Gloeobacter

Gloeobacter
Scientific classification
Kingdom:	Bacteria
Phylum:	Cyanobacteria
Class:	Gloeobacteria
Order:	Gloeobacterales
Family:	Gloeobacteraceae
Genus:	Gloeobacter Rippka, Waterbury, & Cohen-Bazire, 1974
Species
Gloeobacter violaceus Rippka, Waterbury & Cohen-Bazire, 1974 Gloeobacter kilaueensis Saw et al., 2013

Gloeobacter is a genus of cyanobacteria. It is the sister group to all other cyanobacteria. Gloeobacter is unique among cyanobacteria in not having thylakoids, which are characteristic for all other cyanobacteria and chloroplasts. Instead, the light-harvesting complexes (also called phycobilisomes), that consist of different proteins, sit on the inside of the plasma membrane among the (cytoplasm). Subsequently, the proton gradient in Gloeobacter is created over the plasma membrane, where it forms over the thylakoid membrane in cyanobacteria and chloroplasts.

The whole genome of G. violaceus (strain PCC 7421) and of G. kilaueensis have been sequenced. Many genes for Photosystem I and II were found missing, likely related to the fact that photosynthesis in the bacterium does not take place in the thylakoid membrane like in other cyanobacteria, but in the plasma membrane.

Description

G. violaceus produces several pigments, including chlorophyll a, β-carotene, oscillol diglycoside, and echinenone. The purple coloration is due to the relatively low chlorophyll content. G. kilaueensis grows with a few other bacteria as a purple colored biofilm of approximately 0.5 mm thick. Cultivated colonies are dark purple, smooth, shiny and raised. G. kilaueensis is mostly unicellular, capsule-shaped, approximately 3.5×1.5 µm, and inbedded in mucus. They divide over the width of the cell. Cells color gramnegative, and lack vancomycin resistance. They are not motile and do not glide. Growth ceases in complete darkness, so Gloeobacter is very likely obligatory photoautotrofic.

Species and distribution

G. violaceus was found on a limestone rock in the Swiss Canton Obwalden. G. kilaueensis occurred within a lava cave at the Kilauea-caldera on Hawaï. It grew there at a temperature of about 30°C at very high humidity, with moisture condensating and dripping of the biofilm.

Gloeobacteria could have split off from the other cyanobacteria between 3.7 and 3.2 billion years ago. The species of Gloeobacter may have branched 280 million years ago.