Diatoms from small ponds and terrestrial habitats in Deserta Grande Island (Madeira Archipelago)

Abstract Background Freshwater diversity, and diatoms in particular, from Desertas Islands (Madeira Archipelago, Portugal) is poorly known, although the Islands are protected and became a Natural Reserve in 1995. During two field expeditions in 2013 and 2014 to Deserta Grande Island, several freshwater and terrestrial habitats were sampled. The analysis of these samples aims to contribute to the biodiversity assessment of the freshwater biota present in Deserta Grande Island. Here, we present the diatom diversity in Deserta Grande Island resulting from that survey. This study contributes to improve the knowledge of Madeira Archipelago freshwater diversity, particularly in the Desertas sub-archipelago. New information To our knowledge, we present the first diatom data for the Desertas sub-archipelago. This work resulted in a list of 60 diatom taxa for Deserta Grande, from which 57 were identified to species level. From the 60 new records for Desertas sub-archipelago, 30 of them were also new records for Madeira Archipelago. Several specimens could not be assigned to a known species and may be new diatom species not yet described.


Introduction
are an ubiquitous, highly successful and distinctive group of eukaryotic microalgae, essentially unicellular, which are present in almost every aquatic environment (Round et al. 1990). They are major constituents of benthic and planktic algal communities worldwide in terrestrial, freshwater and marine habitats (Mann and Droop 1996). In addition, the ecological specificity of many diatoms species allows them to be used as environmental indicators (Smol and Stoermer 2010). Freshwater diatom communities have been studied from several continents and remote Oceanic Islands (e.g. Van de Vijver et al. 2002, Bouchard et al. 2004, Grenier et al. 2006, Delgado et al. 2012, Gonçalves et al. 2015, Falasco et al. 2016, Bak et al. 2017).
The Desertas sub-archipelago is formed by three remote small islands, belonging to the Madeira Archipelago. Due to their remoteness, small area, harsh environment and lack of freshwater sources, these Islands remained uninhabited to this day. There has always been interest in the geological setting, fauna and flora from Desertas (Lowe 1868, Lockley 1952, Bannerman 1965, Matias 1984, Neves et al. 1992) and, in 1995, these Islands started to be protected under the Natural Reserve of the Desertas. The entire south terrestrial and marine areas are classified as an Integral Reserve and the north marine area as a Partial Reserve. Due to the high conservation value of these Islands (Neves et al. 1992), fauna and flora have been a matter of study in the last years (e.g. Voigt and Leitner 1998, Nunes 2000, Pires and Neves 2001, Crespo et al. 2013, Crespo et al. 2014, Boieiro et al. 2018, Teixeira et al. 2019; however, to our knowledge, freshwater biota has never been studied. The knowledge of microbial diversity in remote areas with reduced human presence, as are Oceanic Islands, is fundamental for the study of biogeography patterns and meta-community structures amongst microrganisms. Although microorganisms have been considered cosmopolitan, based on the hypothesis that "everything is everywhere, but the environment selects" (Baas Becking 1934), several recent studies show that microorganisms, including diatoms, exhibit biogeographical and macroecological patterns (e.g. Foissner 2006, Martiny et al. 2006, Soininen 2007, Vanormelingen et al. 2008, Verleyen et al. 2009). Diatoms are particularly useful for the study of macroecology conceptual frameworks for microorganisms (Benito et al. 2018), but such studies rely on the existence of large species distribution datasets covering a broad geographical scale (Vanormelingen et al. 2008).
This study presents a taxonomical characterisation of the diatoms found in Deserta Grande freshwaters. We aim to contribute to the current knowledge of diatom diversity and distribution in the Macaronesian Archipelagos and to provide diatom distribution records for regional and global diatom meta-community analysis. Study area description: the Desertas sub-archipelago is formed by three uninhabited small islands belonging to the Madeira Archipelago, located 20 km southeast of Madeira Island (Fig. 1). With an age of 3.6 Ma (Schwarz et al. 2005), the Desertas Islands were connected by a land bridge to Madeira Island during the last glacial period (18,000 years BP) (Brehm et al. 2003). Today, the depth of the sea between Ponta de São Lourenço Peninsula (eastern tip of Madeira Island) and Ilhéu Chão is about 90 m (Geldmacher et al. 2001). Deserta Grande is the largest of the three Islands, with an area of approximately 10 km and a maximum altitude of 479 m. The Deserta Grande geomorphology is mostly rugged, with very steep slopes, ridges and peaks. The climate is temperate oceanic and the predominant habitats are rocky slopes and small arid flatlands, with sparse vegetation. Freshwater habitats are reduced to temporary streams in Vale da Castanheira and some very small rock pools scattered across the Island. Madeira Archipelago was included as one of the global biodiversity hotspots, together with Azores and the Canary Islands, due to their unique biodiversity (Médail andQuézel 1999, Myers et al. 2000).

Sampling methods
Study extent: Freshwater habitats ranging from water reservoir, natural pools and temporary streams were sampled during two field campaigns in 2013 and 2014 in Deserta Grande Island (Fig. 2). Five samples were collected at four sites ( Sampling description: Diatom samples were collected in 2013 and 2014 by filtering water or by brushing the bottom and walls of the pools or stream bed. With the help of a toothbrush to remove the biofilm, the sample was placed into a tray with a little water and Table 1. Samples code, date and location of the sampling sites in Deserta Grande Island. the resulting suspension was collected in a plastic container, fixed with alcohol and stored prior to analysis. Samples were treated with warm nitric acid and mounted with Naphrax©, according to European and national recommendations (Kelly et al. 1998, INAG 2008. Diatom slides were examined under differential interference contrast light microscopy using a ZEISS AXIOIMAGE A1 microscope with an immersion Plan-Apochromat 100x objective (NA 1.40). Step description: The data have been published as a Darwin Core Archive (DwC-A), which is a standardised format for sharing biodiversity data as a set of one or more data tables. The core data

Taxonomic coverage
Description: All diatoms were identified to genus or species level. A total of 60 taxa were found, from which 57 were identified to species and three to genus level. The species found belong to 22 families, 13 orders, five subclasses and three classes (  The name of the first or species epithet of the scientificName. infraspecificEpithet The name of the lowest or terminal infraspecific epithet of the scientificName, excluding any rank designation. taxonRank The taxonomic rank of the most specific name in the scientificName.

Municipality
Name of the municipality where the event occurred.
The diatom flora of Deserta Grande is mainly constituted by cosmopolitan species, but some taxa were impossible to assign to a known species and may belong to undescribed species. The possible existence of endemic species for the Island of Deserta Grande, in particular and the Madeira Archipelago, in general, would not be surprising considering the volcanic origin and remoteness of these Islands, which favours speciation (Whittaker et al. 2008). High levels of island and regional endemisms were found in other Oceanic Islands in the South Atlantic. For instance, aproximately 33% species found in the Falkland Islands were considered island or regional endemisms (Flower 2005), whereas Carter ( Azores (Delgado et al. in press). Thus, a more thorough survey and more detailed analysis of the fine structure of the frustule with a scanning electron microscope in the future is needed to fully describe the diversity and distribution of diatoms in Desertas Islands and this may result in the description of many new taxa.
Additionally, we thank the reviewers for their constructive and valuable comments that helped to improve this paper.

Author contributions
VG, HM, DT, PMR and CR conceived the study and PMR and DT carried out the sampling campaign in Deserta Grande. HM prepared the microscope slides and HM and VG identified the diatoms. CR and PMR wrote the paper with inputs from all authors. All authors agree with the final version of the paper.