Large sulfur bacteria
Studying complex macroscopic bacteria to understand the rules of life and the evolution of biological complexity
credit: T Tyml
From the smallest bacteria to the largest sequoia trees, the biovolume of organisms spans 24 orders of magnitude. Bacteria and Archaea seem to be mostly constrained to the microscopic world while the entire macroscopic landscape is exclusively made of organisms composed of highly compartmentalized eukaryotic cells. Virtually all the key features of the eukaryotic cell, such as multicellularity, cell differentiation, or even phagocytosis, can be found individually in modern Bacteria or Archaea, yet the transition to complex macroscopic organisms happened only once in nearly 4 billion years of evolution?
- What underlies the emergence and the evolution of biological complexity? - is one of the most fundamental yet unanswered questions in biology. While we cannot go back in time to sample the microbes at the origin of the eukaryotic cell, we can look at modern microbes outside of the street light and study macroscopic bacteria which display a higher level of cellular complexity.
My research focuses on non-model large sulfur bacteria which often display complex traits such has forming macroscopic multicellular structures, social behaviors, and even bacteria-bacteria symbiotic associations. We recently described a new Thiomargarita species, Ca. Thiomargarita magnifica, which forms centimeter-long single-cells which are visible to the naked eye. These cells grow orders of magnitude over theoretical limits for bacterial cell size, display unprecedented polyploidy of more than half a million copies of a very large genome, and undergo a dimorphic life cycle with asymmetric segregation of chromosomes into daughter cells.
ribosomes in membrane-bound organelles which we named pepins after the small seeds in fruits.
I believe that investigating further the biology of Ca. T. magnifica, its energy metabolism and the precise role of pepins, may help us better understand the emergence and the evolution of biological complexity.
from: Levin, 2022
While investigating the ultrastructure of these bacterial giant, we discovered that unlike most bacteria which have their DNA free-floating in their cytoplasm, Ca. T. magnifica compartmentalises its genomic material and
Cultivation of chemosynthetic symbioses
We are developing an open-source, standardized, and low-cost cultivation system for chemosynthetic symbioses
credit: JM Volland
In the last decades, symbiosis went from a biological curiosity to a ubiquitous strategy that has shaped life as we know it today. From the human gut to coral reefs, microbial symbiosis is a central mechanism allowing the establishment and maintenance of complex ecosystems. The breakdown of symbiosis may have very deleterious effects. Gut dysbiosis in humans, for instance, leads to many chronic and degenerative diseases, and coral bleaching leads to the loss of entire ecosystems with catastrophic ecological and economical consequences. I am interested in chemosynthetic symbioses, a type of partnership where invertebrate animals and protists host bacteria capable of harnessing the energy stored in chemical compounds to fix carbon.
Chemosynthetic symbioses, even while being ubiquitous and evolutionarily relevant, have received far less attention than heterotrophic (e.g. rumen) and photosynthetic (e.g. coral) symbioses and their research has been hampered by the lack of model systems. Symbiotic and free-living Sulfur Oxidizing Bacteria (SOB) are hypothesized to have played an important role in the evolution of life and biogeochemical cycling of sulfur, and they fuel some of the most productive ecosystems on Earth, such as deep-sea hydrothermal vent communities. In light of their ecological and evolutionary relevance, we aim to develop model systems involving related SOB (yet unclassified Gammaproteobacteria) that are interestingly capable of forming “promiscuous” interactions with diverse eukaryotes (ciliates, cnidaria) as well as prokaryotes.
The only fast reproducing chemosynthetic symbiosis which has been cultivated in the laboratory is Zoothamnium niveum. This giant colonial ciliate thrives in sulfidic marine shallow waters and is entirely covered with a monolayer of sulfur-oxidizing bacteria, Ca. Thiobius zoothamnicola. In this mutualistic symbiosis, the ciliate host benefits from carbon transfer, while the bacteria benefit from a competition-free environment and enhanced access to an electron donor (hydrogen sulfide) and acceptor (oxygen). The current state-of-art requires cultivation using custom-made flow-through chambers which are needed to provide the symbiosis with a continuous supply of sulfide and oxygen. Unfortunately, these high-maintenance chambers are expensive and not readily available to the community, limiting the potential of Z. niveum to become a true model system.
We aim to overcome these obstacles and help establish Z. niveum as the first fast reproducing, easy-to-culture chemosynthetic symbiosis model system in the laboratory that is reproducible and experimentally tractable. As a first step towards this goal, we have developed a new low-cost cultivation device that includes a modified tesla valve ensuring the mixing of sulfide and oxic seawater. High-fidelity complex computational fluid dynamics simulations were performed to determine the ideal fluidic and structural parameters. These devices can easily be produced in any laboratory using 3D models of printable molds for casting slide-bound polydimethylsiloxane (PDMS) chambers.
