Giant Bacteria

Abstract

For almost all bacteria, it is convenient to be very small, but a few highly specialist groups of bacteria have evolved to be orders of magnitude larger than ordinary bacteria. While some of these giant microbes are abnormally large in all dimensions, others are long and thin or consist of assemblages of multiple cells. These giant bacteria are spread across the domain Bacteria and have evolved multiple ways to combat diffusion constrains. Giant bacteria often thrive in nutrient and carbon‐rich environments and/or in steep redox gradients. Genomically, giant bacteria seem all to be polyploid, that is containing multiple copies of their genome. Giant bacteria have been described throughout the history of microbial research but only some of them can be traced in current taxonomy and are being actively studied. Therefore, it is likely there are other giant bacteria in nature, awaiting to be explored. Key Concepts Growth and activity of giant bacteria are expected to be constrained by a combination of diffusion of nutrients into the cells as well as intracellular trafficking of solutes by diffusion. Giant bacteria can be divided into ‘truly’ giant, consisting of abnormally large cells on all dimensions, or ‘pseudo‐giant’ consisting of cells abnormally large on one axis or multicellular assemblages of bacteria. ‘Pseudo‐giant’ bacteria may not face the same diffusion limitations as ‘truly’ giant bacteria. ‘Truly’ giant bacteria likely ‘resolve’ diffusion constrains by one or more methods including, minimising cytoplasmic space by intracellular (pseudo)compartmentalisation, storage vacuoles, rapid swimming. Giant bacteria often grow in carbon‐ and nutrient‐rich environments and/or in steep redox gradients, thus additionally minimising diffusion‐related constraints. Current knowledge suggests that all giant bacteria contain multiple copies of their genomes being oligoploid (3–10 copies) or polyploid (10–1000s of copies). Gigantism in bacteria occurs in multiple phylogenetic lineages covering both Gram‐negative and Gram‐positive taxa. Early descriptions of giant bacteria suggest that the currently studied taxa do not encompass the entire diversity of abnormally large bacteria.

The role of mitochondrial energetics in the origin and diversification of eukaryotes

“…Such a reduced mitochondrial genome could only have evolved after the invention of a protein import machinery that sped up gene transfer to the nucleus or main genome (i.e., by allowing import of transferred gene products). In addition, only the presence of active intracellular transport (i.e., a dynamic cytoskeleton and motor proteins that bypassed diffusion constraints) would have allowed the nuclear or main genome not to scale up with cell volumes (unlike in prokaryotes (42,47)). Thus, a great degree of evolutionary change (and time) separates the two types of cells compared here.…”

“…4). The cause of this scaling might be the need to either bypass a diffusion constraint in the absence of active intracellular transport (42) or maintain genomes physically adjacent to respiratory membranes for efficient regulation (3,18). We compiled data for several prokaryotes that show that the cell volume per genome does not exceed 2 µm 3 in prokaryotes (𝑉 𝑔𝑠𝑒𝑟𝑣 in Eq.…”

Heterozygous, Polyploid, Giant Bacterium,Achromatium, Possesses an Identical Functional Inventory Worldwide across Drastically Different Ecosystems

“…Similarly to the extreme allelic divergence (Ionescu et al 2017), Achromatium challenges our understanding of genomic evolution in general and particularly that of polyploid organisms. In light of a plethora of large, polyploid bacteria (Ionescu and Bizic 2019), an urgent question that remains open is whether these, or other bacteria, share some or all of these newly discovered features.…”

CaveThiovulaceaediffer metabolically and genomically from marine species

“…In Movile Cave, the oxidation of reduced compounds such as H2S, CH4, and NH4 + is the only primary energy source, There, Thiovulum sp., a large sulfur oxidizer, often found in close proximity to sediments, microbial mats or surfaces, is part of the chemoautotrophic microbial community involved in in situ carbon fixation that represents the food base for the cave's abundant and diverse invertebrate community. These Thiovulum cells, exceeding 15 µm in diameter, are larger than most known sulfur-oxidizers: 5-10 µm (Fenchel, 1994) and 5 µm (Thar and Fenchel, 2005) and, together with the larger Thiomargarita namibiensis (Schulz et al, 1999) and Achromatium oxaliferum (Babenzien, 1991), belong to the group of giant sulfur bacteria (Ionescu and Bizic, 2019). Here we investigated the morphological and genomic aspects of what appears to be a fully planktonic Thiovulum sp.…”