- Environmental DNA (eDNA) sampling is the first and one of the most important steps in an eDNA study. Environmental DNA refers to genetic material released by organisms into their surroundings through cells, tissues, mucus, saliva, feces, urine, gametes, hair, scales, and other biological materials. By collecting environmental samples and subsequently analyzing their DNA, researchers can obtain evidence of organisms without necessarily capturing or directly observing them.
- The quality of an eDNA study depends strongly on the quality of the sampling design. Even highly sensitive molecular techniques cannot compensate for poorly collected, contaminated, insufficient, or unrepresentative samples. Sampling therefore needs to consider the target organism, habitat, environmental conditions, expected DNA concentration, spatial distribution, temporal variation, and the objectives of the study.
- Water sampling is one of the most widely used eDNA sampling approaches. It is particularly important in freshwater, marine, and other aquatic ecosystems because organisms continuously release DNA into the surrounding water. Researchers may collect water from rivers, lakes, ponds, streams, wetlands, reservoirs, estuaries, coastal areas, and oceans. The collected water is commonly filtered to concentrate DNA-containing particles before DNA extraction and molecular analysis.
- The location where water is collected can strongly influence eDNA detection. Samples may be collected near the surface, at different depths, close to vegetation, near suspected habitats, upstream or downstream of particular locations, or at multiple points across a water body. Water movement can transport DNA away from its original source, so eDNA transport and hydrological conditions need to be considered when interpreting results.
- Soil eDNA sampling provides an important approach for studying terrestrial organisms and below-ground biodiversity. Soil can contain DNA from plants, animals, fungi, bacteria, and other organisms. Samples may be collected from forests, grasslands, agricultural land, wetlands, deserts, caves, or other terrestrial environments. Soil sampling can provide information about both organisms currently interacting with an environment and DNA remaining from biological material that has accumulated over time.
- Sediment sampling is particularly useful in aquatic environments. DNA can become associated with particles and settle on the bottom of rivers, lakes, ponds, oceans, and wetlands. Sediments can therefore act as reservoirs of environmental genetic material. However, sediment may contain DNA from different time periods, making interpretation more complex than sampling from the water column. Sediment eDNA can be useful for investigating present biodiversity as well as longer-term ecological changes when combined with appropriate chronological and molecular approaches.
- Airborne eDNA is an emerging sampling approach in which genetic material is collected from particles suspended in the atmosphere. Organisms can release DNA-containing material into the air through pollen, spores, skin cells, feathers, hair, tissue fragments, and other biological particles. Air sampling has created new possibilities for terrestrial biodiversity monitoring, particularly in forests and other ecosystems where conventional surveys can be difficult.
- Researchers may also collect eDNA from biofilms, plant surfaces, animal-associated material, and other environmental substrates. Biofilms can contain DNA from microorganisms and larger organisms interacting with a surface, while plant surfaces can accumulate DNA from insects, microorganisms, animals, and surrounding biological material. The appropriate sampling material depends on the organism and ecological question being investigated.
- An important part of eDNA sampling is sampling design. Researchers must determine how many samples should be collected, where they should be collected, how far apart they should be, and when sampling should occur. A small number of poorly distributed samples may fail to represent the biological diversity of an ecosystem. Replicated sampling at multiple locations can increase the probability of detecting target organisms and can provide information about spatial variation.
- Spatial replication involves collecting multiple samples from different locations within the study area. This can be particularly important when organisms or their DNA are unevenly distributed. For example, a species may be abundant in one section of a lake but absent from another. Sampling multiple locations helps distinguish genuine spatial patterns from random sampling variation.
- Temporal replication involves collecting samples at different times. Environmental DNA concentrations can change according to season, temperature, reproductive activity, organism abundance, rainfall, water flow, and other environmental factors. Repeated sampling can therefore provide a more reliable picture of biodiversity than a single sampling event.
- The volume of environmental sample collected is another important consideration. For water eDNA studies, researchers may collect relatively small or large volumes depending on the expected DNA concentration, environmental conditions, filtration method, and target organism. A larger volume can potentially increase DNA recovery when target DNA is scarce, although excessive particulate material can clog filters and introduce inhibitors.
- Filtration is commonly used to concentrate eDNA from water. During filtration, environmental water passes through a membrane or other filter that retains particles containing DNA. The filter can then be preserved and transported for DNA extraction. Filter pore size, membrane composition, filtration volume, and filtration technique can influence DNA recovery and therefore need to be selected carefully.
- Different eDNA filters can have different characteristics. Smaller pore sizes may capture smaller DNA-containing particles but can clog more rapidly, particularly in turbid water. Larger pore sizes may permit faster filtration but may have different DNA-retention characteristics. Researchers therefore select filtration conditions according to the environmental matrix and research objectives.
- Sample preservation is essential because DNA can degrade after environmental material has been collected. Preservation methods may involve cooling, freezing, chemical preservatives, specialized storage solutions, or rapid processing. The appropriate approach depends on the sample type and laboratory workflow. Maintaining consistent preservation conditions between samples helps reduce variation introduced after collection.
- Field contamination is one of the major challenges in eDNA research. Because eDNA methods can detect extremely small quantities of DNA, DNA from researchers, equipment, clothing, boats, containers, previous sampling locations, or other sources can potentially contaminate samples. Field teams therefore use carefully controlled procedures to reduce the possibility of contamination.
- Field blanks or negative controls can be used to monitor contamination introduced during sampling. These controls can contain molecular-grade water or another appropriate control material that is exposed to the sampling environment and equipment without intentionally collecting environmental material. If DNA from a target organism appears in a field blank, researchers can investigate whether contamination may have affected the environmental samples.
- Equipment decontamination is another important component of eDNA sampling. Sampling bottles, filtration equipment, tubing, forceps, gloves, and other equipment should be cleaned or replaced according to an appropriate contamination-control protocol. Disposable equipment can reduce cross-contamination, while reusable equipment requires validated cleaning procedures.
- Researchers should also consider sample order and field workflow. Sampling locations can be visited according to a predetermined plan, and equipment should be handled consistently. In some studies, locations with low expected DNA concentrations may be sampled before locations with high concentrations to reduce the risk of carrying DNA between sites.
- Negative controls are not the only controls required in an eDNA study. Positive controls can help determine whether molecular procedures are capable of detecting the target DNA, while extraction controls can help identify contamination introduced during laboratory processing. A combination of field, extraction, and molecular controls provides stronger quality assurance.
- Environmental conditions can substantially affect eDNA sampling. Temperature, ultraviolet radiation, pH, salinity, turbidity, microbial activity, water flow, dissolved organic matter, and sediment characteristics can influence DNA persistence and recovery. Recording relevant environmental parameters alongside genetic samples can therefore improve interpretation of eDNA results.
- Water flow and hydrodynamics are particularly important in aquatic eDNA studies. DNA released by an organism can be transported downstream or dispersed through a water body. Consequently, detection of eDNA at a particular location does not always mean that the organism itself was physically present at exactly that location. Understanding water movement helps researchers interpret the spatial meaning of eDNA detections.
- Seasonal variation can also affect sampling results. Many organisms reproduce during particular seasons, potentially releasing increased quantities of DNA through eggs, sperm, larvae, or other biological material. Population abundance and activity can also change throughout the year. Consequently, the optimal sampling period depends on the target species and research objective.
- For species-specific studies, researchers may develop sampling strategies designed around the ecology of the target organism. A rare amphibian, for example, may require a different sampling strategy from a widespread fish or terrestrial plant. Knowledge of species ecology can therefore be as important as molecular expertise when designing an eDNA survey.
- For broad biodiversity studies, eDNA metabarcoding can be used after sampling to examine multiple taxa simultaneously. In this type of study, researchers generally collect environmental samples using standardized procedures and later analyze DNA from entire communities. Sampling design should therefore account for the taxonomic groups of interest and the environmental characteristics that influence their DNA distribution.
- A major advantage of eDNA sampling is that it is often non-invasive. Researchers can obtain genetic information without necessarily capturing, handling, or disturbing organisms. This can be particularly valuable for endangered, protected, elusive, or difficult-to-capture species. However, eDNA sampling itself still needs to be conducted responsibly to avoid environmental disturbance.
- eDNA sampling can also be more efficient than some traditional biodiversity survey methods. A water sample may contain DNA from numerous organisms, potentially allowing researchers to investigate multiple taxa from the same environmental material. Nevertheless, eDNA is best considered a complementary method rather than a universal replacement for traditional ecological surveys.
- Sampling standardization is important when eDNA data are compared between locations, seasons, laboratories, or studies. Differences in water volume, filtration, preservation, equipment, sampling locations, and processing procedures can produce differences in detection that are unrelated to actual biological differences. Standardized protocols therefore improve reproducibility and comparability.
- Replication provides another way to improve confidence in eDNA results. Multiple independent samples from the same location can help distinguish consistent biological signals from stochastic detection. Replication is particularly important when target DNA concentrations are expected to be low.
- The chain of custody and sample tracking should also be carefully managed in large eDNA studies. Each sample should have a unique identifier and associated information such as collection location, date, time, environmental conditions, sample type, volume, preservation method, and other relevant metadata. Accurate metadata allow researchers to connect molecular results with their environmental context.
- Modern eDNA research increasingly combines field sampling with automated environmental sampling. Autonomous or semi-autonomous systems can collect environmental material repeatedly over extended periods. Such systems may increase sampling frequency and reduce the dependence on individual field visits, potentially providing higher-resolution information about temporal changes in biodiversity.
- Portable molecular technologies are also changing the potential workflow of eDNA studies. Advances in portable sequencing and field-based molecular analysis may allow some stages of genetic analysis to occur closer to the sampling location. Although laboratory-based workflows remain important, field-deployable technologies could make rapid biodiversity assessment increasingly practical.
- The interpretation of an eDNA detection must always consider the possibility of false positives and false negatives. A false positive may result from contamination, DNA transport, or non-specific molecular detection. A false negative may occur when DNA is present but below the detection threshold or when sampling and molecular procedures fail to recover it. Good sampling design, replication, controls, and appropriate molecular methods can reduce these problems.
- An important distinction is that eDNA detection generally provides evidence that genetic material associated with an organism is present or has recently been present in an environment. It does not necessarily prove the organism is currently alive at the exact sampling location. DNA can persist after an organism has left or died, and environmental transport can move DNA between locations. This distinction is fundamental to responsible interpretation of eDNA results.
- Overall, eDNA sampling provides the foundation for environmental DNA research. The process involves selecting appropriate environmental materials, designing representative sampling strategies, collecting samples under controlled conditions, preventing contamination, preserving genetic material, recording environmental metadata, and preparing samples for downstream molecular analysis. The reliability of later DNA extraction, PCR, qPCR, DNA metabarcoding, sequencing, and bioinformatics depends substantially on how carefully the original samples were collected.
- As eDNA technology continues to develop, sampling approaches are becoming more diverse and sophisticated. Water, soil, sediment, air, biofilms, and other environmental substrates can provide valuable genetic information about biological communities. Improvements in automated sampling, preservation, portable molecular technologies, and standardized protocols are expected to further increase the usefulness of eDNA for biodiversity assessment, species detection, invasive species monitoring, conservation biology, ecosystem monitoring, and environmental genomics.
