EFFECTS OF NITROGEN SOURCES ON P‐LIMITED GROWTH OF <i>ANABAENA FLOS‐AQUAE</i>¹

Abstract

ABSTRACT The effects of N 2 , nitrate and ammonia as N sources were investigated in P‐limited and nutrient‐sufficient cultures of Anabaena flos‐aquae (Lyngb.) Bréb. The maximum growth rate (μ m ) was highest at 1.34 d −1 with ammonia, compared to 1.18 with nitrate and 0.95 d −1 with N 2 . There was no difference in P requirement between N 2 and nitrate cultures. Under P‐limited conditions, the increase in cell P with growth rate (μ) was identical. With N 2 as the N source, cell‐N concentrations in P‐limited cells increased with μ as did cell P, and the cellular N:P ratio remained the same (14) within the range of μ examined. With nitrate, however, cell N concentrations were high and independent of n, except at a low μ. It appears that this organism fixes atmospheric N 2 only at the minimum concentration required to maintain a μ. The acetylene reduction rate increased with μ in both N 2‐ and nitrate‐grown cells, but the rate was lower in nitrate. Under P‐limitation, there was no difference in net C‐fixation rate per cell between N 2 and nitrate cultures at a given μ. However, the rate per unit of chlorophyll a (chl a ) was higher in N 2 than in nitrate cultures, and the rate was independent of μ with N 2 but was a linear function of nitrate supplied. The maximum C‐fixation rate in nutrient sufficient cells was highest with ammonia, followed by nitrate and N 2 . The cellular chl a concentration was correlated with the total cell‐N concentrations regardless of H and the source of N.