Diversity of silica-scaled chrysophytes (Chrysophyceae) from Mindanao Island (Philippines) with the description of Mallomonas pseudopustula sp. nov.

81 Diversity of silica-scaled chrysophytes (Chrysophyceae) from Mindanao Island (Philippines) with the description of Mallomonas pseudopustula sp. nov. Evgeniy S. Gusev1, Marina E. Ignatenko2, Chris Rey Lituanas3, Kyla Rose Gumapac3, Nikita A. Martynenko1, Boris D. Efeykin1 1 Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky Prospect 33, 119071 Moscow, Russia 2 Institute for Cellular and Intracellular Symbiosis, Orenburg Federal Research Center, Ural Branch, Russian Academy of Sciences, Pionerskaya Street, 11, 460000, Orenburg, Russia 3 Institute of Biological Sciences, College of Arts and Sciences, Central Mindanao University, Plant Biology Division, Purok 13, Musuan, 8714 Maramag, Bukidnon, Philippines Corresponding author: Evgeniy S. Gusev (evser[email protected]) Copyright: © Evgeniy S. Gusev et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract This study presents the first comprehensive investigation of chrysophytes from the Philippines. The flora of silica-scaled chrysophytes from freshwaters of Mindanao Island has been studied by means of scanning and transmission electron microscopy. A total of thirty-six taxa were recorded, including three belonging to Paraphysomonas, two to Spiniferomonas, one to Lepidochromonas, twenty-nine to Mallomonas and one to Synura. A new species, Mallomonas pseudopustula sp. nov., was discovered and described. Several unidentified Mallomonas taxa were also recorded. The flora of silica-scaled chrysophytes of the Mindanao Island is similar to the flora of neighbouring countries — Vietnam, Indonesia and Malaysia. Key words: Chrysophyceae, electron microscopy, morphology, new species, siliceous scales, taxonomy Introduction Silica-scaled chrysophytes comprise heterokont algae from the class Chrysophyceae and different evolutionary lineages belonging to three orders: Paraphysomonadales, Chromulinales and Synurales (Škaloud et al. 2013a). These organisms can be colonial or unicellular. They possess a covering of silica scales, bristles and spines, the ultrastructure of which is species-specific. A robust morphological species concept has been established for this group, which is congruent with molecular phylogenetic data (Scoble and Cavalier-Smith 2014; Siver et al. 2015; Pusztai et al. 2023). Modern morphological studies of silica-scaled chrysophytes are based on electron microscopy, which began in the 1950s (Škaloud et al. 2013a). The information accumulated over more than 70 years allows chrysophytes to be considered a model group for biogeographical studies (Kristiansen 2001; Řezáčová and Neustupa 2007; Siver and Lott 2012). Academic editor: Dmitry Kapustin Received: 1 November 2025 Accepted: 29 November 2025 Published: 19 December 2025 Citation: Gusev ES, Ignatenko ME, Lituanas CR, Gumapac KR, Martynenko NA, Efeykin BD (2025) Diversity of silica-scaled chrysophytes (Chrysophyceae) from Mindanao Island (Philippines) with the description of Mallomonas pseudopustula sp. nov. PhytoKeys 268: 81–102. https://doi.org/10.3897/ phytokeys.268.176758 PhytoKeys 268: 81–102 (2025) DOI: 10.3897/phytokeys.268.176758 82 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Although silica-scaled chrysophytes in the Tropics have been studied since the 1970s (Compère 1974; Takahashi and Hayakawa 1979), data are currently available from only 23 countries (approximately one-third of all countries with a tropical climate). A diverse tropical flora has been documented from the tropical region (Compère 1974; Takahashi and Hayakawa 1979; Dürrschmidt and Croome 1985; Dürrschmidt and Cronberg 1989; Cronberg 1989; Hansen 1996a; Wujek and Saha 1996; Wei and Yuan 2001; Wei et al. 2014; Gusev et al. 2022b, 2023a). However, data on the diversity of silica-scaled chrysophytes from tropical islands are rather limited. Furthermore, many studies of island floras have been accompanied by descriptions of new species. In total, 57 taxa of silica-scaled chrysophytes were recorded from 14 localities on two small tropical islands, Phu Quoc and Con Son, in Vietnam (Gusev et al. 2022d). From these localities, four new species of the genus Mallomonas were described (Gusev et al. 2019a, d, 2023b; Gusev and Kulikovskiy 2020). Dürrschmidt and Cronberg (1989) identified 29 taxa from Sri Lanka, including two new species and one new variety of Mallomonas and a new species of Paraphysomonas. In Madagascar, 42 taxa have been recorded (Hansen and Kristiansen 1995; Hansen 1996a, b), including three new species of Mallomonas and one of Spiniferomonas. Wei et al. (2014) reported 36 taxa of silica-scaled chrysophytes from Hainan Island (China) and described a new species of Paraphysomonas. During studies of silica-scaled chrysophytes on the island of Java, 19 taxa were identified, including a new species of Chrysosphaerella (Kapustin and Gusev 2019). In Indonesian Papua, a new species of Mallomonas was described, with 24 other taxa reported (Kapustin et al. 2019; Gusev et al. 2022b). Many taxa described from islands remain endemic to this day. The numerous islands of the Philippines belong to the Indo-Malaysian-North-Australian phycogeographical region, one of the ten regions defined by Krieger (1932), based on the composition of desmid floras. There were roughly 287 species of microalga documented in the Philippines which were dominated by diatoms (Lituañas 2019; Martinez-Goss 2021; Santos et al. 2023; Argueles 2025). Since then, there were no studies conducted on Chrysophyceae in general and particularly the investigations of silica-scaled chrysophytes in the Philippines. Algal floristic studies in this poorly-studied region are essential for understanding the biodiversity and biogeography of different algal groups in the Asian tropics. This paper presents the results of a study on the flora of silica-scaled chrysophytes from Mindanao Island, one of the largest islands of the Philippines. Materials and methods Samples for this study were collected in November–December 2024 and June 2025 on Mindanao Island, Philippines (Table 1, Fig. 1). A total of 13 freshwater localities were studied across three provinces. Mindanao is classified under types II, III and IV in the Modified Coronas Climate Classification System (MCCS) used by the Philippine weather agency (PAGASA). The study area lies in the Type IV climate zone, which is characterised by rainfall being more evenly distributed throughout the year, without a pronounced dry or wet season. Plankton samples were collected from the surface water layer using a plankton net with a mesh size of 20 μm. Fifteen samples were collected and processed. All samples were immediately fixed with Lugol’s solution. 83 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Water-specific conductance, pH and temperature were measured in situ using a Hanna HI 9828 multiparameter device (Hanna Instruments, Inc., Ann Arbor, Michigan, USA). Table 1. Basic characteristics of the studied localities (GPS—coordinates, Cond.—specific conductance, µS/cm, T— temperature, °C). No. Localities (year of sampling) GPS pH Cond T 1 Fullbright Pond, CMU Campus, Musuan, Maramag, Bukidnon (2024) 7°51.410'N, 125°2.998'E 7.3 157 28 2 Pulangi Reservoir, Barangay Bayabason, Maramag, Bukidnon (2025) 7°49.230'N, 125°1.457'E 6.8 156 28 3 Lake Apo, Barangay Guinoyuran, Valencia City (2024) 7°52.776'N, 125°00.487'E 7.6 52 28 4 Lake Apo, Barangay Guinoyuran, Valencia City (2025) 7°52.803'N, 125°00.441'E 8.2 43 28 5 Lake Pinamaloy, Barangay Pinamaloy, Don Carlos, Bukidnon, st. 1 (2025) 7°40.436'N, 125°00.063'E 6.9 53 29 6 Lake Pinamaloy, Barangay Pinamaloy, Don Carlos, Bukidnon, st. 2 (2025) 7°40.421'N, 125°00.141'E 6.6 49 31 7 Opalon Spring, Barangay Butong, Quezon, Bukidnon (2025) 7°47.807'N, 125°03.984'E 7.1 202 27 8 Opalon Stream, Barangay Butong, Quezon, Bukidnon (2025) 7°47.777'N, 125°03.989'E 7.1 200 27 9 Opalon Swamp, Barangay Butong, Quezon, Bukidnon (2025) 7°47.781'N, 125°03.979'E 7.4 155 39 10 Forest waterbody in Barangay Lunotan, Gingoog City, Misamis Oriental (2025) 8°42.054'N, 125°01.044'E 5.5 14 24 11 Pond in Barangay Lunotan, Gingoog City, Misamis Oriental (2025) 8°42.263'N, 125°00.923'E 6.8 19 24 12 Water pool 1 near pond in Barangay Lunotan, Gingoog City, Misamis Oriental (2025) 8°42.263'N, 125°00.923'E 5.2 13 22 13 Water pool 2 near pond in Barangay Lunotan, Gingoog City, Misamis Oriental (2025) 8°42.263'N, 125°00.923'E 6.9 169 30 14 Pawan Resort, Barangay Sampagar, Damulog, Bukidnon (2025) 7°30.964'N, 124°57.460'E 7.4 369 29 15 Abandone Ricefield, Barangay Sampagar, Damulog, Bukidnon (2025) 7°30.976'N, 124°57.421'E 8.3 251 38 Figure 1. Schematic map of the studied area. 84 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines For electron microscopy, an aliquot of each sample was rinsed with deionised water via centrifugation. Drops of the rinsed sample were either air-dried or digested with sulphuric acid and potassium dichromate. For scanning electron microscopy (SEM), prepared samples were placed on SEM stubs and sputter-coated with gold using an ion-plasma sputtering system (Quorum Q150R S plus; Quorum Technologies Ltd., London, UK). Observations were carried out with a TESCAN MIRA3 microscope (TESCAN BRNO, s.r.o., Brno, Czechia) at the Gagarin Center for the Identification and Support of Talented Children (Orenburg Oblast, Russia). For transmission electron microscopy (TEM), Formvar-coated copper grids (EMS FF200-Cu-50; Electron Microscopy Sciences, Hatfield, PA, USA) were used. TEM observations were conducted with a JEOL JEM-1011 microscope (Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok, Russia). Twenty-seven scales of the new species were measured to prepare the description. Results A total of thirty-six taxa of silica-scaled chrysophytes were recorded in the freshwaters of the Philippines (Mindanao Island), including three belonging to Paraphysomonas De Saedeleer emend. Scoble & Cavalier-Smith, two to Spiniferomonas E. Takahashi, one to Lepidochromonas Kristiansen, twenty-nine to Mallomonas Perty and one to Synura Ehrenberg (Table 2). All identified taxa are new records for the Philippines. Three organisms belonging to the genus Mallomonas could only be identified to the genus level and may represent new species, pending further detailed study. Furthermore, a new species of the genus Mallomonas was discovered (Fig. 2) and its description is provided below. Several other noteworthy taxa are characterised in the following section (Figs 3–5). Mallomonas pseudopustula E.S. Gusev, Ignatenko, Lituanas, Gumapac & Martynenko, sp. nov. Fig. 2 Description. Scales oval, 5.8–7.1 × 4.1–4.6 µm. A wide posterior rim (0.6–1.1 µm) encircles approximately one-half of the scale’s perimeter on its posterior part. The anterior part of the scale is bordered by a narrow, troughshaped hyaline belt along a narrow, thickened edge. A thick secondary siliceous layer with an internal reticulation pattern of rounded mesh covers most of the scale surface, typically from two-thirds to four-fifths of it. This area contains numerous base plate pores and papillae on the surface of the secondary silica layer. In the proximal part of the scale, the secondary silica layer is significantly thinner and is absent around a large rimmed pore located centrally in this area. This central rimmed pore is surrounded by a “cloud” of small pores. Additional pores penetrate the entire proximal part of the scale, where the secondary siliceous layer is thinner. These pores are smaller in size than those in the distal part of the scale covered with the thick secondary layer. Cysts and bristles were not observed. Type. PhiliPPines • Mindanao Island, Bukidnon, Don Carlos, Barangay Pinamaloy, Lake Pinamaloy; 7°40.436'N, 125°00.063'E; 21 June 2025; E.S. 85 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Gusev leg. Holotype. Portion of a single gathering of cells on SEM stub 70_I_1, deposited at the Herbarium of the Steppe Institute of the Ural Branch of the Russian Academy of Sciences, Orenburg (ORIS). Fig. 2A is a representative scale from the type specimen. Table 2. List of silica-scaled chrysophytes found in the waterbodies of the Mindanao Island (Philippines) (“+” indicates the presence of taxon). TAXON 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Mallomonas cetosa + M. ceylanica + + + M. favosa + M. foveata + + + + M. fragariformis + + M. furtiva + + + + + M. gemina + + + M. guttata var. guttata + + + M. guttata var. simplex + M. hippocrepica + + M. kapustinii + + + M. kenyana + + M. kornevae + M. mangofera apud Dürrschmidt, 1983 + + M. minuscula + M. morrisonensis + + + M. multisetigera + + + + + + + M. paragrandis + + + M. aff. paragrandis + M. parvula + + M. peronoides + M. pseudomatvienkoae + + M. pustula + M. rasilis + + + + M. tropica + Mallomonas pseudopustula sp. nov. + + + + Mallomonas sp. 1 + + + + Mallomonas sp. 2 + Mallomonas sp. 3 + Synura longitubularis + + + + + Spiniferomonas cf. bourrellyi + S. trioralis + + Lepidochromonas eiffelii + Paraphysomonas cf. corynephora + P. uniformis subsp. hemiradia + P. vulgaris subsp. vulgaris + + + + + + + + 86 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Etymology. Epithet “pseudopustula” is derived from the species Mallomonas pustula E.S. Gusev, Safronova, Podunay, Hoan Tran & Martynenko and Greek “pseudo” (= false). Distribution and ecology. In addition to the type locality, this species has been observed in the Fullbright Pond (Bukidnon, Maramag, Musuan, CMU Campus, 7°51.410'N, 125°2.998'E) and a water pool (Misamis Oriental, Gingoog City, Barangay Lunotan, 8°42.263'N, 125°00.923'E). This species has also been previously recorded in Vietnam, Dong Nai Province (Gusev et al. 2022a, fig. 4f, p. 210 as Mallomonas sp. 1). Mallomonas pseudopustula sp. nov. was found under the following environmental conditions: pH 6.6 to 7.3, specific conductance of 49 to 169 µS cm−1 and a temperature range of 28 to 31 °C (Table 1). Discussion Our study revealed a relatively rich chrysophyte flora in the relatively small number of waterbodies investigated. In terms of species richness, the flora of Mindanao Island (36 taxa) is comparable to the known floras of silica-scaled chrysophytes of other tropical regions, such as Indonesia (45 taxa, Kapustin et al. (2019); Gusev et al. (2022b)), Madagascar (42 taxa, Hansen (1996a)), Malaysia (40 taxa, Figure 2. Mallomonas pseudopustula sp. nov. A–D, G, H. Surface view of the scale; E, F. Undersurface view of the scale; Arrows indicate: (a) Posterior rim; (b) Hyaline belt; (c) Thick secondary layer with papillae and pores; (d) Large central rimmed pore in the proximal part of the scale surrounded by a “cloud” of small pores; (e) Pores in the proximal part of the scale; (f) Pores in the distal part of the scale; A–F. SEM; G, H. TEM. Scale bars: 5 µm (G); 2 µm (A–F, H). 87 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Dürrschmidt and Croome (1985)), the tropical areas of Brazil (47 taxa, Couté and Franceschini (1998); Franceschini and Kristiansen (2004) etc.), Sri Lanka (29 taxa, Dürrschmidt and Cronberg (1989)) and more than in such Asian countries as Papua New Guinea (19 taxa, Vyverman and Cronberg (1993)), Bangladesh (14 taxa, Takahashi and Hayakawa (1979)) and Singapore (9 taxa, Neustupa and Řezáčová (2007)). A greater number of species is known from regions with more extensive research efforts, such as Vietnam (108 taxa, Gusev et al. (2023a) etc.), India (58 taxa, Saha and Wujek (1990); Wujek and Saha (1996)) and the tropical areas of China (58 taxa, Wei and Yuan (2001); Wei et al. (2014)). Since all reported taxa are new records for the flora of the Philippines, we provide brief comments on their distribution patterns, grouping them by sections or species complexes. The most represented group in terms of the number of taxa was the Mallomonas mangofera species complex. Mallomonas mangofera K. Harris & D.E. Bradley was previously considered a cosmopolitan species (Kristiansen 2000). It was originally described from Great Britain without a designated type (Harris and Bradley 1960) and its description was later expanded to include a morphotype from Chile (Dürrschmidt 1983). A recent revision, however, has shown that the morphotypes from Great Britain and Chile represent distinct species (Gusev et al. 2024b, c). Mallomonas mangofera sensu stricto appears to be restricted to the temperate zone of Eurasia and Japan, while findings from other regions identified as M. mangofera represent different morphotypes and species (Gusev et al. 2024b, c). The morphotype described from Chile, M. mangofera apud Dürrschmidt (Dürrschmidt 1983), was found on Mindanao Island (Fig. 3T). This morphotype is widely distributed in the tropical region, but requires revision and formal description as a new species (Gusev et al. 2023a). One widely-distributed taxon from the M. mangofera complex, M. foveata (Dürrschmidt) E.S. Gusev, was recorded in the Philippines (Fig. 3F–H). Recently, a number of new species have been described from the Tropics, based on both morphological and molecular criteria. Mallomonas cetosa Sterlyagova, Ignatenko, Nguyen Thi Lan & E.S. Gusev (Fig. 3A) is a recently described species that is widespread in the tropics, where it had been misidentified for many years under various names, most commonly as M. mangofera var. reticulata (Sterlyagova et al. 2025). Mallomonas fragariformis Gusev, Kapustin, Shkurina & Martynenko (Fig. 3I) and M. minuscula Gusev, Guseva, Kezlya & Kulikovskiy (Fig. 4A, B) are other species recently described from Vietnam, based on scale ultrastructure, but with a more limited known distribution (Gusev et al. 2019a, 2022c, 2023a). Mallomonas fragariformis, besides Vietnam, is also known from Indonesia (Gusev et al. 2022c) and Australia (Němcová and Diaz-Pulido 2023), while the M. minuscula has recently been reported from Australia (Němcová and Diaz-Pulido 2023). Mallomonas hippocrepica E.S. Gusev, Martynenko, Hoan Tran & Podunay (Fig. 3P) and M. kornevae E.S. Gusev, Martynenko, Hoan Tran & Podunay (Fig. 3S) were also recently described, based on morphological and molecular data (Gusev et al. 2024c). The finding of M. hippocrepica (Fig. 3P) in the Philippines is its first record outside of Vietnam, where it is known from only two localities (Gusev et al. 2024c). Mallomonas kornevae (Fig. 3S) is widespread in Vietnam and has already been found in Florida (Knotek et al. 2025), suggesting it may have a broad distribution in the Tropical Region. Another species similar in ultrastructure to the Mallomonas mangofera complex is M. favosa K.H. Nicholls (Fig. 3E), which has been considered cosmopolitan (Kristiansen 2000; Kristiansen and Preisig 2007). Mallomonas favosa was 88 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines Figure 3. Mallomonas taxa from Mindanao Island (Philippines). A. Mallomonas cetosa; B–D. Mallomonas ceylanica, body scales (B, C), apical scale (D); E. Mallomonas favosa; F–H. Mallomonas foveata, body scale (F), rear scale with a small spine (G), apical scale (H); I. Mallomonas fragariformis; J. Mallomonas furtiva; K, L. Mallomonas gemina, body scale (K), apical scale (L); M. Mallomonas guttata var. guttata; N. Mallomonas guttata var. simplex; O. Mallomonas sp. 1; P. Mallomonas hippocrepica; Q. Mallomonas kapustinii; R. Mallomonas kenyana; S. Mallomonas kornevae; T. Mallomonas mangofera apud Dürrschmidt. Scale bars: 2 µm (B, C, G, J–L, N, O, R–T); 1 µm (A, D–F, H, I, M, P, Q). originally described from Canada (Nicholls 1984) and later reported from other parts of the world (Kristiansen and Preisig 2007). However, our unpublished data indicate that morphotypes of M. favosa from the temperate zone of Eurasia (Czechia) and Vietnam differ significantly in their SSU rDNA and ITS rDNA nucleotide sequences, indicating they are different species (Gusev et al. 2024d; Škaloud et al. 2025). The M. favosa morphotype found in the Tropics is distrib- 89 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines uted in the freshwaters of Vietnam (Gusev et al. 2023a), Malaysia (Dürrschmidt and Croome 1985), China (Wei et al. 2014), Papua New Guinea (Vyverman and Cronberg 1993) and Brazil (Couté and Franceschini 1998; Franceschini and Kristiansen 2004). Another species from this group, Mallomonas gemina (Fig. 3K, L), is widely distributed in tropical Asia (Gusev et al. 2024d). Morphotypes similar to Mallomonas matvienkoae sensu lato are widely distributed in the Tropics. For a long time, M. matvienkoae B. Asmund & Kristiansen, like M. mangofera, was considered a cosmopolitan taxon (Kristiansen 2000; Kristiansen and Preisig 2007). However, initial molecular studies revealed the heterogeneity of this group, leading to the description of four new species (Jo et al. 2013). The typification of M. matvienkoae and subsequent distribution analysis have shown that this taxon has so far been recorded only in Europe, Russia and South Korea (Gusev et al. 2024a; Škaloud et al. 2025). Findings from other regions belong to different species, representing a complex of morphologically similar species (Gusev et al. 2024a). Amongst the species of the Mallomonas matvienkoae/pseudomatvienkoae group, the following were recorded on Mindanao Island: M. pseudomatvienkoae Jo, Shin, Kim, Siver & Andersen (Fig. 4L, M), M. pustula E.S. Gusev, Safronova, Podunay, Hoan Tran & Martynenko (Fig. 4N), M. kapustinii E.S. Gusev, Safronova, Podunay, Hoan Tran & Martynenko (Fig. 3Q), M. paragrandis Gusev (Fig. 4G) and M. pseudopustula sp. nov. (Fig. 2). We also provide images of a putative new species, designated here as Mallomonas sp. 2 (Fig. 4R) and Mallomonas sp. 3 (Fig. 4S), which requires further study. They differ from the known species by the development of the secondary silica layer surrounding the rimmed pore on the proximal part of the scale. Mallomonas pseudomatvienkoae is a widely distributed species. It was described from South Korea (Jo et al. 2013) and its presence has been confirmed by molecular data in Vietnam (Gusev et al. 2024e). Morphotypes of this taxon have also been reported from India (Saha and Wujek 1990), Indonesia (Kapustin and Gusev 2019), Australia (Němcová and Diaz-Pulido 2023), Russia (Gusev et al. 2019b), France (Němcová et al. 2012) and Czechia (Němcová 2010), although these records require molecular confirmation. Mallomonas pustula and M. kapustinii were described from waterbodies in Vietnam (Gusev et al. 2024e). Scales of M. pustula also reported from other tropical regions (Gusev et al. 2024e). The record of M. kapustinii from Mindanao Island is the first outside Vietnam. Mallomonas paragrandis, also described from Vietnamese waterbodies a decade ago, is a species widespread in the Tropics and is frequently reported in Asian freshwaters (Gusev et al. 2023a). Scales corresponding to Mallomonas paragrandis (Fig. 4G), as well as scales morphologically similar to M. paragrandis, but differing by their larger size (6.3–7.0 × 3.7–4.1 µm vs. 4.8–6.0 × 2.7–4.0 μm), were discovered in waterbodies on Mindanao Island. The latter were identified as Mallomonas aff. paragrandis (Fig. 4H). Further molecular studies are required to determine whether these scales represent a new, previously unknown taxon or fall within the morphological variability of M. paragrandis, a species that may require a revision and an expansion of its morphometric characterisation. As a result of this study, a new species, Mallomonas pseudopustula sp. nov. (Fig. 2), was described. M. pseudopustula belongs to section Planae. In scale shape and ultrastructure, M. pseudopustula resembles taxa from the M. pseudomatvienkoae and M. matvienkoae species complexes. Mallomonas pseudopustula is most 96 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. Gusev et al.: Silica-scaled chrysophytes from Philippines composition shows the greatest similarity to nearby regions, such as Malaysia, Indonesia and Vietnam. Our study documented two taxa recently described from Vietnam and recorded here for the first time outside that country, described one new species and identified three morphotypes requiring further detailed investigation. Further research in the Philippines will undoubtedly reveal a richer flora and novel species of chrysophytes. Acknowledgements The authors are grateful to the staff of the Centre of Electron Microscopy of the IBIW RAS for help in TEM studies. The SEM studies were conducted using Joint Usage Center “Instrumental methods in ecology” at the IEE RAS and the Gagarin Center for the Identification and Support of Talented Children (Orenburgskaya Oblast). The authors are grateful to the Central Mindanao University Administration, headed by Dr. Rolito G. Eballe, CMU President and DENR 10, for the support. Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. Funding The study was supported by the Ministry of Science and Higher Education of the Russian Federation in the frames of Agreement Nº 075-15-2025-654. M.E. Ignatenko was supported by the Governmental Task NºFUUG-2022-0007 of ICIS RAS. Author contributions Evgeniy S. Gusev: supervision, conceptualisation, methodology, investigation (sampling, species description, identification), writing — original draft, writing — review and editing. Marina E. Ignatenko: investigation (SEM studies, species description, identification), visualization, writing — original draft, writing — review and editing. Chris Rey Lituanas: investigation (sampling), project administration (organising expeditions in the Philippines, permits). Kyla Rose Gumapac: investigation (sampling), project administration (organising expeditions in the Philippines, permits). Nikita A. Martynenko: investigation (sampling, species identification), writing — original draft. Boris Efeykin: writing — review and editing, resources, funding acquisition. Author ORCIDs Evgeniy S. Gusev https://orcid.org/0000-0002-7397-5808 Marina E. Ignatenko https://orcid.org/0000-0002-4451-7816 Chris Rey Lituanas https://orcid.org/0000-0002-8584-3757 Kyla Rose Gumapac https://orcid.org/0009-0008-8147-5820 Nikita A. Martynenko https://orcid.org/0000-0002-7286-003X 97 PhytoKeys 268: 81–102 (2025), DOI: 10.3897/phytokeys.268.176758 Evgeniy S. 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