During a survey of the Indonesian diatoms, five Luticola D.G.Mann taxa that could not be identified, based on the available literature were discovered. Based on light microscopy, scanning electron microscope observations and comparisons with similar taxa, all of them are described as new species. All taxa were found on mosses growing on tree trunks and concrete on the islands of Banda Besar and Seram and from spring on Java Island. Luticola insularis sp. nov. is most similar to L. aequatorialis and L. simplex, but it can easily be distinguished from both taxa, based on the lower striae density, the narrower valves and the well-developed silica ridges on the valve face/mantle junction. Luticola bandanensis sp. nov. resembles L. frequentissima, but they can be easily distinguished, based on their valve widths and the direction of the grooves located on the distal and proximal raphe endings. Luticola elliptica sp. nov. is most similar to L. sparsipunctata, L. tenuis and L. bryophila. Amongst all the species compared, L. elliptica sp. nov. is the only one with a highly asymmetrical central area, with the isolated pore located on the wider side. Luticola malukuana sp. nov. shares similarities with L. dismutica and L. areolata, but it has a notably higher stria density. From L. areolata, it can also be separated by the morphology of striae and the lack of ghost areolae in the central area. Luticola poliporea sp. nov. is unique in the whole genus due to the presence of multiple isolated pores.

Diatoms, morphology, Southeast Asia, taxonomy

The study of terrestrial diatoms in Southeast Asia began at the end of the 19^th century (Grunow 1865) and continues to this day. The result of this work is over 100 scientific publications focusing on diatoms in this area (Glushchenko et al. 2021). Despite a relatively long history of diatom research, terrestrial and aerophytic diatoms have received much less attention. The first and, for a long time, the only mention of this ecological group in Southeast Asia was the work of Kolkwitz and Krieger (1936), in which they mention the results from a single soil sample collected under trees, in which they found just a few diatom species. However, the situation has been changing in recent years, reflected in the descriptions of many new species from terrestrial environments of this region like soils or tufts of mosses (Kezlya et al. 2020a, b, 2022a, b; Rybak et al. 2020, 2022a, b, c; Glushchenko et al. 2022).

Many genera of diatoms are recorded in terrestrial and aerophytic environments and Luticola D.G. Mann (in Round et al. (1990: 148)) seems to be one of the most diverse (Chattová 2018; Bishop et al. 2021; Rybak et al. 2021a, 2023; Radhakrishnan et al. 2022; Chattová et al. 2022). However, representatives of this genus can also be found in other environments such as fresh-, brackish and marine water and even as epizoic on turtles (Wetzel et al. 2010; Levkov et al. 2013; Wu and Bergey 2017; Rybak et al. 2021b). The genus Luticola was distinguished from Navicula to accommodate species included in the Naviculae sect. Punctatae with Luticola mutica (Kützing) D.G.Mann (in Round et al. (1990: 532)) selected as the generitype. Features common to all species are the distinctly punctate, uniseriate striae composed of rounded to elongated areolae covered internally by perforated hymenes, an evident and morphologically unique isolated pore in the central area and a marginal longitudinal channel positioned internally between the valve face and the valve mantle (Round et al. 1990; Levkov et al. 2013). Since the publication of the Levkov’s et al. (2013) monograph on the genus, in which almost 200 species are treated, the genus has attracted the attention of many taxonomists from around the world. As a result of their work, the number of currently-known taxa has increased to 262 (Guiry and Guiry 2023), amongst which 23 species have been described from tropical Asia (Glushchenko et al. 2017; Liu et al. 2017; Lokhande et al. 2020; Rybak et al. 2021b; Yang et al. 2022) or transferred to the genus (Glushchenko and Kulikovskiy 2015; Kale et al. 2017).

In this paper, five new Luticola taxa are described from terrestrial and water mosses in Indonesia and separated from other similar taxa, based on their combinations of morphological features as documented with light and scanning electron microscopy.

The Maluku Islands (Spice Islands or Moluccas) are an archipelago in the northeast of Indonesia. The climate of the study area is almost entirely tropical and is dominated by a tropical rainforest climate with wet and dry seasons. Samples of terrestrial mosses from concrete and tree trunks were collected, placed in paper envelopes and left to dry. Moss samples from springs were collected with a spoon and placed in a plastic container. Three samples, in which unidentified Luticola taxa were observed, were selected for this study:

• 2018/440 – 4°31'29.93"S, 129°56'51.69"E, terrestrial orthotropic mosses collected from the base of a tree trunk on Banda Besar, Indonesia, in a forest near the shore at an elevation of 19 m a.s.l. The pH measured was 5.8 and conductivity was 1250 µS/l/cm.
• 2018/447 – 3°19'12.2"S, 128°56'6.94"E, terrestrial plagiotropic mosses collected from concrete in Amahai, Seram, Indonesia, at an elevation of 14 m a.s.l. The pH measured was 6.7 and conductivity was 680 µS/l/cm.
• 2023/81 – 7°50'26.2"S, 112°31'43.0"E, plagiotropic mosses from an unnamed spring collected in Malang, East Java, Indonesia.

The samples were used for preparation of diatom slides and filtrates for pH and conductivity measurements. The filtrates were obtained by soaking pieces of moss in deionised water (at a 1:10 weight ratio) for 24 h. pH and electrical conductivity were measured with a MARTINI pH56 pH meter and a MARTINI EC59 conductivity meter (Szeged, Hungary).

For diatom slides preparation, a small part of each moss sample was digested with a mixture of sulphuric acid and potassium dichromate. After dissolving all organic matter, the suspension was centrifuged at 2500 rpm to remove the dissolving mixture and subsequently washed 3–5 times with centrifugation in distilled water. The cleaned diatom suspension was pipetted on to coverslips, left to dry overnight at room temperature and then mounted with Naphrax (Brunel Microscopes Ltd, Wiltshire, U.K.). Identification, counting and the measurements of the diatoms’ basic morphological features were performed under a Carl Zeiss Axio Imager A2 light microscope (LM), equipped with a 100× Plan Apochromatic objective with differential interference contrast (DIC) for oil immersion (NA 1.4) and captured with a Zeiss AxioCam ICc5 camera. For scanning electron microscope (SEM) observations, several drops of the samples were placed on a polycarbonate membrane filter with a 3 μm mesh, attached to aluminium stubs and sputtered with 20 nm of gold using a Turbo-Pumped Sputter Coater Quorum Q 150OT ES. The diatoms were observed using a Hitachi SU 8010 SEM. Diatom terminology follows Barber and Haworth (1981), Round et al. (1990) and Levkov et al. (2013).

Five Luticola observed and described in the study show specific features that separate them from all other described Luticola species so far. Tables 1–5 present morphological comparisons amongst all of the new species and the most similar taxa worldwide.

Luticola insularis sp. nov. is morphologically most similar to L. aequatorialis (Heiden) Lange-Bertalot & T.Ohtsuka and L. simplex Metzeltin, Lange-Bertalot & García-Rodríguez, since they share similar valve outlines, areolae densities and distal raphe ending morphologies (Metzeltin et al. 2005; Levkov et al. 2013). However, L. insularis sp. nov. can be easily distinguished from the other two taxa by a lower striae density and narrower valves (Table 1). Moreover, L. insularis sp. nov. shows well-developed silica ridges along the valve face/mantle junction (Fig. 4D–F), which are absent in both L. aequatorialis and L. simplex (Metzeltin et al. 2005; Levkov et al. 2013). Additionally, the European L. pseudoimbricata Levkov, Metzeltin & Pavlov shows some degree of similarity and the dimensions of the two taxa overlap. However, L. insularis has different proximal and distal raphe endings. In L. insularis, the external proximal raphe endings are deflected with small, shallow grooves (Fig. 4A–E), while in L. pseudoimbricata, proximal raphe endings are short, curved and slightly expanded (Levkov et al. 2013; pl 16, figs 1–3, 5). Additionally, L. insularis has a lower striae density (16–20/10 µm vs. 20–24/10 µm) (see Table 1).

Figure 4. 

SEM microphotographs of Luticola insularis sp. nov. external (A–F) and internal (G, H) views. Arrow indicate a longitudinal channel. Scale bars: 5 µm.

Luticola bandanensis sp. nov. resembles two European species: L. frequentissima Levkov, Metzeltin & Pavlov, L. pitranensis Levkov, Metzeltin & Pavlov and also with L. rapanuiensis M.Rybak, Peszek, A.Witkowski & Lange-Bertalot, which was recently described from Easter Island (Table 2). Luticola frequentissima seems the most morphologically similar to L. bandanensis sp. nov. The two taxa can be separated, based on the valve width (4.5–6.5 µm in L. bandanensis sp. nov. vs. 6.5–9.0 µm in L. frequentissima; Levkov et al. (2013)). Additionally, L. bandanensis sp. nov. has symmetrical valves, while the valves of L. frequentissima are commonly more bulged on the side opposite the isolated pore (Levkov et al. 2013; Noga et al. 2017). Both species show irregular grooves on both distal and proximal raphe endings. In L. bandanensis sp. nov., these grooves are present on the side opposite to the isolated pore, while in L. frequentissima, the grooves extend to the isolated pore side (Levkov et al. 2013: pl. 9, figs 1–4, Noga et al. 2017: fig. 3). Luticola pitranensis can be distinguished from L. bandanensis sp. nov., based on its lower striae density (18–21/10 µm vs. 21–23/10 µm), more cuneate valve apices and, as in L. frequentissima, the grooves on the proximal raphe endings that extend to the isolate pore side (Levkov et al. 2013: pl. 9, figs 1–5). Luticola rapanuiensis can be separated from L. bandanensis sp. nov. by its lower striae density (16–19/10 µm vs. 21–23/10 µm), central area that is more rectangular in shape (Peszek et al. 2021: fig. 3A–P) and the presence of irregular silica ridges on the valve face/mantle junction (Peszek et al. 2021: fig. 3U, X–Z) (absent in L. bandanensis sp. nov.).

Figure 5. 

SEM microphotographs of Luticola bandanensis sp. nov. external (A–F) and internal (G, H) views. Arrow indicate a longitudinal channel. Scale bars: 5 µm (A–G); 3 µm (H).

Luticola elliptica sp. nov. shows some degree of similarity to L. bryophila M.Rybak, Czarnota & Noga, Luticola sparsipunctata Levkov, Metzeltin & Pavlov, L. tenuis Levkov, Metzeltin & Pavlov and an unidentified species from the Île Saint-Paul (Chattová et al. 2017: p. 10, figs 72–82). The European L. sparsipunctata can be distinguished from L. elliptica sp. nov., based on its wider valves with lower striae and areolae density (see Table 3) and a more lanceolate valve shape, especially in larger specimens (Levkov et al. 2013: pl. 32). Both L. tenuis and L. bryophila have a lower striae density than L. elliptica sp. nov. The distal endings in L. bryophila are short and deflected to the side opposite the isolated pore (Rybak et al. 2023: fig. 2Y–AA), while the raphe endings in L. elliptica sp. nov. are weakly hooked and continue on to the valve mantle (Fig. 5A–G). Finally, Luticola elliptica sp. nov. is the only species with a strongly asymmetrical central area. The side bearing the isolated pore is almost two times wider than the other side. This very rare feature was only observed in a single unnamed species from Île Saint-Paul Island in the south Indian Ocean (Chattová et al. 2017). Both taxa overlap in valve shape, basic dimensions and raphe ending morphology (Table 3). However, because of the lack of SEM documentation for Luticola sp., it is impossible to determine definitively whether it is an isolated population of the same taxon or a similar, but different species.

Luticola malukuana sp. nov. most closely resembles L. dismutica (Hustedt) D.G.Mann and L. areolata V.Lokhande, Lowe, Kociolek & B.Karthick. The basic feature separating it from both species is the notably higher density of striae (Table 4). L. malukuana sp. nov. can also be distinguished from L. dismutica based on valve outline, which is more undulate in the latter species (see Levkov et al. 2013, pl. 151, figs 15–33, pl. 155, figs 26–35). Luticola malukuana sp. nov. and L. areolata share a similar, slightly undulating valve outline; however, L. areolata has more elongated, narrower valve apices (Lokhande et al. 2020: figs 52–61). Additionally, these taxa differ in striae morphology. Striae of L. areolata are composed of fewer areolae (2–3 vs. 3–4) with a clearly reduced external opening (Lokhande et al. 2020: fig. 76). Luticola areolata also commonly has ghost areolae in the central area (Lokhande et al. 2020: 52–61) and these are absent or rare in L. malukuana sp. nov. (Fig. 7A–D).

Figure 6. 

SEM microphotographs of Luticola eliptica sp. nov. external (A–G) and internal (H) views. Arrow indicate a longitudinal channel. Scale bars: 4 µm (A, C–G); 5 µm (B, H).

Figure 7. 

SEM microphotographs of Luticola malukuana sp. nov. external (A–F) and internal (G, H) views. Arrow indicate a longitudinal channel. Scale bars: 5 µm (A–G); 3 µm (H).

Figure 8. 

SEM microphotographs of Luticola poliporea sp. nov. in external views. * – indicates valves with triple isolated pores. Scale bars: 10 µm (A–E); 5 µm (F, G).

Figure 9. 

SEM microphotographs of Luticola poliporea sp. nov. in internal views (A–F) and external details of valve mantle and girdle band (G). * – indicates valves with triple isolated pores, arrow indicates a longitudinal channel. Scale bars: 10 µm (A, C); 5 µm (B, E–G); 2 µm (D).

Luticola poliporea sp. nov. most closely resembles taxa from the group of Luticola goeppertiana (Bleisch) D.G.Mann ex Rarick, S.Wu, S.S.Lee & Edlund. Amongst them, Luticola tujii Levkov, Metzeltin & Pavlov and Luticola burmensis Metzeltin & Levkov (Levkov et al. 2013) are most similar to the new taxon (Table 5). Luticola poliporea sp. nov., together with the taxa it most closely resembles, are characterised by having the raphe located on a clearly visible raphe sternum (Levkov et al. 2013: pl. 63, figs 1–7, Pl. 69, figs 1–5). However, due to the presence of multiple isolated pores in the central area, Luticola poliporea sp. nov. is distinguished from both mentioned taxa. Additionally, Luticola tuji in contrast to newly-described species, has narrower and usually shorter valves with denser striae (Levkov et al. 2013). Luticola burmensis can be distinguished from L. poliporea sp. nov. by less rostrate apices and mostly wider valves (Levkov et al. 2013). Moreover, multiple isolated pores make this species unique amongst the entire genus. So far, all taxa described in the genus Luticola have a single isolated pore on each valve (Round et al 1990).

Species of the genus Luticola described in recent years, especially those representing the L. goeppertiana group (Simonato et al. 2020; Yang et al. 2022) clearly show that the morphological boundaries of the genus are much wider than previously documented (Round et al. 1990; Levkov et al. 2013). The original description of the genus by Round et al. (1990) indicated several distinguishing features of the genus Luticola, such as: mainly solitary life habit, single plastid with two lobes, single central pyrenoid, mantle margin notched halfway between the apices and valve middle, uniseriate striae, single row of areolae on valve mantle, poroids on valve face and mantle covered by hymenes, presence of a longitudinal canal, narrow raphe-sternum and short stauros, presence of single isolated pore (called stigma) with lipped external opening, deflected raphe endings and open girdle bands with 1 or 2 row of poroids. Since the genus was described, several species with multiple rows of areolae on the mantle have been described or transferred to the genus (Mayama and Kawashima 1998; Pavlov et al. 2009; Wetzel et al. 2010; Da Silva Lehmkuhl et al. 2019; Simonato et al. 2020).

Levkov et al. (2013), in their monograph of the genus, draw attention to the great heterogeneity of the genus in terms of the shape of frustules, areolae and raphe ends. They also draw attention to individual taxa that deviate from the general pattern of frustule structure due to the presence of distinct ridges or spines on the edge. Additionally, Levkov et al. (2013) noted that L. mutica (which is the type species of the genus), unlike the other species known at that time, does not have simple areoles, but has a recessed cribrum (Levkov et al. 2013). However, since the publication of the monograph of the genus, several additional species possessing a cribrum have been described, i.e. Luticola ivetana Chattova & B.Van de Vijver, L. cribriareolata M.Rybak, Witkowski, Risjani & Yunianta and L. jinshaensis L.Yang & Q.X.Wang (Chattová et al. 2017; Rybak et al. 2021b; Yang et al. 2022). Additionally, Simonato et al. (2020) shed some light on the morphological variability of the genus, pointing out the occurrence of pseudosepta in some species group, as well as the absence of longitudinal/marginal channels in others. Observations of various species show that the range of morphological variability of the genus is much broader than originally assumed by Round et al. (1990), within which taxa demonstrating certain differences from many characteristic valve features are found (i.e. taxa without longitudinal/marginal canals, having additional rows of areolae on the mantle, additional isolated pores, cribra-bearing). Despite the significant expansion of knowledge about the diversity of morphological features found within Luticola, observations of living cells so far show that features, such as the shape of the plastid, the number of its lobes and the presence of a single pyrenoid, seem to be constant (Denys and De Smet 1996; Poulíčková 2008; Bagmet et al. 2023).

A significant number of new diatom taxa have been described in the last few years from both terrestrial and freshwater environments in Southeast Asia. The discovery of five new species of Luticola in just three samples presented herein, shows that the poorly-explored terrestrial environments hide interesting and yet undescribed species.