The origin of life on Earth has long been a mystery, with one theory suggesting that complex life forms have evolved from the ancient, single-celled organism archaea. Recently discovered microbes from the Black Sea may explain how bacteria and Archaea may have developed a symbiotic relationship to evolve into other life forms.
Researchers from the Royal Netherlands Institute for Sea Research (NIOZ) and Utrecht University published their study in Nature's ISME Journal. They describe unique tiny bacteria from the depths of the sea that are capable of making membrane lipids.
Lipid membranes are molecules that form a barrier around cells while influencing the maintenance of energy metabolism, noted by the authors. They also help scientists classify different species. For example, bacteria and single-celled organisms have membranes composed of fatty acids, while archaea membranes are "made of isoprenoid alkyl chains" or hydrocarbons.
Lipid Membranes of Bacteria, Eukaryotes, and Archaea
It is believed that the two types of the lipid membrane, also called a "lipid divide," may have originated from the Last Universal Common Ancestor (LUCA). The microbe LUCA existed nearly four billion years ago, which may be the starting point of the tree of life.
Archaea, bacteria, and eukarya comprise the three-domain system, dividing types of cellular life. Archaea are prokaryotic cells that typically survive in harsh and extreme environments. Bacteria commonly exist around other living beings and often form symbiotic relationships. Eukarya are uniquely made of eukaryotic cells, which are the basis of plants, animals, fungi, and protists such as algae.
Lipid membranes also protect cells from shifting environments such as climate, similar to how the skin adjusts to heat or cold. Laura Villaneuva, a NIOZ scientist, explained that when cells die, "these lipids preserve like fossils and hold ancient-old information on Earths' early environmental conditions."
She went on to say that for complex life-forms to be created, "the archaeal membrane must have made a switch to a bacterial type membrane." For the transition to be possible, it means that there was a period when both archaeal and bacteria/eukaryote lipid membranes were mixed.
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Mixed Membrane Lipids
Evidence of the mixed membrane has not been found until the team discovered a group of microbes in the Black Sea under the phylum Candidatus Cloacimonetes. The microbes contain genetic evidence of fatty acids aw well as enzymes associated with archaeal membrane lipids. They also "thrive in the deep-sea, absent of oxygen and with high sulfide concentration," said Villaneuva.
To validate the presence of mixed lipids, the team experimented on how the genetic codes were expressed in the environment. Then, they extracted the lipids using freeze-dried glass fiber filters, which had the genetic markers of both types of lipid membranes.
Then, they experimented if the mixed membrane would form and support E.coli and living cells. In conclusion, the authors wrote that "this discovery provides further support for the existence and potential feasibility of 'mixed membranes' in natural environments and over a long period in evolutionary history, bridging the lipid divide."
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