Conclusions and perspectives
On the basis of the findings in the present study, we summarized the
performance of eDNA-based abundance estimation depending on eDNA
particle size and size fraction (Figure 4). As shown in this figure, the
relationship between eDNA concentration and species abundance could
complicatedly rely on the cellular structure of eDNA; the cellular
structure of eDNA (e.g., intra-/extra-cellular, particle size,
dissolved/adsorbed) might multifacetedly impact the processes of its
degradation and persistence in water and the heterogeneity of spatial
dispersion and distribution, eventually influencing the correlation
between the steady-state eDNA concentration and species abundance. What
is especially important in this study is that better understanding and
utilizing the characteristics and dynamics of eDNA can promote the
refinement of its application for biomonitoring. We here proposed a
hypothesis that the accuracy and sensitivity of eDNA-based abundance
estimation could be improved by targeting “appropriately” larger eDNA
particles (indicated as 3-10 µm in diameter).
Nonetheless, given the substantial lack of knowledge on its production
source and persistence state in water, it remains unknown in what
physiological origin and physiochemical structure eDNA exists in each
size fraction. To explain the mechanism underlying the findings in this
study, future empirical research on the production source and
persistence state of eDNA is required. For instance, detecting
tissue-specific messenger RNA in water could identify the production
source of eDNA (Tsuri et al., 2021). Alternatively, visualization of
eDNA particles via fluorescence in situ hybridization (FISH)
techniques and immunostaining (e.g., Kogure et al., 2006; Amann &
Fuchs, 2008) may directly offer information on its cellular and the
molecular structure. Although different particle sizes of eDNA can
result in other transport and diffusion dynamics, the effect of the eDNA
state (e.g., particle size, weight) on its transport and diffusion
dynamics has not been elucidated (Shogren et al., 2016; Jo et al.,
2021a). The additional information would explain the heterogeneous
distribution of eDNA in water, associating its multiple states, and help
support our hypothesis above. Clumped eDNA particles, such as aggregates
of cells and tissues, might show more limited diffusion but more rapid
settling.
Furthermore, toward a practical application of our findings, evaluating
whether the use of appropriately larger eDNA particles can be adequate
for improved abundance estimation in the field is required. Various
environmental parameters can complicatedly influence the production,
transport, and degradation processes of different eDNA particles
(Harrison et al., 2019; Jo & Minamoto, 2021; Yates et al., 2021),
consequently driving the relationship between eDNA concentration and
species abundance. It may also cause the potential gap between eDNA
yields and its particle size distribution between controlled and natural
environments, possibly complicating the improvement of abundance
estimation accuracy via eDNA sampling focused on appropriately larger
eDNA particles in the field. Therefore, even if our findings were proper
in principle, the significance of eDNA sampling considering its state
for improved species abundance estimation should be further assessed in
mesocosm experiments and natural environments. Additionally, the eDNA
particle size may recount a spatiotemporal range of biological
information inferred by eDNA. For instance, if a larger eDNA particle
shows the presence of target species near a given eDNA sampling site, it
may be suitable for the accurate estimation of instantaneous and local
species abundance. Conversely, if a research interest is a comprehensive
abundance estimation at the ecosystem level (i.e., possibly on
timescales of weeks to months), the use of a smaller pore size filter
and multiple particle sizes of eDNA may be acceptable (Jo et al., 2021;
Yates et al., 2021). Addressing these issues and optimizing the
methodology of eDNA analysis will ensure the reliable and sensitive
monitoring of species distribution and abundance, contributing to the
efficient management of biodiversity conservation and fisheries
resources.