Figure 1: Summary of eDNA states, the processes that convert eDNA between states (cell lysis and adsorption/desorption), and the chemical reactions (intra- and extracellular breakdown, microbial utilization, or stabilization) that degrade or alter eDNA in different states making it inaccessible to capture and detection.
The simplest form in which extra-organismal eDNA is present is a purely dissolved state. DNA is a highly water soluble polyelectrolyte due to the negatively charged phosphodiester groups in the DNA backbone. However, dissolved DNA interacts with and may adsorb to the surfaces of mineral and organic particles and colloids suspended in the water. Particle-adsorbed DNA is therefore a second state. Existing literature on DNA adsorption (e.g.,17–24) suggests that DNA-particle interactions are mainly controlled by electrostatics (which may either be attractive or repulsive for positively and negatively charged particle surfaces, respectively) as well as inner-sphere complex formation on some mineral surfaces25–30. The bases connected to deoxyribose (i.e., cytosine, adenine, guanine and thymine) likely only play a small, modulating role on DNA adsorption processes (i.e., these bases are involved in H bonding between the two complementary DNA strands). DNA can also remain associated with cells that are shed by organisms into the water, either as intracellular DNA (third state) or intraorganellar DNA (fourth state) such as skin cells from mucus or cells from the intestinal tract during defecation. The types of cells shed from any organism and their source remain mostly undescribed, but recent advances using messenger RNA typing may allow us to gain a better understanding of the sources and types of cells that make up eDNA29, for instance, intraorganellar DNA may be present in mitochondria and chloroplasts. In fact, many extra-organismal eDNA studies target genes found in organelles due to their high copy number per cell which should increase probability of eDNA detection.