3. Limitations and perspectives
We noted some potential biases and limitations of the dataset used in
our meta-analyses. Firstly, studies estimating the decay rates of
nuclear eDNA were substantially fewer when compared with those on
mitochondrial eDNA, particularly in freshwater systems (Figure 3), which
might limit our ability to infer the effect of water source on eDNA
degradation between the target genes. In addition, eDNA decay rates
targeting longer DNA fragments (>200 bp) and taxa other
than fish were relatively scarce. Moreover, estimation of eDNA decay
rates using a 0.7 µm pore size filter appeared to be relatively more
common, which suggests greater knowledge of eDNA persistence in this
filter pore size, and a potential bias in our meta-analyses. It is
expected that eDNA analysis will be applied to ecological monitoring of
more varied taxa and environments in the future, and will have to be
developed accordingly to determine the spatiotemporal scale of eDNA
signals and to maximize the biological information obtained from eDNA
samples. More information on eDNA persistence and degradation should
therefore be collected, by targeting different taxa and environments and
using various collection and analysis methods.
Although our findings and their implications require further
verification, this study is the first to propose that the persistence of
eDNA from macro-organisms can be determined by the state of the eDNA and
its complex interactions with environmental conditions, i.e. the
mechanism of eDNA persistence and degradation cannot be fully understood
without knowing not only the environmental biotic and abiotic factors
involved in eDNA degradation but also the cellular and molecular states
of eDNA occurring in water. If our findings are correct, the
spatiotemporal scale and intensity of eDNA signals would be different
depending on the eDNA particle size and state. The fact that Weibull or
biphasic exponential decay models fit better to eDNA degradation implies
the differences in eDNA persistence depending its state (e.g., intra- or
extra-cellular, living or dead cells, particulate or dissolved)
(Eichmiller et al., 2016; Bylemans et al., 2018), which support our
results linking eDNA persistence to its state. In addition, the study by
Jo et al. (2020c), where it was reported that the genomic information
obtained from eDNA samples can differ depending on the filter pore size,
can further support the link between eDNA state and persistence.
Experimental verification of our findings and implications will
highlight the importance of clarifying the characteristics and dynamics
of aqueous eDNA, and will contribute substantially to the development of
eDNA analysis in the future.