Aerobic nitrogen fixation is confined to a subset of cells in the non‐heterocystous cyanobacterium Symploca PCC 8002

Abstract

Symploca PCC 8002 Ku$ tzing is a filamentous cyanobacterium that lacks the specialized cells, known as heterocysts, that protect nitrogenase from O # in most aerobic N # -fixing cyanobacteria. Nevertheless, Symploca is able to carry out N # fixation in the light under aerobic conditions. When cultures were grown under light\dark cycles, nitrogenase activity commenced and increased in the light phase and declined towards zero in the dark. Immunolocalization of dinitrogenase reductase in sectioned Symploca trichomes showed that the enzyme was present only in 9 % of the cells. These cells lacked any obvious mechanical protection against atmospheric O # and their ultrastructural characteristics were similar to those of cells that did not contain any dinitrogenase reductase. The nitrogenase-containing cells possessed carboxysomes that were rich in ribulose-1,5-bisphosphate carboxylase\oxygenase and phycoerythrin, a light harvesting pigment of PS II. This indicates that these cells had a capacity for both N # fixation and photosynthesis. The significance of the localization pattern for dinitrogenase reductase is discussed in the context of N # fixation in Symploca PCC 8002.

The Ecology of Nitrogen Fixation in Cyanobacterial Mats

“…Symploca PCC8002 was originally isolated from tidal sediment and assigned as Microcoleus chthonoplastes. Fredriksson et al (1998) reported that nitrogenase activity in this organism was confined to the light and showed that nitrogenase was present in a subset of the cells similar to the supposed diazocytes of Trichodesmium. However, this pattern may not be a general characteristic of this genus (Kumazawa et al 2001).…”

Nitrogen fixation and growth rates of Trichodesmium IMS-101 as a function of light intensity

“…This pattern has been reported for only 2 other cyanobacteria, Lyngbya majuscula and Symploca sp. (Jones 1990, Fredriksson et al 1998. Trichodesmium is very abundant and is recognized as one of the most important nitrogen fixers in tropical and subtropical oligotrophic oceans (Capone et al 1997).…”

Nitrogen‐fixation strategies and Fe requirements in cyanobacteria

“…1). Active photosynthetic components (such as PSI and PSII complexes, Rubisco, carboxysomes) are found in all cells, even those harboring nitrogenase (Janson et al 1994;Fredriksson et al 1998;Berman-Frank et al 2001a). Hence, spatial aggregation of nitrogenase into certain zones (or cells) is not sufficient to protect against oxygen.…”