At Jonah, we typically provide our aquatic eDNA clients with 1 µm nylon filters.
But what a “1 µm filter” actually means may not be obvious at first.
Most filtration membranes are not like colanders with identical circular pores punched through a sheet.
They are more like dense, fibrous fabrics with tortuous flow paths:
• Pores vary in size
• Channels twist and narrow
• Particles encounter multiple constrictions before exiting
This matters immensely for environmental DNA.
A 1-µm rating for a filter does not mean that everything smaller than 1 µm passes through.
A membrane rating (like 1 µm or 0.45 µm, ) typically refers to the largest pore size in the membrane — meaning particles larger than that size should not pass.
It does not imply perfectly uniform, straight-through holes.
No manufacturer actually measures every pore. Instead, ratings are typically assigned using a bubble point test — the pressure required to force air through the largest wetted pore in the membrane. That pressure is inversely related to pore diameter.
A 1 µm membrane just behaves like a membrane that has a single pore that is 1 µm in diameter. It says nothing about all the other pores.
And remember the scale we’re talking about: a typical 47 mm diameter 1 µm nylon membrane contains on the order of ~1 billion pores. These are not a handful of precision-drilled holes — they are a vast, heterogeneous network.
When we filter water, DNA-containing particles aren’t just “sieved” by size. They can be retained by:
• Size exclusion
• Adsorption to membrane material
• Electrostatic interactions
• Hydrophobic interactions
• Capture within complex pore pathways
In other words: filtration is both a physical and chemical process.
And here’s something even more interesting.
As a filter accumulates material:
• Flow rate declines
• The effective largest pore size decreases
• Smaller particles are increasingly likely to be trapped
• The membrane becomes functionally “tighter”
The filter you started with is not the filter you end with.
This has real biological consequences.
For example, when sequencing phytoplankton communities, the first volume of water filtered often captures larger green algae and diatoms. As more water passes through and the membrane loads, the effective pore space narrows.
Relative abundance of DNA from smaller organisms — like picocyanobacteria — can increase because the filter begins functioning like a smaller-diameter membrane.
In other words, filtration is dynamic — and community composition can shift as a function of volume filtered.
We’ll expand on what to consider when choosing filters later, but to start, it’s important to remember:
Filters aren’t simple sieves.