Figure 2: (a) Fish eDNA decay in relation to temperature. Data
for marine and freshwater fish were included. The natural logarithm of
the decay constant k is plotted against the reciprocal values of the
temperature expressed in Kelvin [1/T], analogous to the temperature
dependence of reaction rates presented in the Arrhenius equation. (b)
eDNA data from amphibians, fish and crustaceans in relation to water pH.
eDNA half-life (in hours) is plotted against pH of water where the
organisms were present. Data were extracted from46,47,51and visualized using the R package ggplot2 v3.3.3.
Lastly, microbial abundance and activity are expected to play an
important role in animal and plant eDNA decay in water
(6and references therein). While studies have been performed on soil and
sediments52–55,
no systematic experiment has been conducted to determine the relative
importance of abiotic versus biotic DNA degradation in water. Several
studies have suggested higher microbial activity contributes to the
faster DNA degradation observed at higher
temperatures43,46,56,57,
which appears to be supported by a mesocosm experiment that examined the
influence of microbial activity on fish eDNA degradation. However, the
experiment did not control bacterial abundance independently of
temperature or
time58.
Another study examining bacterial abundance in relation to eDNA used
radio-labelling as opposed to PCR amplification of natural seawater
samples7,
thus results are based on total eDNA as opposed to animal and/or plant
eDNA. Bacteria are known to graze on DNA for nutrients in aquatic
ecosystems through extracellular enzymes and ectoenzymes (e.g.,
nucleases on the surface of their cells that hydrolyse
DNA11,15).
Active DNA degrading enzymes have been found in filtered water fractions
containing bacteria, cyanobacteria, algae, fungi, and single- and
multicellular plankton animals, but some enzyme types (e.g.,
5’-nucleotidase) have only been found on the surface of bacteria
cells59.
Another study employed antibiotics to decrease bacterial loads and found
that antibiotics decreased eDNA decay rates to smaller values than
measured under higher bacterial loads in untreated
samples45,
suggesting that microbial decay is the main driver. However in both of
these studies, there was no control without bacteria to determine the
relative importance of abiotic reactions. If enzymes secreted by cells
are the main driver of hydrolysis of DNA, the subsequent nutrient
utilization (N and P) by microbial cells is a plausible mechanism for
the shorter decay rates (hours to days) observed for animal eDNA in
natural water compared to abiotic reactions which occur over much longer
timescales (Box
1)7.
This would lead to environment-specific rates of eDNA decay requiring an
understanding of both N and P limitation, and the parameters that
control eDNA state (discussed in the “State conversion processes”
section).