Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species Author Costa, Lívia F. 0000-0001-7148-9964 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & livia. fcosta @ hotmail. com; https: // orcid. org / 0000 - 0001 - 7148 - 9964 livia.fcosta@hotmail.com Author Wetzel, Carlos E. 0000-0001-5330-0494 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation Department (ERIN), Observatory for Climate, Environment and Biodiversity (OCEB), 41 rue du Brill, 4422 Belvaux, Luxembourg. & carlos. wetzel @ list. lu; https: // orcid. org / 0000 - 0001 - 5330 - 0494 carlos.wetzel@list.lu Author Maquardt, Gisele C. 0000-0002-2721-1487 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & Universidade Guarulhos, Centro de Pós-Graduação, Pesquisa e Extensão CEPPE, Praça Tereza Cristina 88, 07023070 Guarulhos, SP, Brazil. & giselecmarquardt @ gmail. com; https: // orcid. org / 0000 - 0002 - 2721 - 1487 giselecmarquardt@gmail.com Author Zanon, Jaques E. 0000-0002-9619-4393 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & jaques. zanon @ gmail. com; https: // orcid. org / 0000 - 0002 - 9619 - 4393 jaques.zanon@gmail.com Author Ector, Luc 0000-0002-4573-9445 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation Department (ERIN), Observatory for Climate, Environment and Biodiversity (OCEB), 41 rue du Brill, 4422 Belvaux, Luxembourg. & https: // orcid. org / 0000 - 0002 - 4573 - 9445 Author Bicudo, Denise C. 0000-0002-4248-3261 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & denisecbicudo @ gmail. com; https: // orcid. org / 0000 - 0002 - 4248 - 3261 denisecbicudo@gmail.com text Phytotaxa 2022 2022-12-13 575 3 187 223 http://dx.doi.org/10.11646/phytotaxa.575.3.1 journal article 207914 10.11646/phytotaxa.575.3.1 9921851b-40b9-4dba-aaa7-e7c5975b77cf 1179-3163 7431242 Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector in Wetzel et al. (2019: 345) ( Figs 144–165 ) Morphometric features:— 7.6–12.3 µm long; 2.0–2.7 µm wide; 29–34 striae in 10 µm; striae mainly composed of 3–5 areolae. Taxonomical remarks:— Our population is in agreement with the type species, besides it has slightly narrower valves when compared to the French population (2.8–3.3 μm, Wetzel et al. 2019 ). This species has narrow valves with subcapitate to small capitate apices, similar to A. macrocephalum (Hustedt) Round & Bukhtiyarova (1996: 349) , although the latter taxon presents broadly capitate apices. Despite being difficult to separate, a reevaluation of A. macrocephalum type material was recently published in the same study as A. peetersianum ( Wetzel et al. 2019 ) . Using SEM micrographs, the authors showed that A. peetersianum presented predominantly rounded areolae and had a higher density per stria, while A. macrocephalum striae are composed of 1–2 elongated areolae of different lengths. Moreover, the axial area of the rapheless valve of the latter taxon is expanded, widening towards the central area. Recently, Yu et al. (2019) described two small species that resemble ours: A. subtilissimum P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 161 ) and A. limosua P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 158 ). The first species is likely a synonym of A. macrocephalum , and the second presents visibly larger valves (3.2–4.0 µm) than A. peetersianum . Achnanthidium indistinctum also bears several similarities with A. peetersianum . They can be distinguished in girdle view by the apparently thickened striae in the first taxon, which is not visible in our specimens (see Figs 158, 159 ). Concerning the valve view, A. indistinctum presents rostrate apices and an almost nonexistent central area that never forms a fascia. On the other hand, A. peetersianum exhibits subcapitate to capitate apices and a small rounded central area that rarely forms a fascia. Notably, SEM analysis can also help separate both taxa due to the distal raphe ends straight in A. indistinctum and bent in our specimens. Despite the slightly narrower valves in the latter taxon (1.8–2.2 µm, Van de Vijver & Kopalová 2014 ), the metric features of these two species overlap. FIGURES 144–165. LM and SEM images of Achnanthidium peetersianum from Rifaína city, São Paulo state (SP371175). 144–154. LM views of raphe valves. 155–157. LM views of rapheless valves. 158, 159. Girdle views of the taxon. 160–162. SEM external view of raphe valves showing striation pattern, axial and central areas and the terminal raphe fissures deflected. 163. SEM internal view of the raphe valve. 164, 165. SEM external view of the rapheless valves showing striation pattern, axial and central areas. LM scale bar = 10 μm. SEM scale bars indicated in each figure. Achnanthidium reimeri (Camburn) Ponader & Potapova (2007: 235) has a similar valve outline with our population, however, it presents longer and wider valves and a rhomboid central area on the rapheless valve ( Ponader & Potapova 2007 ). Distribution and ecological information:— Recently described from Le Sauvigny River in France , it was one of the most well-distributed species found in this study (40 % of occurrence). It is plausible that this taxon was previously identified in Brazilian publications as A. macrocephalum . In this study, A. peetersianum occurred in all of the sampled habitats. Its ecological preference was for slightly acidic (pH optimum of 6.6–6.7), electrolytic-poor (optimum of 28.2–49 μS∙ cm-1 ) and oligo- to mesotrophic waters (TP optimum of 14.9–27.1 μg∙L- 1 , TN optimum of 374.4–519.4 μg∙L- 1 ). We also observed this species in enriched waters (TP> 50 μg∙L- 1 ).