Comparative study of thylakoid membranes in terminal heterocysts and vegetative cells from two cyanobacteria, Rivularia M-261 and Anabaena variabilis, by fluorescence and absorption spectral microscopy

Abstract

Heterocyst is a nitrogen-fixing cell differentiated from a cell for oxygen-evolving photosynthesis (vegetative cell) in some filamentous cyanobacteria when fixed nitrogen (e.g., ammonia and nitrate) is limited. Heterocysts appear at multiple separated positions in a single filament with an interval of 10-20 cells in some genera (including Anabaena variabilis). In other genera, a single heterocyst appears only at the basal terminal in a filament (including Rivularia M-261). Such morphological diversity may necessitate different properties of heterocysts. However, possible differences in heterocysts have largely remained unexplored due to the minority of heterocysts among major vegetative cells. Here, we have applied spectroscopic microscopy to Rivularia and A. variabilis to analyze their thylakoid membranes in individual cells. Absorption and fluorescence spectral imaging enabled us to estimate concentrations and interconnections of key photosynthetic components like photosystem I (PSI), photosystem II (PSII) and subunits of light-harvesting phycobilisome including phycocyanin (PC). The concentration of PC in heterocysts of Rivularia is far higher than that of A. variabilis. Fluorescence quantum yield of PC in Rivularia heterocysts was found to be virtually the same as those in its vegetative cells, while fluorescence quantum yield of PC in A. variabilis heterocysts was enhanced in comparison with its vegetative cells. PSI concentration in the thylakoid membranes of heterocysts seems to remain nearly the same as those of the vegetative cells in both the species. The average stoichiometric ratio between PSI monomer and PC hexamer in Rivularia heterocysts is estimated to be about 1:1.

Heterocyst Thylakoid Bioenergetics

“…This misconception stems partly from early investigations, where harsh isolation methods unintentionally damaged the photosynthetic enzymes more than the nitrogenase, and resulted in the misunderstanding that heterocysts are brownish rather than green in color and completely lacking in chlorophyll- a fluorescence emission [21]. More recently, single cell spectroscopy has shown that heterocysts emit chlorophyll- a (Chl- a ) fluorescence to lower but still comparable levels, as in vegetative cells [18,22]. Intact PSII complexes have been found in heterocysts from the cyanobacterium Nostoc punctiforme (PCC 73102/ATCC 29133, henceforth referred to as N. punctiforme ).…”

Confocal Laser Scanning Microscopy for Spectroscopic Studies of Living Photosynthetic Cells

“…By means of known reference spectra, this equation can be solved for the intensities of each pigment. The reference spectra can either be loaded from a spectra database, or from literature [40][41][42][43][44], or directly extracted from the lambda stack. Obtained linear unmixing function will generate a multi-channel image, in which each channel represents only one pigment.…”

Bioenergy production using Trichormus variabilis – a review

“…These have been applied in studying the effects of altering various physicochemical factors such as light and nutrient supply, on T. variabilis phototrophic or heterotrophic growth. A large number of studies have investigated T. variabilis growth rate as a function of time, cell density, or photosynthesis components, but only a few have directly surveyed the interrelation between multiple parameters. Such data can be used for modeling growth kinetics.…”