Algal biomass production,N uptake and N2 fixation in a synthetic medium

An experiment was conducted in the growth chamber to quantify the biomass production, N removal and N₂ fixation from a synthetic medium by Chlamydomonas reinhardtii and Anabaena flos-aquae. Nitrogen was supplied at a concentration of 100 mg liter^?1 of NO₃^??¹⁵N and NH₄⁺?¹⁵ (3·5 atom %), respectively. After 21 days Chlamydomonas reinhardtii removed an average of 83·8 and 78·7 mg N liter^?1 as NO₃^? and NH₄⁺, respectively. Averages of 0·89 and 0·71 g liter^?1 (first batch), 1·63 and 0·95 g liter^? (second batch) algal biomass were collected from NO₃^? and NH₄⁺ media, respectively. Uptake rates of 0·11 mg ¹⁵N g^?1 algae day^?1 from NO₃^? medium and 0·10 mg ¹⁵N g^?1 algae day^?1 from NH₄⁺ medium were calculated. Algal cells grown in NO₃^? and NH₄⁺ medium contained 71 and 65 g N kg^?1 (first batch), 39 and 58 g N kg^?1 (second batch), respectively. Anabaena flos-aquae produced averages of 0·58 and 0·46 g liter^?1 (first batch), 0·55 and 0·48 g liter^?1 (second batch) after 14 days of growth from NO₃^? and NH₄⁺ media, respectively. Blue-green algal biomass contained higher N (81–98 g kg^?1) than green algae. Isotope dilution method for determining N₂ fixation indicated that 55% and 77% of total N of blue-green algae grown in NO₃^? and NH₄⁺ media, respectively, was derived from the atmosphere.