Mullus surmuletus environmental DNA intraspecific metabarcoding Next-Generation Sequencing data

Four 250-liter aquariums were bleached clean one day prior to be used (filled with seawater; fish transfer) in Montpellier (France). Seawater collected by the French Research Institute for Exploitation of the Sea at Palavas-les-Flots (France) was first stored in a 1,000 L tank for two weeks, under UV treatment to avoid any contamination. The aquariums were then filled with 120 L of this water. Each aquarium had a closed-circuit water circulation and was equipped with an air bubbles exhauster in a tube that brought up the water on a neutral synthetic foam filter. The aquariums were thus oxygenated and the coarsest suspended matter was filtered out. The remaining seawater in the tank was used as a negative control (Aquarium 1). Nine to eleven fish were added to each of the four aquariums (Fig. 1). The aquarium water was sampled six hours after introducing the fish into the aquariums using an Athena peristaltic pump (SPYGEN, Le Bourget-du-Lac, France) with a nominal flow of 1.0 L/min to filter 30 L, and VigiDNA 0.22 μm crossflow filtration capsules (SPYGEN) with disposable sterile tubing. After filtration, 80 mL of CL1 conservation buffer (SPYGEN) was added before storing the samples at ambient temperature.

We reanalyzed here two eDNA samples of 30 L replicate each, collected in  the Mediterranean Sea, at Banyuls (France, coordinates: 42.41568, 3.17110) and Calvi (France, coordinates: 42.62964, 8.89161) published in a previous metabarcoding analysis and known to contain M. surmuletus sequences (detected with the metabarcode teleo 12S) (Boulanger et al. 2021). These two Mediterranean eDNA samples were amplified and sequenced using the primers developed for this study and then analyzed using the best-performing pipeline as determined by our evaluation. These two samples were used as proof of concept of the possibility to estimate within site variability in real conditions.

DNA extraction and amplification from eDNA samples were performed by the company SPYGEN (Le Bourget du Lac, France) in separate, dedicated rooms following the protocol described by Polanco Fernández et al. (2020). The amplification was performed in a final volume of 25 μL including 1 U of AmpliTaq Gold DNA Polymerase (Applied Biosystems, Foster City, CA, USA), 10 mM of Tris-HCl, 50 mM of KCl, 2.5 mM of MgCl2, 0.2 mM of each dNTP, 0.2 μM of each primer, 0.2 μg/μL of bovine serum albumin (Roche Diagnostics, Basel, Switzerland) and 3 μL of DNA template. The PCR mixture was denatured at 95°C for 10 min, followed by 50 cycles of 30 s at 95°C, 30 s at 47°C and 1 min at 72°C and a final elongation step at 72°C for 7 min.  The primers were 5’-labelled with an eight-nucleotide tag unique to each DNA sample, allowing each sequence to be assigned to the corresponding sample during the sequence analysis. Twelve replicate PCRs were run per sample. Two libraries were prepared using the MetaFast protocol (Fasteris 2020, https://www.fasteris.com/dna/) and the sequencing was performed by Fasteris (Geneva, Switzerland) on two separate runs on an Illumina MiSeq (2x250 bp) (Illumina, San Diego, CA, USA) and the Miseq Kit v3 (Illumina) following the manufacturer’s instructions. Two negative extraction controls and one negative PCR control (12 replicates of ultrapure water) were amplified and sequenced to monitor for possible contaminants (Polanco Fernández et al., 2020).