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.