Cyclotella alvarniensis (Wuthrich) Straub & Aboal is considered a fossil species of the centric order with a very characteristic triangular shape with rounded poles and an irregular central space (Fig. 1). The first record of C. alvarniensis was from samples collected in the “Tourbière du Cachot” (Switzerland), where several fossil species were found (Wuthrich 1979Wuthrich M. 1979. Note sur une forme tripolaire de Fragilaria leptostauron (Ehr.) Hust. Bulletin de la Société Neuchâteloise des Sciences Naturelles 102: 69-71.). It was initially named as Fragilaria leptostauron var. alvarniensis Wuthrich, a former synonym of C. alvarniensis. To verify the origin of this diatom, they placed aerial traps on the peat bog and concluded that C. alvarniensis frustules, along with other fossil species, came from nearby dumpsites. According to their hypothesis, some residues with diatomite, such as electrical appliances, were responsible for releasing fossil diatoms into the atmosphere. For this reason, the appearance of C. alvarniensis frustules in other parts of Europe has been attributed to air transport in its fossil form. For example, C. alvarniensis in the French Pyrenees was attributed to air transport from areas of southern Spain (Bertrand & al. 2022Bertrand J., Coste C. & Garrigue J. 2022. Première contribution à l’étude des diatomées de la réserve naturelle nationale de la forêt de la Massane (Pyrénées-Orientales). Carnets natures 9: 45-66.). This is because large numbers of this triangular fossil species have been found in many Miocene diatomite spots from Murcia and Elche de la Sierra (Straub & Aboal 1992Straub F. & Aboal M. 1992. Note sur une forme tripolaire de Cyclotella (Bacillariophyceae) du sud-est de l’Espagne et révision du taxon Alvarniensis Wuthrich 1979. Bulletin de la Société neuchâteloise des sciences Naturelles 115: 23-29.; Foucault & al. 1987Foucault A., Calvo J.P., Elízaga E., Rouchy J.M. & Servant-Vildary S. 1987. Place des depôts lacustres âge Miocène supérieur de la region d’Hellín (province de Albacete, Espagne) dans l’évolution géodynamique des Cordillères Bétiques. Comptes Rendus de l’Academie des Sciences Paris 305: 1163-1166.; Servant-Vildary 1986Servant-Vildary S. 1986. Fossil Cyclotella species from Miocene deposit of Spain. In Ricard M. (ed.) Proceedings of the Eighth International Diatom Symposium: 495-511. Koeltz, Koenigstein. ). However, the causes of its aerial dispersal and its geographical distribution are not clear (Bertrand & al. 2022Bertrand J., Coste C. & Garrigue J. 2022. Première contribution à l’étude des diatomées de la réserve naturelle nationale de la forêt de la Massane (Pyrénées-Orientales). Carnets natures 9: 45-66.). Frustules of C. alvarniensis also appeared in a recent study of subfossil microflora in Thiaroye, Senegal (Sow 2004Sow E.H. 2004. Les diatomées subfossiles de la mare de Thiaroye (Dakar-Sénégal): inventaire floristique et paléoenvironnement. Vie et Milieu/Life & Environment 171-180.). This is considered the first record of this species in Africa. Although its presence could be explained by wind transport and/or birds, it raised doubts about its geological origin. As the authors comment, even accepting the detrital origin of the species, its dating requires further investigation since the Miocene outcrops in the study area are mainly composed of volcanic rocks.

Our samples were collected in the Torrent de la Font del Llor (Sant Llorenç del Munt i l’Obac Natural Park, Barcelona, Catalonia) (Fig. 2). The first frustule recorded was from biofilm growing in running waters in 2019 spring (Fig.1l). Lately from early September to early October of 2021 eleven frustules of Cyclotella alvarniensis were identified. In this period, this temporary stream was constituted by dry riverbeds alternated with disconnected pools. Cyclotella alvarniensis was found in three different type of samples: one frustule was found in biofilm samples from natural disconnected pools (Fig. 1e), another one in rehydrated sediment samples from the dry riverbed (Fig. 1b), and nine more from four artificial pools (mesocosms) placed on the dry riverbed sections and filled with bottled water (from an experiment which aimed to study organisms dispersion and colonization in temporary rivers) (Figs. 1a, c, f-k). No living individuals were found, nevertheless low abundant species are highly difficult to find in raw samples.

In relation to the measurements of our specimens (Fig. 1), the number of striae was 10-16 for 10 µm and a base length of 6-16 µm (Fig. 3). These values were slightly different from those initially established for the species, with 10-12 striations for 10 µm and a base length of 10-18 µm (Wuthrich 1979Wuthrich M. 1979. Note sur une forme tripolaire de Fragilaria leptostauron (Ehr.) Hust. Bulletin de la Société Neuchâteloise des Sciences Naturelles 102: 69-71.). However, Estrada (1972)Estrada M. 1972. Distribucions de mesures en una diatomea fòssil. Treballs de la Societat Catalana de Biologia, 32, 137-145. also recorded a wider range of measurements in diatomite samples from Murcia (Spain), suggesting that more studies are needed to better characterize this species.

The appearance of Cyclotella alvarniensis in our samples raises questions about whether it is an exclusive fossil species. Firstly, frustules of this species were found in four different types of substrates sampled (running waters biofilm, natural disconnected pools, rehydrated dry sediments and artificial pools). Secondly, they were found to be part of the current communities of freshwater diatoms. Thirdly, no frustules of C. alvarniensis or any other fossil species have appeared in any other tributary or river in the Natural Park, formerly intensively sampled. Finally, although we cannot rule out the wind as a passive dispersal mechanism, this would be a process difficult to explain. Besides there are not known nearby dumpsites that could have released diatomite into the atmosphere, the closest diatomite deposit in Catalonia is located more than 120 kilometers away (in Santa Eugènia de Nerella, la Cerdanya). If this were its origin, it would be highly likely that the wind would have spread C. alvarniensis also in other basins closer to the area of exploitation. On the other hand, the great Murcia deposit -more than 600 kilometers away- has been proposed as an important area for the emission of aerial fossil individuals (Aboal 1989Aboal M. 1989. Aportaciones al conocimiento de las algas del SE de España. IV. Las diatomeas (Bacillariophyceae). Acta Botánica Malacitana 14: 13-40.). By the same token, no frustules of this diatom have appeared between Catalonia and Murcia despite this area have been intensely sampled for diatoms (Cambra & Ortiz-Lerín 2005Cambra J. & Ortiz-Lerín R. 2005. Red de intercalibración, red de referencia y red básica de diatomeas en la Cuenca del Ebro. Informe Verano 2005. Ministerio de Medio Ambiente, Confederación hidrográfica del Ebro.; Flor-Arnau & al. 2008Flor-Arnau N., Vegas-Lozano E., Galán Cubero A. & Cambra Sánchez J. 2008. Red de referencia y red básica de diatomeas en la Cuenca del Ebro. Informe verano 2005. Ministerio de Medio Ambiente, Confederación hidrográfica del Ebro.; Durán & Ormad 2012Durán C. & Ormad Melero M.P. 2012. Investigación del estado de las masas de agua superficiales de la cuenca del Ebro utilizando indicadores biológicos y físico químicos: primera aproximación a la aplicación de la Directiva Marco del Agua. Doctoral dissertation, Universidad de Zaragoza, Zaragoza.; Alvarez & Rojo García-Morato 2020Alvarez M. & Rojo García-Morato C. 2020. Data set of benthic diatom taxa identified in Serranía de Cuenca (central Spain), Summer Report 2017. Website: https://roderic.uv.es/handle/10550/75666 [accessed: 12 Nov. 2022].). Precisely, the only record of C. alvarniensis until 2003 in the entire Iberian Peninsula was in Murcia (Aboal & al. 2003Aboal M., Alvarez-Cobelas M., Cambra J. & Ector L. 2003. Floristic list of non marine diatoms (Bacillariophyceae) of Iberian Peninsula, Balearic Islands, and Canary Islands. Updated taxonomy and bibliography. Diatom Monographs 4: 1-639.). No more recent bibliographic citations of this species are found in Catalonia or the rest of the Iberian Peninsula.

For all the exposed reasons, in our opinion there are modern and successful populations of Cyclotella alvarniensis thriving both in Catalonia and in other regions of Europe. Since its discovery, the few records of C. alvarniensis have automatically assumed its fossil form, not contemplating other modern forms. As diatoms samples are usually treated to their taxonomical identification, living samples are rarely observed, thus losing the opportunity to verify the current presence of living individuals. We encourage researchers to look at their untreated samples when recording C. alvarniensis in their samples.