Abstract
In the context of the sixth wave of extinction, reliable surveys of biodiversity are increasingly needed to infer the cause and consequences of species and community declines, identify early warning indicators of tipping points, and provide reliable impact assessments before engaging in activities with potential environmental hazards. DNA metabarcoding has emerged as having potential to provide speedy assessment of community structure from environmental samples. Here we tested the reliability of metabarcoding by comparing morphological and molecular inventories of invertebrate communities associated with seagrasses through estimates of alpha and beta diversity, as well as the identification of the most abundant taxa. Sediment samples were collected from six Zostera marina seagrass meadows across Brittany, France. Metabarcoding surveys were performed using both mitochondrial (Cytochrome Oxidase I) and nuclear (small subunit 18S ribosomal RNA) markers, and compared to morphological inventories compiled by a long-term benthic monitoring network. A sampling strategy was defined to enhance performance and accuracy of results by preventing the dominance of larger animals, boosting statistical support through replicates, and using two genes to compensate for taxonomic biases. Molecular barcodes proved powerful by revealing a remarkable level of diversity that vastly exceeded the morphological survey, while both surveys identified congruent differentiation of the meadows. However, despite the addition of individual barcodes of common species into taxonomic reference databases, the retrieval of only 36% of these species suggest that the remaining were either not present in the molecular samples or not detected by the molecular screening. This finding exemplifies the necessity of comprehensive and well-curated taxonomic reference libraries and multi-gene surveys. Overall, results offer methodological guidelines and support for metabarcoding as a powerful and repeatable method of characterizing communities, while also presenting suggestions for improvement, including implementation of pilot studies prior to performing full “blind” metabarcoding assessments to optimize sampling and amplification protocols.
Highlights
The sixth wave of extinction has already begun, far in advance of the completion of comprehensive biodiversity inventories [1,2]
The list details the presence of Cytochrome Oxidase I (COI) and 18S barcode sequences matching the genera and/or species available in the public sequence database GenBank (NCBI), as well as those families, genera or species that match to MOTU sequences in the environmental metadatasets
From the COI environmental metadataset, seven MOTUs blasted up to the species level (14% of all common species and 50% of the barcodes added to the COI reference database), and five to the family level
Summary
The sixth wave of extinction has already begun, far in advance of the completion of comprehensive biodiversity inventories [1,2]. Awareness of this situation has led to the establishment of transnational conservation programs whose effectiveness rely upon the ability to thoroughly assess biodiversity and provide indicators of ecosystem health within a time frame that counteracts the initial delayed response [3]. The construction of biological inventories has traditionally and primarily relied upon morphological identifications of taxonomic groups, morphological discrimination of a given community is a time consuming task that requires meticulous taxonomic expertise that is becoming more rare [4,5]. DNA barcoding has become especially useful for analyses of environmental collections (water, soil, mud, feces, etc. . .), for which ‘environmental metagenetics’ is implemented when community sorting and morphological descriptions are challenging due to the large number and small sizes of possible taxon, as well as to the state of conserved specimens in samples [11,12,13,14,15,16]
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