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.