In trophic systems photosynthetic organisms are the primary producers. These trophic interactions shape patterns of biodiversity globally. Lindeman emphasized the important role of decomposer organisms in a trophic system of classification. Food webs depict energy flow via trophic linkages. Soil microorganisms are influenced by and feed back into the trophic dynamics of the ecosystem. These interactions can be trophic or symbiotic. Each trophic level transforms energy into biomass. PCB concentrations within an organism also change depending upon which trophic level they occupy. Food chains are nested within the trophic links of food webs. Food webs have trophic levels and positions. There may be a cascade of coextinction across the trophic levels. Many of the barriers to top-down trophic cascades also promote stability. One way of classifying predators is by trophic level. A common metric used to quantify food web trophic structure is food chain length. These primary consumers form an important link between the primary producers and higher trophic levels.