Quantitative relationship between bacteriochlorophyll content, cytoplasmic membrane structure and chlorosome size in Chloroflexus aurantiacus

Abstract

Continuous cultures of Chloroflexus aurantiacus were cultivated in a chemostat in the light with varying bacteriochlorophyll (BChl) a/c ratios by changing the growth rate. Under these culture conditions all cells were homogeneously and reproducibly equipped with chlorosomes. In order to determine the number and size of chlorosomes in relation to different BChl contents morphometric measurements were performed on electron micrographs. The linear increase of BChl a contents coincided with an increasing number of chlorosomes per membrane area and per bacterium rather than with an enlargement of the average size of chlorosomes. The numbers of chlorosomes and therefore the percentage of chlorosome-covered cytoplasmic membrane increased linearly with increasing BChl a contents. The average size of the baseplates was largely constant in all cultures (mean 3,222±836 nm2). However, within individual cells the size of baseplates varied by a factor of 3.0, especially by the variation of the length. The exponential increase in BChl c contents coincided with an increasing number of chlorosomes (up to a factor of 2.3) and an enlargement of the average chlorosome volume (up to a factor of 1.9). The number of BChl a molecules per chlorosome was about 1,484±165, thus the number of reaction centers per chlorosome was 58±12. The data suggest, firstly, that BChl a is confined to areas (cytoplasmic membrane plus baseplate) as represented by the chlorosome attachment sites; secondly, that the degree of packing of BChl c molecules within chlorosomes increases with increasing BChl c contents.