Nanobacteria (singular nanobacterium) is the name of a proposed class of living organisms, specifically cell-walled microorganisms with a size much smaller than the generally accepted lower limit size for life (about 200 nanometres for bacteria). The status of nanobacteria is controversial, with some researchers suggesting they are a new class of living organisms capable of incorporating radiolabeled uridine and others attributing to them a simpler, abiotic nature.
The term 'calcifying nanoparticles' (CNPs) has also been used as a conservative name regarding their possible status as a life form. The most recent research tends to agree that these structures exist, and probably replicate in some way. Their status as living entities is still hotly debated, though some researchers now claim that the case that they are nonliving crystalline particles is conclusively proven.
They were first described in 1981 by Torella and Morita and were discovered because of their implication in the formation of both kidney stones and arterial plaque. Early in 1989, geologist Robert L. Folk found what he later identified as nannobacteria (written with double "n"), that is, nanoparticles isolated from geological specimen in travertine from hot springs of Viterbo, Italy. He proposed that nanobacteria are the principal agents of precipitation of all minerals and crystals on Earth formed in liquid water, that they also cause all oxidation of metals, and that they are abundant in many biological specimens. In 1996, NASA scientist David McKay published a study suggesting the existence of nanofossils — fossils of Martian nanobacteria — in ALH84001, a meteorite originating from Mars and found in Antarctica.
According to the Finnish researcher Olavi Kajander and Turkish researcher Neva Ciftcioglu, the particles self-replicated in microbiological culture, and the researchers further reported having identified DNA in these structures by staining.
A paper published in 2000 led by John Cisar further tested these ideas and claimed that what had previously been described as "self-replication" was a form of crystalline growth. The only DNA detected in his specimens was identified as coming from the bacteria Phyllobacterium mysinacearum, which is a common contaminant in PCR reactions. An article in PLoS in 2008 focused on the comprehensive characterization of nanobacteria. The authors say that their results rule out the existence of nanobacteria as living entities, instead revealing that they are a unique self-propagating entity and that they are self-propagating mineral-fetuin complexes. Another 2008 PNAS article reported that blood nanobacteria are not living organisms and stated that "CaCO3 precipitates prepared in vitro are remarkably similar to purported nanobacteria in terms of their uniformly sized, membrane-delineated vesicular shapes, with cellular division-like formations and aggregations in the form of colonies."
The growth of such "biomorphic" inorganic precipitates was studied in detail in a 2009 Science paper, which showed that unusual crystal growth mechanisms can produce witherite precipitates from barium chloride and silica solutions that closely resemble primitive organisms. The authors commented on the close resemblance of these crystals to putative nanobacteria, stating that their results showed that evidence for life cannot rest on morphology alone.
This once again relates back to the definition of life and how we determine if something is alive if we are dealing with an alien life form. Another example is that of viruses, in which some scientists, like the paleontologist Peter Ward, classify as living. He says that our current tree of life leaves no room for viruses which he suggests are living things, and will likely be an entirely useless concept for classfying alien life. Indeed, some evolutionary biologists today believe the current phylogenetic tree to be incapable of classifying life in an accurate way because it is a human construct that is organized not necessarily according to classifications defined by nature itself, but by ease of convenience for human understanding. Thus the search for alien life needs a restructuring of how we think about life itself.