Downloaded from www.microbiologyresearch.org by IP: 54.70.40.11 On: Mon, 03 Dec 2018 13:59:29 Journal of General Microbiology (1992), 138, 2673-2678. Printed in Great Britain 2673 Heterocyst envelope thickness, heterocyst frequency and nitrogenase activity in Anabaena flos-aquae: influence of exogenous oxygen tension N. KANGATHARALINGAM,' JOHN C. PRISCU'* and HANS W. PAERL~ Department of Biology, Montana State University, Bozeman MT 5971 7, USA Institute of Marine Sciences, University of North Carolina-Chapel Hill, Morehead City, NC 28557, USA (Received 24 February 1992; revised 15 July 1992; accepted 27 July 1992) The heterocyst envelope of the N,-fixing cyanobacterium Anabaenaflos-aquae thickened as exogenous 0, partial pressure (PO,) was increased from 5 to 40 kPa. The majority of the thickening occurred in the glycolipidlayer area of the envelope. Such thickening appears to be an 0,-induced mechanism for providing a greater 0, diffusion barrier against 0, inhibition of nitrogenase.Nitrogenase activity decreased at PO, levels above ambient (20 kPa), indicating that a thicker envelope is not completelyeffective as a barrier to 0, diffusion. However, when cultures grown at 10 kPa and 40 kPa PO, were transferred to ambient PO,, the 40 kPa PO, cells showed higher nitrogenase activity 24 h after transfer compared to those grown at 10 kPa or ambient pOz, indicating O,-protection of nitrogenase by thicker heterocyst envelopes. Heterocyst frequency was lowest at 20 kPa 0,. Introduction In oxygenic phototrophic cyanobacteria, exogenous oxygen tension is the net product of consumption through respiration, release through oxygenic photosyn- thesis and physical processes such as diffusion. The ni trogenase enzyme complex responsible for the conver- sion of molecular nitrogen to ammonium in diazotrophs is inactivated by oxygen (Fay & Cox, 1967; Robson & Postgate, 1980). Various physiological, morphological and ecological strategies exist among different diazo- trophs to overcome this dilemma (e.g. Carlton & Paerl, 1989 ; Carpenter & Price, 1976 ; Kangatharalingam et al., 199 1). An important structural feature providing protec- tion against oxygen inactivation of nitrogenase in certain cyanobacteria is the multi-component envelope of the heterocyst (Wolk, 1982). Despite the presence of heterocysts, filamentous cyanobacteria such as Anabaena and Aphanizomenon exhibit some degree of nitrogenase inhibition when exogenous oxygen levels are elevated above ambient. This indicates that the oxygen diffusion barrier afforded by the heterocyst envelope is not completely effective against increases in external oxygen tension (Murry et al., 1984; Murry & Wolk, 1989). Structures analogous to heterocysts, called vesicles in ~~~ ~~ ~ Author for correspondence. Abbreviations : Chl a, chlorophyll a; TEM, transmission electron microscopy. the actinorhizal diazotroph Frankia, have thin walls in cultures grown at low partial pressure of 0, (PO,) while having thicker walls at high p 0 2 (Parsons et al., 1987). This phenomenon has not been reported among cyano- bacterial heterocysts, except for the observation that in Anabaena spp. the heterocyst envelope is poorly developed under low-PO, and anaerobic conditions (Kulasooriya et al., 1972; Rippka & Stanier, 1978). Only short-term effects of PO, or 02-supersaturation on nitrogenase activity in cyanobacteria have been reported (Murry et al., 1984; Paerl, 1978). Physiological (environmental) factors such as light intensity, combined nitrogen availability and assimilable organic carbon sources affect the frequency of heterocyst formation (Fogg, 1949). Heterocyst frequency of Anabaena cylindrica under highly elevated C02 levels with no O2 was shown to be higher than that in air (Kulasooriya et al., 1972). However, we are unaware of reports on the effect of varied PO, alone on heterocyst frequency. Because heterocysts are the primary sites of nitrogenase synthesis (Elhai & Wolk, 1990), their frequency is often positively related to corresponding nitrogenase activity (Smith et al., 1987). We have examined the influence of external PO, on heterocyst envelope thickness, heterocyst frequency and nitrogenase activity in Anabaena tflos-aquae. This study provides information on the inter-relationships of these factors and the physiological implications of exogenous PO,. 0001-7421 0 1992 SGM