Beggiatoa

Genus of bacteria

Beggiatoa is a genus of Gammaproteobacteria belonging to the order Thiotrichales, in the Pseudomonadota phylum. These bacteria form colorless filaments composed of cells that can be up to 200 μm in diameter, and are one of the largest prokaryotes on Earth. Beggiatoa are chemolithotrophic sulfur oxidizers, using reduced sulfur species as an energy source. They live in sulfur rich environments such as soil, both marine and freshwater, in the deep sea hydrothermal vents, and in polluted marine environments. In association with other sulfur bacteria, e. g. Thiothrix, they can form biofilms that are visible to the naked eye as mats of long white filaments; the white color is due to sulfur globules stored inside the cells.

Beggiatoa was originally described as a type of blue green algae (today known as Cyanobacteria) by the botanist Vittore Trevisan in 1842, who named it in honor of the Italian doctor and botanist Francesco Secondo Beggiato (1806 1883), from Venice. Later, it was shown that Beggiatoa in their natural habitat of sulfur springs accumulate sulfur globules in their cells. The Ukrainian microbiologist Sergei Winogradsky, working in the laboratory of Anton de Bary, showed that these intracellular sulfur globules were formed when Beggiatoa oxidized hydrogen sulfide (H2S) as an energy source, with oxygen as the terminal electron acceptor and CO2 used as a carbon source. Winogradsky referred to this form of metabolism as "inorgoxidation" (oxidation of inorganic compounds), today called chemolithotrophy. The finding represented the first discovery of lithotrophy.

The genus Beggiatoa is diverse, with representatives occupying several habitats and niches, both in fresh and salt water. In the past, they have been confused as close relatives of Oscillatoria spp. (phylum Cyanobacteria) because they have similar morphology and motility, but 5S rRNA analysis showed that members of Beggiatoa are phylogenetically distant from Cyanobacteria, and are instead members of the phylum Gammaproteobacteria. Despite their diversity, only two species of Beggiatoa have been formally described: the type species Beggiatoa alba and Beggiatoa leptomitoformis, the latter of which was only published in 2017. The capability to oxidize sulfide and store sulfur are the main features which define Beggiatoa and its close relative Thioploca as filamentous colorless sulfur bacteria, in contrast to other filamentous bacteria like cyanobacteria and the non sulfur oxidizing Cytophaga and Flexibacter. Another defining feature is the ability to store nitrate inside the vacuoles of the wide marine species' cells. 16S rRNA sequences base studies inferred that this characteristic is shared between members of a monophyletic clade nested in the Beggiatoa genera; this clade also includes members of Thioploca and Thiomargarita, both presenting only slight differences with Beggiatoas: whereas the former grows sharing a common slime sheath, the latter has not conserved filamentous growth and forms chains of rounded cells. Since the phylogenic history do not reflect the nomenclature, there is a need for a new denomination of genera and species. The Neo type strain is the B18LB and it settled the criteria for identification of the freshwater species Beggiatoa alba.

Because of the lack of a pure culture, little is known about the genetics of Beggiatoa. Beggiatoa alba has a GC content between 40 and 42.7 mol%, two or three similar plasmids, and a genome size of about 3 Megabase (Mbp) (strain B18LD).

Beggiatoa group is mainly composed by chemolithotrophic, sulfide oxidizing bacteria. However, the range of possible metabolic pathways is very diversified, varying from the heterotrophy to the chemolithoautotrophy. Because of this huge variability the diverse bacteria of this genus can differ greatly from each other.