Four billion years ago
Researchers from the Cnr with Bruno D’Argento and Giuseppe Geraci, geology and molecular biology professors from the university of Naples, discovered in 2001 some microorganisms inside ten meteorites whose genetic make-up is slightly different from the approximately 28 thousand known terrestrial bacteria. These organisms, that are very similar to the familiar ones, have been found inside 4,5 billion-year old meteorites, a discovery which may be confirmed only following exams and very accurate verifications.
Researchers from the University of Naples and from the Geomare Institute of the Cnr, state that the possibility that the samples may have been contaminated by terrestrial bacteria is very low. However, following the revelation of these discoveries, it is impossible to ignore the doubts that many scientists have about these meteorites not having been contaminated by terrestrial bacteria when they fell to earth.
The crystalmicrobs have the same characteristics as other bacteria which are already known and have been thoroughly studied in the past 40 years: the extremophile bacteria. As the very word says, these organisms are able to live and reproduce in environmental conditions where others would suffer. Some of these particular bacteria are part of the archaebacteria group, that existed way back in the time when life started on earth.
The “extraterrestrial bacteria” discovered in the meteorites that are kept in the Mineral Museum of Naples, have been clonated and breed plentifully in test-tubes in the labs of the Federico II University. Following their reproduction, their DNA has been analyzed and a new type was discovered that is totally different from the other 18 thousand genetic code types known until now. Besides the same bacteria types called “crystalmicrobs” or “Cryms”, have been found by Campania researchers also in about 50 terrestrial rock samples, some dating back to up to 3,8 billion years and taken from different parts of each continent of our planet. Researchers analyzed about 50 different samples of sedimentary, igneous and metamorphic rocks, minerals and other solid natural materials; then they extracted microorganisms that when placed in a physiological solution generally used in microbiology labs become visible under the microscope and are active again. When bacteria regain their metabolic capacities they are clonated and studied.
These kinds of microbes are called extremophile because they are able to survive in environments that until now were considered impossible, we could call them sterile in the sense that no other form of life can develop there. We will give examples of other environments where extreme bacteria live and reproduce.
In waters with a very high saline concentration we can find the Helobacterium salinarum, while the record for being the “saltiest” is held by Halophilic bacterium that is able to live in water with a 30% salt concentration (seawater has a 3,5% salt concentration).
Rocks embedded a few kilometers below the surface are an ideal habitat too: some microorganisms live in tiny interstitial spaces of rocks which are 3,2 km beneath the subsoil and they are able to tolerate high pressure levels, radiation and extreme heat. While organisms belonging to the “Bacillius infernus” species live 2800 meters underground at a 75-degree temperature, the Staphylothermus marinus colonizes environments at the bottom of the ocean where there are temperatures that reach 115C°. On the contrary microbes belonging to the Chroococcidiopsis and Crypotendoliths families find their ideal living conditions at -15°C, but in rocks of the Antarctic continent there are others who tolerate temperatures that reach -50°C.
Another peculiarity of extreme environments is that there is a pH which is either extremely acid or basic: the most basic microorganism is the Alkaliphic that lives in alkaline minerals with a ph of 11 that were sedimented following massive water evaporation.
Some American researchers wanted to study the effects of intense solar radiation on microorganisms in the emptyness of outer space: the first experiment was run by NASA in order to see to what extent solar radiation can influence living cells. According to their previsions ultraviolet radiation was supposed to harm all bacteria at an altitude of 320 km, except for the Deinoccocus radiodurans that normally lives in the ground.
In fact the discovery of these microbes dates back to 1950 when some scientists that were searching for the best methods of food conservation found out that it was practically impossible to kill them.
It is assumable that these intrepid bacteria could survive on other planets since following the experiments they were found to have no fear of radiations, high temperatures, dehydration or chemical agents able to destroy the DNA.
Primordial and resistant
The prokaryotes that live in habitats with extreme conditions (high salinity, low oxygen concentration, high temperatures and extreme pH values) are grouped in the division of the Archaebacteria. If we take into consideration the characteristics of the environment in which they live, the archaebacteria are an apparently mismatched group, yet they have many characteristics in common. These characteristics are to be found in the composition of the cell walls and in the base sequence of their RNA. In particular, the cell wall isn’t formed by peptide-glycol, a typical molecule of a bacterium cell wall.
The archaebacteria are divided into 3 groups: the thermoacidophiles, the methanogens and the narrow halophiles.
The thermoacidophiles archaebacteria prefer high temperature conditions with an acid Ph and they colonize environments where few others are able to survive. The Solfolobus is a typical representative of this bacteria group that lives near by hot sulphurous springs at 70-75°C and they cannot survive in temperatures below 55°C. This bacterium is able to maintain an internal ph around 7, and the ideal acid environment in which it thrives is between 2 and 3. Some archaebacteria belonging to the Bacillus acidocaldarius and Sulfolobus solfataricus groups have been discovered in the solfataras of the Flegrei fields in Pozzuoli (Naples)
The methanogen archaebacteria are prokaryotes that live without oxygen and the key tool for their metabolism is the reaction that produces methane starting from carbon dioxide. Another group, the Methanopyrus, lives at the bottom of the ocean near volcanic fractures and thrives in temperatures ranging from 110 to 84°C.
The narrow halophiles live only in extremely salty environments where few other organisms are able to live because they would die from dehydration. Some of these bacteria have found their ideal environment in the Dead Sea where the salt concentration is about 10 times higher than that of the other seas (340g/l).
Why are salt pits red?
Some halophile archaebacteria, that love salt, dye the crystalized water of the salt pits red. Several bacteria species belonging to the Halobacteriaceae family are responsible for these red-pinkish dyes because their cell membranes contain pigments deriving from beta-carotene. These groups are being studied with great care because they are potentially useful in the biotechnology field both for the production of carotenoids as well as to spot active enzymes in very concentrated saline solutions.
The environmental factors
The size and density of a population is influenced by several factors that are different for one group of organisms to another. In general, for example, we can say that for some bacteria the ideal ph range in which they thrive is between 6,5 and 7,5, but we also know that there are other groups that live in acid environments with a ph as low as 4,5. An acid ph would not be ideal for the first bacteria group that we mentioned because they wouldn’t be able to grow and survive. In fact the tolerance limits of organisms to factors such as light, temperature, water availability, salinity and oxygen availability are very important. If an essential element is scarce or some other is over the tolerance limit, even if the other requirements are met, that group will not be able to thrive.
For example, bacteria that live in the Antarctic regions find their ideal living conditions in temperatures ranging from -7°C to -20°C, while thermophile bacteria that live near by thermal springs thrive and reproduce at 90°C. As for the presence of oxygen in the environment, many prokaryotes are optional anaerobia, which means that they can obtain energy to live either through cell respiration or through fermentation.
Others that are compulsory anaerobia can live only through fermentation which means they can’t thrive in the presence of oxygen; on the other hand there are compulsory aerobic groups that cannot survive for too long without oxygen.
There is life in salt too!
Another environmental factor is the saline condition of the waters which can limit the growth of most bacteria but it isn’t a problem in the case of another group called the hypersalinphyles.
In the period between 2001 and 2004 four basins of the Mediterranean Sea have been monitored and studied, located 100 km west of Crete, that were formed in the Miocene period.
These basins are characterized by areas that can reach a depth of 3000 m., where the waters are stagnant and very still, almost devoid of oxygen, with a manganese concentration 100 times higher than the Mediterranean one and a high saline concentration (about 470 g/l of manganese chloride).
In spite of the inhospitability of this type of environment, several bacteria have been discovered to have a metabolism which has adapted to high saline concentrations. A team of European scientists, within a research program on microbic ecosystems, is trying to discover how the metabolism of these organisms works.
The study of life in extreme conditions requires a joint effort by the different scientific categories such as microbiologists, geologists and molecular biologists. Researchers are interested in discovering the unique ability that extremophile microorganisms developed to live and thrive in environmental conditions which are so hostile for other organisms. Besides these extremophile organisms are particularly interesting also because they are able to produce molecules that can be useful in many ways, in biotechnologies as well as several different industrial fields (nutrition, environmental, medical diagnostic and for the development of new medical therapies).
For instance, to understand what it is that allows Colwellia psychrerythraea to resist to freezing conditions, some scientists have sequenced and analyzed its genome and discovered that some genes codify the filling of cell membranes with fatty acids that resist freezing, polyester composites that work as energy reserves and altered enzymes that function even in freezing cold salty water. Instead in the early 80’s, with the purpose of analyzing and replicating DNA both in medical diagnostic and in legal medicine, they started using a thermophile and thermostable enzyme (Taq-polymerases) This polymerases had been isolated by the Thermus aquaticus bacterium discovered in the hot springs of Yellowstone National Park.
The ability of methanogen bacteria is exploited for the production of biogas which is useful for heating and cooking.
From the origin of life to the future
The prokaryotes are the most ancient and numerous group in the world: the first fossil remains of this group date back to 3,5 billion years ago and these ancient traces prove that there were already many substantial differences between prokaryotes in the Achaean period as well. These organisms reigned on earth for over 2 billion years adapting to many different environments and to the many changes that took place during that time spreading into every possible habitat on earth, thus colonizing other organisms as well.
Modern day prokaryotes are very different from the ones of 3,5 billion years ago and they are the result of the many independent evolution lines that branched off from one another hundreds of millions of years ago. In fact, starting from a common prokaryiote background, each different line has followed different evolution paths and each one of them has transformed to adapt to the changes in their environment with the result that diversity within each line is greater compared to the verifiable differences within other groups.
The prokaryote group includes bacteria, unicellular organisms, without nucleus or other cytoplasmic structures which are typical of eukaryote cells.
One of the last theories to become known on the beginning of life was published on Nature magazine by a molecular biologist called James A. Lake. His genomic studies (the science of mapping, sequencing and analyzing genomes, which are the contents of DNA) have allowed him to come up with an hypothesis which goes a step forward with regard to what was known so far on this topic. Apparently 2 billion years ago, the fusion of some protobacteria (ancient photosynthetic unicellular microorganisms) with some archaebacteria may have given birth to the first multicellular prokaryote organism. This theory gives archaebacteria a very important role, that had never been thought of before, in the evolution sequence from unicellular organisms to the most advanced multicellular ones.
Over a billion years ago some prokaryotes invaded similar organisms (or were encompassed by them) which created the parasite-host relationship (or prey-predator). This relationship was consolidated over the evolution periods and probably constituted the origin of protists, characterized by eukaryote cells with a real nucleus.
From a taxonomic point of view based on the type of cell wall, the prokaryotes are divided into 4 groups: archaebacteria, gram positive bacteria, gram negative bacteria and mycoplasm.
written by Eliana Marchisio