These organic molecules essential for growth are called growth factors and fall into three major categories: 1) amino acids (building blocks of protein), 2) purines and pyrimidines (building blocks of nucleic acid), and 3) vitamins (enzyme cofactors). Other microbes require that certain organic compounds exist within their environment. ![]() Some microbes can synthesize certain organic molecules that they need from scratch, as long as they are provided with a carbon source and inorganic salts. ![]() In fact, it has taken years to cultivate some of the most numerous marine taxa, with success coming once sterilized sea water was used as a growth medium. Cultivation of the oligotrophs has been somewhat challenging as they have very low metabolic rates, therefore slow generation times. Such conditions are characterized as oligotrophic, and the organisms that require such low nutrient levels are oligotrophs. The traditional g/L amounts of major macronutrients support the growth of many bacteria and archaea, it is often toxic to isolates from marine environments where, beyond the coastal areas, nutrient concentrations are extremely limiting. These are present as contaminants of glassware and water and are typically structural components of enzymes or cofactors. Micronutrients (e.g.cobalt, copper, manganese, zinc) are also required, but at ~1000-fold lower levels. These can be further differentiated into major macronutrients (C, H, O, N, P, S) which have traditionally been added to microbiological growth media at g/L amounts, and minor macronutrients (K +, Mg 2+ and Fe), traditionally added at mg/L amounts. Collectively these elements are referred to as the macronutrients. Required at slightly lower levels are cations: potassium (K+), which is needed for enzymes, and magnesium ions (Mg 2+), which is used to stabilize ribosomes and membranes, iron (Fe 2+) which is needed for electron transport systems, as well as for enzymes and calcium (Ca 2+). Sulphur is necessary for a few amino acids, as well as several vitamins. Phosphorous is also a crucial component of nucleic acids it is also required for the synthesis of phospholipids, and adenosine tri phosphate or ATP. In particular, cells need nitrogen for the formation of proteins, nucleic acids, and a few other cell components. In addition to requiring a source of carbon, hydrogen and oxygen to build these molecules, cells need a source of three other elements at relatively high levels: nitrogen, phosphorus, and sulphur. Organic molecules contain, minimally, carbon and hydrogen, however oxygen is typically also present. The majority of dry weight of cells comes from organic molecules. So although the golden rule for laboratory cultivation is to try to replicate the conditions from the natural environment, this is not always effective and cultivation conditions are always selective. Another problem is the “patchiness” of the physicochemical conditions of an environment – that is conditions may change over microscopic distances. ![]() Culturing many microorganisms is challenging because of highly specific nutritional and environmental requirements, and the diversity of these requirements among different species. The study of microorganisms is greatly facilitated if we are able to culture them, that is, to keep reproducing populations alive under laboratory conditions.
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