Study: Larger filter pores improve animal eDNA capture in seawater shotgun sequencing

Filter Size Matters: New Study Reveals Impact on Marine eDNA Capture

A new study published in Metabarcoding and Metagenomics has provided critical insights into how the choice of filter pore size can significantly influence the results of environmental DNA (eDNA) analysis in marine environments. The research, conducted in Skovshoved Harbour, Denmark, tested a range of filter sizes to determine their effect on capturing genetic material from different organisms.

"Filter pore sizes are expected to influence results, since eDNA may be present in many different states."
— Lead author Dr. Adrián Gómez-Repollés

The study found a stark contrast in what was captured depending on the filter used.

• Larger pores (5.0 µm and 8.0 µm) were far more effective at capturing eukaryotic reads. These filters retained 49% eukaryotic DNA, compared to only 31% bacterial DNA.
• Smaller pores (0.2 µm and 1.2 µm) showed the opposite effect, retaining a much higher proportion of bacterial reads (63% bacterial vs. 28% eukaryotic).

This distinction is crucial. Using larger filters increased the abundance of animal (metazoan) reads significantly. In fact, all but one of the 19 detected metazoan phyla were more abundant when larger pore sizes were used.

The study also compared two common sequencing approaches. Both shotgun sequencing and traditional 18S rDNA metabarcoding detected 39 out of 54 eukaryotic phyla, showing substantial overlap in their ability to identify broad taxonomic groups.

Despite the promising results, the researchers highlight several limitations that readers should consider.

• Database Limitations: A major challenge was that only 0.78% of total shotgun reads could be assigned at the superkingdom level. This is due to incomplete and sparsely populated reference databases for many marine organisms.
• Detection of Exotic Taxa: Some exotic taxa were detected, which the authors attribute to the low resolution of shotgun sequencing. Higher read counts for local, known taxa may help distinguish authentic findings from erroneous matches, but further testing is required.
• Sampling Constraints: The study was limited to a single location and included no field controls to assess potential cross-contamination or airborne DNA.

The authors are optimistic that these limitations will lessen over time.

"As global genomic databases expand, taxonomic coverage and resolution should improve, likely enhancing shotgun sequencing for future eDNA research."
— Senior author Philip Francis Thomsen

As genomic databases grow, the accuracy and utility of shotgun sequencing for monitoring marine biodiversity are expected to increase significantly, making it an even more powerful tool for conservation and ecological research.