Most nutrient media supply energy in the form of carbohydrates. Almost all industrial media contain sugars as the main source of energy. For products sold in large amounts at relatively low prices, e.g., ethanol, the cost of the sugar is a major factor in determining profitability.
Photosynthetic cells derive energy from sunlight. When it is dark,
they can use biochemical reserves that they accumulated during the day
or can accept nutrients from their medium. Some cells can also use light
energy to convert nitrogen in the air to the nitrogenous part of organic
compounds needed for life.
Hydrogen and oxygen compose organic compounds and also come from water. Neither is in short supply in terms of the cell's composition. However, cells that require oxygen for breakdown of their biochemicals for metabolic energy can very quickly exhaust the oxygen dissolved in the medium. Supplying oxygen at an adequate rate is a very common challenge for engineers.
A source of nitrogen for incorporation into biochemicals is essential. While many cells can use ammonium compounds and some can accept nitrates, it is very common to have a requirement for nitrogen that is already in organic compounds. Furthermore, many cells must have specific biochemicals that contain nitrogen. For example, certain amino acids must be in the diets of humans.