Identifying information needed to understand eDNA states, decay
and implications for species detection
We have discussed four eDNA states (dissolved, particle-absorbed,
intracellular, inter-organellar) from eukaryotic organisms that are
likely to be present in aquatic environments. Processes responsible for
conversion between states and eDNA decay are detailed and well
understood. Studies of eDNA decay in natural and artificial aquatic
systems to date provide evidence that environmental parameters affect
DNA decay rates in
water3.
We have made the case that chemical reactions that cause eDNA decay are
likely to be state-specific and decay rate constants are influenced by
the physical and chemical properties of aquatic environments. Thus, the
next step is to form a greater understanding of what states are present
for analysis in natural systems.
With this in mind, a meta-analysis of published eDNA studies targeting
single species was performed to assess whether we could ascertain what
eDNA states are being analysed overall, and whether the detection of the
species’ DNA from a specific environmental context could inform which
eDNA state was present. We note that this literature review may
potentially be biased by methodologies that resulted in a positive eDNA
detection from water samples (as non-detections are less likely to be
published). Details of the search and analysis are presented in the
Supporting Information. Because of the chemical properties of eDNA
states, we know that molecular purification protocols can select and
potentially isolate different states from a water sample (e.g.,60).
Thus, we focused our literature review on methods used to isolate eDNA
from a water sample and infer what states were likely analysed. We also
recorded what environmental parameters were co-measured at the time of
sampling.
Our literature review of 74 peer-reviewed articles showed that most eDNA
studies are broadly employing the same molecular methods for DNA capture
(i.e., filtration) and extraction (i.e., enzyme and chemical) albeit in
different combinations (Figure 3).