MARTINUS BEIJERINCK (1851-1931)
Martinus Beijerinck was a Dutch microbiologist and botanists who began examining the role of non-infectious microorganisms in the soil. He reported that microorganisms play important role in nutrient recycling in the ecosystem (particularly those of nitrogen, carbon and sulphur) as well as in the process of nitrogen-fixation by symbiotic or free-living soil bacteria. Martinus Beijerinck showed in 1888 that some beneficial microbes in the soil (for example, Rhizobium) inhabit the root of leguminous plants and provide such plants with functional nitrogen through the process of nitrogen fixation. Through the process of nitrogen fixation, atmospheric (molecular) nitrogen (N2) can be converted into ammonia (NH3) and ammonium (NH4), which is the forms in which plants can utilize them for their metabolic activities and growth.
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Beijerinck also discovered viruses and sulphate-reducing bacteria. His work into soil bacteria and viruses made him one of the founding fathers of soil (environmental) microbiology and a contributing scientist to the development of the field of virology. Beijerinck began his work by studying the microorganisms that were present in and around plants, and he soon began experiments with microbes in the soil which led him to develop enrichment media for culturing microorganisms. Before Beijerinck’s discovery of the enrichment media, microorganisms were previously cultivated on medium consisting of potatoes or extracts of leftover animal renderings which supported the growth of many different bacteria, with chance and population density dictating what became dominant in the culture medium.
However, he discovered that by adding or removing certain compounds from the medium or incubating under different conditions, it was possible to favour the growth of certain microbes and prevent the growth of other unwanted bacteria. His invention of the enrichment culture media enabled him to isolate many microorganisms in their pure state including Azotobacter chroococum, sulfur-reducing and sulfur-oxidizing bacteria, Rhizobium, Lactobacillus species, and some green algae. Beijerinck referred to this method of specific bacterial isolation as selective culture techniques. This technique have since then been applied to many different groups of microorganisms, allowing them to easily be brought into pure culture forms. Beijerinck’s work awakened great enthusiasm for identifying and classifying the bacteria inhabiting our natural world particularly the soil. And today, there is a branch of microbiology known as geomicrobiology and environmental microbiology which studies microorganisms that are found in the earth’s crust and other parts of the environment.
SERGEI WINOGRADSKY (1856-1953)
Sergei Winogradsky was a Ukrainian-Russian microbiologist, ecologist and soil scientist who was among the first to delve into a different area of microbiology that involved the investigation of microbes in the environment. Winogradsky and Beijerinck focused majorly on the study of soil microorganisms that did not cause disease. Winogradsky pioneered the concept of biogeochemical cycles in which bacteria (like nitrifying and purple-sulphur bacteria) mediate the cycling of some important compounds (nutrients) such as nitrogen and sulphur compounds in the environment so that they can be made available for other forms of life.
He discovered the first known form of lithotrophy during his research with a bacterium called Beggiatoa in 1887, and reported that Beggiatoa oxidized hydrogen sulfide (H2S) as an energy source and formed intracellular sulfur droplets. Lithotrophy is the process in which some bacteria derive their energy source from the oxidation of inorganic compounds. These organisms are generally called lithotrophs. Winogradsky showed in his work that certain bacteria are linked to specific biogeochemical transformations, and this provided the first example of lithotrophy, but not autotrophy, in which the organisms (known as autotrophs) derive their own energy source from the sunlight.
His studies on sulphur-oxidizing bacteria also proposed the concept of chemolithotrphy, which is the oxidation of inorganic compounds linked to energy conservation. Sergei Winogradsky was one of the first to isolate microorganisms responsible for the conversion of elements like nitrogen and sulphur in the soil, and obtaining pure cultures of bacteria capable of oxidizing ammonia to nitrate. Winogradsky also studied the consumption of hydrogen sulfide gas by sulfur-oxidizing bacteria directly in their natural habitat. He showed that nitrifying bacteria obtained their carbon from carbondioxide (CO2), and like phototrophic organisms, that nitrifying bacteria were also autotrophs.
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