7.1: Nutrients

Nutrients are chemicals that organisms need to create their biological molecules but cannot make themselves. As such, they obtain them from their environment. Nutrients can be divided into two broad categories: macronutrients and micronutrients. Macronutrients are substances needed in relatively high abundance. Carbon, nitrogen, and phosphorus are the macronutrients needed in the largest abundance, and those we focus on here (Fig. \(7.1\)). Micronutrients are nutrients needed only in trace quantities, although they are certainly still essential.

Carbon is the major element in all biological macromolecules and accounts for about 50% of the dry mass of cells (Madigan et al., 2003). Autotrophs assimilate carbon mainly from carbon dioxide. In contrast, heterotrophs assimilate carbon from organic sources and have been observed using a wide variety of organic compounds, including amino acids, fatty acids, organic acids, sugars, nitrogen bases, aromatic compounds, and more (Madigan et al., 2003).

Nitrogen is the next most abundant element in biological macromolecules. It accounts for about 12% of the dry weight of cells and is needed to make proteins, nucleic acids, and more. The most abundant nitrogen sources are ammonia, nitrate, and dinitrogen. Most bacteria can obtain nitrogen from ammonia, some can use nitrate, and only nitrogen fixing bacteria can use dinitrogen. Ammonia can be supplied from nitrogen fixation, dissimilatory nitrate reduction to ammonia (DNRA), or ammonification, nitrate can be supplied by nitrification, and both ammonia and nitrate can be supplied by fertilizers.

Phosphorus is primarily used to make nucleotides and phospholipids. Phosphorus is ultimately derived from mineral sources such as apatite \(\left(\text{Ca}_{5} \left(\text{PO}_{4}\right)_{3} (\text{F, Cl, OH})\right)\), which are not generally abundant in natural environments. Therefore, microbial communities recycle phosphorus from decaying biomass and this source accounts for most of the phosphorus used by plants and the microbiomes of most ecosystems unless phosphorus is supplied as chemical fertilizer (Turner, 2008). Where organic sources of phosphate are limited and fertilizers are not applied, microorganisms must acquire phosphate from minerals (Welch et al., 2002) (Fig. \(7.1\)). Such conditions are common during the early stages of rock weathering (Welch et al., 2002).

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