Diurnal nitrogen fixation in tropical marine cyanobacteria: A comparison between adjacent communities of non-heterocystous lyngbya sp. and heterocystous calothrix sp.

Abstract

Nitrogenase activity (ARA) in adjacent marine tropical communities of a non-heterocystous Lyngbya sp. and a heterocystous Calothrix sp. was investigated in the light and the dark. For both cyanobacteria, in situ ARA increased throughout the day except for afternoon reductions during cloud cover. ARA rates varied on different days but Calothrix sp. was always more active during daylight hours than Lyngbya sp. Both cyanobacteria showed only very low rates of ARA when incubated in the dark in situ during normal daylight hours. Nocturnal ARA, at 82% the daylight rate in Lyngbya sp., and 18% the daylight rate in Calothrix sp., was three times higher in Lyngbya sp. than in Calothrix sp. In both, nocturnal ARA rose from a low rate after dusk to a peak around midnight and declined towards dawn. In each organism ARA in the light was higher than in the dark regardless of the time of day. The addition of DCMU, an inhibitor of photosystem II, generally inhibited nitrogenase activity in the heterocystous Calothrix sp. and stimulated it in the non-heterocystous Lyngbya sp., but there were differences dependent on the time of the day when DCMU was added. In short-term nitrogenase assays the temperature optimum for Lyngbya sp. was 35°C compared with 25°C for Calothrix sp. and the salinity optimum was 35%o for both. Calothrix sp., but not Lyngbya sp., rapidly revived after desiccation in the hot sun. © 1992 The British Phycological Society.