intensities and conditions and was highly correlated with the rate of nitrate uptake. Chaetoceros curvisetus, STX-200, is capable of rapid nitrate uptake over a wide range of light intensities ranging from 0-5000 μein/m²/sec and will probably never experience light-inhibition of nitrate uptake under natural conditions