Authors: Mallory Bedwell and Sarah Stinson
On any given day in the San Francisco Estuary (SFE) it’s a common sight to see scientists checking water quality, surveying the diverse species that live there, or conducting a myriad of other monitoring and management activities. The SFE is truly one of the most intensely studied ecosystems in the world. Recently, a new monitoring tool has gained traction among scientists as a promising way to complement traditional monitoring and research approaches. By collecting DNA from the environment and analyzing it with molecular techniques, scientists can detect any number of target species of interest. Have you heard of environmental DNA? If not, then consider this a brief introduction.
What is eDNA?
Environmental DNA (abbreviated eDNA) describes the genetic material that an organism sheds or excretes into its environment (e.g., skin cells, hair, mucus, blood, gametes, waste products, pollen, leaves, fungal spores). Once released, eDNA can be collected and extracted from environmental samples such as soil, sediment, water, snow or air. Once extracted, eDNA can be analyzed by several genetic methods. Depending on the method used, researchers can choose to target a single species (e.g., invasive or endangered), a particular community (e.g., fishes), or even multiple communities (e.g., all animals). For studies targeting multiple species, a common approach is to use a reference database to link the genetic sequence obtained in the eDNA sample to a particular species or taxonomic group. By scanning a reference sequence database for a match, scientists can identify which organism(s) the DNA in their sample came from. This is akin to scanning the barcode on any item in your local grocery store. When the item is scanned, information in the database links the barcode with the price of the item, etc. If the item isn’t listed, they won’t know how much to charge you.
Figure 1. eDNA is the genetic material of living things shed into the environment. We can collect from different substrates including soil, water, and even air.
How is it used?
In recent years, eDNA has been used in a wide array of applications.
Some closely related species can be difficult to differentiate in the field, and accurate identification often requires collecting tissue from the organism, followed by several days of processing in a molecular biology lab. A new technique called SHERLOCK (Specific High-sensitivity Enzymatic Reporter unLOCKing) is a sensitive, rapid method that can provide species identification (such as the difference between endangered Delta Smelt and the visually similar, non-native Wakasagi) without invasive sampling and can be done in the field in an hour or less. Pairing eDNA or other non-invasive DNA sampling with SHERLOCK allows managers to make rapid and accurate decisions, a critical necessity for protecting listed species.
To understand the migratory dynamics of salmonids in a creek just south of the SFE, researchers from the Monterey Bay Aquarium Research Institute (MBARI) deployed an autonomous robotic sampler (Video) to collect 750 eDNA samples over the course of one year. The creek provides habitat for endangered Coho Salmon and Steelhead Trout but is at risk from non-native species including Striped Bass. This sampler will not only filter eDNA samples but perform the molecular reactions and transmit the results, ultimately giving scientists the ability to monitor species of concern in near real time.
California encompasses many unique ecosystems considered to be biodiversity “hotspots,” or areas with high biodiversity. To protect these areas, California recently developed a “30 x 30” initiative to conserve 30% of California’s lands and coastal waters by 2030. Cataloging the biodiversity of an entire ecosystem, especially for the many diverse hotspots of California, is a tall order. To aid this effort, scientists are enlisting the help of community members and students to participate in “Bioblitz” events. Teams collect eDNA samples from diverse areas throughout the state, including the SFE. This is part of the CALeDNA program run by the University of California Conservation Genomics Consortium. By involving the community in monitoring efforts, scientists not only increase their ability to collect precious data, but the community gets to learn about these innovative new technologies and gain a deeper appreciation of their local ecosystems.
The previous three examples provide a snapshot of the different types of applications for using eDNA to monitor the diverse ecosystems of the SFE and beyond. Outside of the ecological applications of eDNA, there are additional important uses. For example, to aid efforts to mitigate and monitor COVID-19 dynamics, human health researchers have begun incorporating eDNA techniques to track the virus in wastewater. Far from being a comprehensive list of eDNA applications, these studies are just the tip of the iceberg.
How eDNA can help management?
Environmental DNA sampling can be an effective management tool in the SFE. Due to its sensitivity, eDNA sampling methods can be used to help find rare and hard to find species that are difficult to detect with traditional sampling methods, like trawls or seines. Further, it can be used to detect invasive species that may be present in low numbers or to track an invasion front. Anywhere that you can collect water, you can collect eDNA. This approach can be used to sample hard to access areas or where traditional surveys cannot be utilized. An additional benefit of eDNA sampling is that it provides a way to obtain information without the need to visually observe or handle organisms. Collection and handling can have negative effects, particularly for sensitive species. Using genetic identification through eDNA sampling can also help when species are challenging to identify visually. Different scales of analysis allow managers to ask different questions or carry out different management tasks, such as: habitat use of a rare species over space and time; initial site evaluation for a species of interest to see if further, more intensive sampling is needed; and to evaluate habitat restoration on a community wide scale. The utility and ease of sampling for eDNA make it a good compliment to traditional sampling methods and can even be more efficient in terms of time, labor, and expense. For more information about eDNA sampling in estuaries and how it can help with management needs, with a focus on those of the SFE, please see Nagarajan et al. (2022).
What researchers are currently doing with eDNA
Environmental DNA sampling is currently being instituted by different agencies and institutions to answer management questions. For example, the Washington Department of Fish and Wildlife has implemented eDNA detection for several projects to evaluate streams after wildfires for Rocky Mountain Tailed Frog, to identify whether redds belong to Coho Salmon or Steelhead, and early detection of invasive mussels and snails in water bodies. Here in the SFE, Ann Holmes, a graduate student in the Genomic Variation Lab (GVL) at UC Davis, studied eDNA detection patterns of Delta Smelt in cages during the first cage deployment experiments at Rio Vista and the Deep Water Ship Channel (manuscript in prep). The California Department of Water Resources (DWR) has several active studies currently underway focused on endangered Longfin Smelt and other listed species. The Longfin Smelt team at the California Department of Water Resources (DWR), in collaboration with Cramer Fish Sciences, has also been investigating whether eDNA sampling could be used to detect larval Longfin Smelt entrained at the Barker Slough Pumping Plant, filtering water that was collected from vegetation. DWR and Cramer Fish Sciences are also planning on utilizing eDNA sampling to monitor Chinook Salmon during droughts. A collaborative group lead by the GVL at UC Davis is working to build a database of reference sequences for fish and invertebrates in the SFE to be used for eDNA based studies. They are also comparing eDNA metabarcoding, or identifying a large group of taxa, with long term fish and invertebrate catch data around the SFE to evaluate the success of eDNA sampling as survey method in these monitoring locations.
Figure 2. eDNA filtration set up in the lab. Replicate water grab samples are filtered using a peristaltic pump. Sites with lower turbidity take only a few minutes to filter. Filters can be stored in ethanol, frozen, or dried before analysis.
Challenges of eDNA in estuaries
If eDNA detection can be a helpful tool in our management toolbox, what is hindering us from applying it further in the SFE? There are a few challenges when it comes to utilizing eDNA captured in an estuarine system. Tidally influenced systems make sampling eDNA more difficult; although unidirectional flows found in streams or rivers mean eDNA is transported in the same direction, the sloshing of the tides can make it hard to know how eDNA is being moved around. This means that sampling location (shore vs. boat) and sample spacing will likely vary by species and habitat. A better understanding of hydrodynamics in estuaries would help us to understand how best to capture and interpret eDNA results.
Additionally, the higher turbidities that are found in our estuary can cause filters that capture eDNA to quickly clog, decreasing the amount of water that is filtered and thus decreasing the probability of capturing the eDNA of our species of interest. Larger pore sized filters can be used to compensate for less volume being filtered at higher turbidities, but there is a risk of allowing smaller eDNA particles to slip through the larger pores. Turbidity also can inhibit reactions in the lab, leading to incorrectly assuming eDNA was not found in the samples (false negatives). An additional clean up step to remove inhibitors may be helpful.
Figure 3. eDNA is happy to help with your monitoring questions!
Considerations for eDNA applications
The field of eDNA is still relatively new and is most powerful when used to answer questions for which it is well-suited. There are many factors that can impact the distribution and detection of eDNA in the environment such as life stage, temperature, flow rate (in aquatic environments), and degradation by microbes. Scientists are still working to understand how these factors play into the ecology of eDNA, and how to optimize their methods to account for them. Another important consideration for any study involving eDNA is the availability of reference databases. The reference databases for various organisms are constantly expanding and improving, but many species are still missing. While eDNA has been used to determine species presence or absence for some time, there is still uncertainty about its ability to estimate biomass or abundance. As the field of eDNA research continues to expand, many of these limitations will likely be overcome, which will improve the utility of the tool.
The California Department of Water Resources (DWR) has initiated a new Genetic Monitoring (GeM) lab that will conduct genetic monitoring and molecular ecological studies using eDNA for water management decision-making within the SFE. As part of the State Water Project and Interagency Ecological Program (IEP), GeM research will prioritize the needs identified within the Incidental Take Permit, Biological Opinions, and water rights decisions for the State Water Project. The new lab will use innovative technology and collaborative partnerships to advance management decision-making critical to the State's water supply operation and planning.
Figure 4. Sampling for eDNA is non-invasive and can allow managers to monitor hard to access locations.
- Miya, M. 2022. Environmental DNA metabarcoding: a novel method for biodiversity monitoring of marine fish communities. Annual review of marine science, 14, 161-185.
- Nagarajan, R. P., Bedwell, M. E., Holmes, A. E., Sanches, T., Acuña, S., Baerwald, M., Barnes, M. A., Blankenship, S., Connon, R. E., Deiner, K., Gille, D., Goldberg, C. S., Hunter, M. E., Jerde, C. L., Luikart, G., Meyer, R. S., Watts, A., and Schreier, A. 2022. Environmental DNA Methods for Ecological Monitoring and Biodiversity Assessment in Estuaries. Estuaries and Coasts. In press.
- Meyer, R. S., Watts, A., and Schreier, A. 2022. Environmental DNA Methods for Ecological Monitoring and Biodiversity Assessment in Estuaries. Estuaries and Coasts. In press.