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Diatoms are among the most commonly used bioindicators. Correct taxonomic identifications are critical to their use as bioindicators because closely related diatom species can respond differently to water physicochemical characteristics and pollutants. However, diatom identification based on morphology can be time consuming, and requires highly specialized taxonomic skills. To optimize diatom identification, DNA metabarcoding is increasingly used because it is generally less time consuming and may be more accurate than morphological identification. To date, however, neither DNA metabarcoding nor DNA barcoding diatom studies have been conducted in Mexico. Thus, we studied epilithic diatoms from streams in Central Mexico with a combination of morphological and metabarcoding techniques, and compared the diatoms identified and quantified by each method. We also assembled a barcode reference library based on clonal culturing. This library is composed of 190 strains that belong to 72 species in 24 genera. The morphological analysis of environmental samples resulted in the identification of 204 infrageneric taxa in 42 genera, but clonal culturing from the same samples retrieved 12 additional infrageneric taxa and 1 additional genus, thereby revealing concealed diversity. The metabarcoding approach resulted in the identification of 266 infrageneric taxa that belonged to 35 genera. Together, these methods detected 49 genera. Of these genera, 14 were identified only by morphology, 29 were identified by both methods, and 6 were only identified by metabarcoding. Of the 266 taxa we retrieved with metabarcoding, we confidently assigned 94 infrageneric taxa because a direct morphological or barcode sequence correlation was possible. Thirty-four of these 94 taxa were only detected with the metabarcoding method. One-fourth (23) of the assignments were only possible because of the barcode reference library we developed during this study, because there were no existing barcode sequences that matched these barcodes in the International Nucleotide Sequence Database Collaboration databases. Large disparities existed between relative abundances based on valve counts and sequence reads of the most abundant taxa, even after we corrected for cell biovolume. Overall, we conclude that the combination of morphological and molecular methods increases the detection and identification of diatoms.

The Lerma-Chapala Basin, in Central Mexico, is geologically heterogeneous, climatically diverse and boasts high biodiversity, lying within two Biodiversity Hotspots, namely Mesoamerica and the Madrean Pine–Oak Woodlands. Epilithon and water samples were collected in the basin from 14 sampling sites three times each, two sampling campaigns during the rainy season and one in the dry season. A total of 274 infrageneric taxa in 48 genera were recorded. The taxonomic composition observed was dominated by taxa from the genera Nitzschia, Gomphonema, Pinnularia, Navicula, Sellaphora and Eunotia. About a third of the taxa found could not be identified to the species level. From those unidentified morphodemes, two are described as new species, namely Brachysira altepetlensis and Sellaphora queretana. Furthermore, Eolimna rhombica is transferred to Sellaphora. Canonical Correspondence Analysis (CCA) revealed that specific conductivity and pH were the main environmental factors driving the community composition observed. Three groups of samples were identified after the CCA: 1) characterized by acidic waters and low conductivity; 2) with circumneutral waters, low specific conductivity and high temperature and phosphorous concentrations; and 3) characterized by circumneutral waters, high conductivity and low nitrogen concentrations. The indicator value method (IndVal), based on the relative abundance and relative frequency of the most abundant taxa was calculated based on the groups observed in the CCA, identifying the characteristic taxa for each of the three groups.

Se presenta el registro de las diatomeas epilíticas de los ríos Dolores y Laja, localizados en la cuenca alta del río Laja, Guanajuato, México. Las muestras se recolectaron en las temporadas seca-fría (enero) y lluviosa (octubre) del 2011. Se identificaron 173 taxa infragenéricos, de los cuales 7 son nuevos registros para México: Fallacia monoculata, Hantzschia abundans, Navigiolum uruguayense, Neidium bisulcatum, Nitzschia solita, Planothidium incuriatum y Sellaphora bacilloides; cabe mencionar que Navigiolum es nuevo registro a nivel genérico. Además, 3 especies son nuevos registros para el estado de Guanajuato: Gomphonema laticollum, G. subclavatum y Gomphosphenia lingulatiformis. Se encontraron formas teratológicas en 6 especies con abundancias relativas que variaron de 0.8 a 8.3%, siendo estas formas más abundantes en los sitios con mayores concentraciones de nutrientes. Derechos Reservados © 2015 Universidad Nacional Autónoma de México, Instituto de Biología. Este es un artículo de acceso abierto distribuido bajo los términos de la Licencia Creative Commons CC BY-NC-ND 4.0.

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