Materials
We identify multiple materials that can be used effectively for passive
eDNA collection. Material choice can influence capture efficiency, and
ideally, the selected material will maximize eDNA capture without
interfering or complicating the extraction process. We investigated the
use of materials which varied in structural complexity and robustness,
and found no significant difference in capture efficiency, apart from
the reduced capacity of hemp and cotton fibres that were contained in
nylon bags. Kirtane et al. (2020) likewise found that the pore size used
to encase the adsorbent had a significant impact on DNA adsorption. They
suggest that restricted flow over the adsorbent was associated with
smaller pore sizes, and that increasing encasing pore size increases
capture. These results highlight the importance of membrane encasing and
suggest the maximum surface area of a material should come in direct
contact with the water.
There was also some indication that the addition of chitosan to a
material could increase capture efficiency, since both chitosan
treatments detected the highest median and maximum fish species
richness. Although this was not statistically significant, our
experiment was conducted in a low diversity, temperate mesocosm. Since
water characteristics, such as pH and temperature, can influence DNA
adsorption to different materials (Lorenz and Wackernagel 1987), it’s
possible the addition of chitosan could result in increased capture
efficiencies in some environments. For example, Bessey et al. (2021)
found that nylon membranes performed as well as conventional filtering
for fish species detection in low-diversity temperate waters but not in
high-diversity tropical waters, presenting a situation where the
addition of chitosan could potentially increase the effectiveness of
nylon materials. Further investigation into material optimization will
be particularly important for high diversity systems.
Practical considerations for material choice will play an important role
since our results reveal many materials are effective for passive eDNA
collection. Cost of material, availability, robustness, ease of
deployment and downstream processing, may all influence material choice.
For example, nanomaterials are more time-consuming and costly to produce
than readily available cellulose ester membranes and aquarium grade
sponges, which are commercially available (Liu 2012). Cellulose ester
membranes require less handling time during downstream processing than
granular materials, which require weighting, or sponge and nylon
materials that require cutting prior to DNA extraction (Bessey et al.
2021). However, in a turbulent, high flow water environment, a more
robust material, such as sponge, may be desirable over the more fragile
cellulose membranes. A challenge for employing passive eDNA collection
will be finding a standard that can be consistently used so that time
series and spatial comparisons are meaningful within and between
studies. This challenge similarly exists for conventional eDNA studies
(Trujillo-Gonzalez et al. 2021).