EVIDENCE FOR CALCIUM‐MEDIATED REGULATION OF HETEROCYST FREQUENCY AND NITROGENASE ACTIVITY IN NOSTOC6720

Abstract

SummaryThe proportion of heterocysts (heterocyst frequency) in cultures of Nostoc 6720 varied with the Ca2+ concentration present in the growth medium, the proportion increasing from 5 to 9 % as the Ca2+ concentration was increased from 10−5 to 10−4 M. The total nitrogenase activity of the culture declined by 20% whereas the activity per heterocyst decreased by 30%. In the 10−4 to 10−3 M range of Ca2+ concentrations, which includes those employed in most standard media, the heterocyst frequency and nitrogenase activity did not change. At Ca2+ concentrations above 2 × 10−3 M the heterocyst frequency decreased and this decrease was associated with a lower growth rate.The presence of the calcium ionophore A23187 increased the heterocyst content of the culture and total nitrogenase activity, but still decreased the nitrogenase activity per heterocyst. The effects of A23187 and Ca2+ concentration were additive and demonstrated a positive interaction between Ca2+ and the ionophore. In contrast, the Ca2+ antagonist, lanthanum, decreased the heterocyst frequency and, at concentrations below 10−8 M, stimulated nitrogenase activity. At higher concentrations of lanthanum, growth, heterocyst frequency and nitrogenase activity were markedly diminished; cell degradation occurred at concentrations in excess of 10−4 M.An analysis of the intra‐ and extracellular binding of 45Ca2+ showed that decreasing the Ca2+ concentrations in the medium from 10−4 to 10−5 M lowered the intracellular Ca2+ content. In the presence of A23187, intracellular Ca2+ increased. The lanthanum treatment was exceptional in that the intracellular Ca2+ was increased about four‐fold.These results are interpreted as favouring the hypothesis that a Ca2+‐mediated regulatory process modulates heterocyst frequency and nitrogenase activity in Nostoc 6720.