Three New Nitzschia (Bacillariophyceae) Species from Highly Acidic Artificial Lakes in Çanakkale, Türkiye

Abstract

Çanakkale is located in Western Anatolia, a region known for extensive mining in the country. In this study, three new species: Nitzschia arslaniana sp. nov., Nitzschia efeiana sp. nov., and Nitzschia szaboiana sp. nov. are described from highly acidic environments in Çanakkale. In morphology, Nitzschia arslaniana is similar to Nitzschia inconspicua but Nitzschia inconspicua is broader, with a lower fibulae density. Nitzschia efeiana is similar to Nitzschia. bremensis, Nitzschia capitellata, Nitzschia homburgiensis, Nitzschia hybrida, Nitzschia palustris, and Nitzschia pellucida. However, Nitzschia efeiana has variable fibulae widths, thickened virgae on its external valve face, and an elevated ridge on the valve margin opposite the keel. Nitzschia szaboiana is similar to Nitzschia pusilla, but the later taxon has a higher striae density. This study highlights the unique species diversity of extreme environments, including anthropogenic extreme environments, by describing three new taxa in the same genus.

1. Introduction

Extreme environments also include new environments created by the anthropogenic impacts of mining. Open-pit mining with subsequent aquatic infilling of pits is common at abandoned sites, typically with elevated acidic levels as a result of the mining [1,2]. The Çanakkale region of western Anatolia is a classic example of an extreme environment created by mining. The bedrock of Çanakkale is magmatic rocks from the Oligo-Miocene [3], which, along with mine processing, has created an environment with low pH and alkalinity. Three lakes/ponds in this region have pH levels < 2.5 with no ion buffering capacity. The resulting fluctuating levels of pH further create instability in nutrient and chemical balances within the lake. Species with resilience to pH and ion stress in unstable environments will flourish, while others will perish. Diatoms will survive with lower diversities in highly acidic environments, especially species within genera like Nitzschia [4].

The genus Nitzschia is currently non-monophyletic, including many clades and subclades that are morphologically cryptic, which makes classical identifications difficult and inconsistent within and across research labs, e.g., [5]. Further homoplasy is evident in classical examinations using LM and SEM (anonyms reviewer). Although DNA studies are revealing interesting systematic relationships at higher taxonomic levels, genetics has not effectively linked clear genetic clades with distinct, definable morphological synapomorphies. It is, however, evident that taxa within the genus Nitzschia have a broad range of autecological optimal conditions that seem to have taxa selectivity. Further research using more populations across environments and incorporating comprehensive studies (morphology, genetics, cellular metabolic functions) will be needed to discern evolutionary and systematic relationships.

Acidic aquatic systems at mining sites have good documentation of diatom species, especially in tailing pond environments. Diatom species richness and diversity are typically low, with 3–5 species dominating. In extreme environments, benthic species from the genera Nitzschia, Eunotia, Frustulia, and Pinnularia are consistently recorded, although taxonomically poorly studied and often subject to force fitting [6,7]. Six Nitzschia species: N. capitellata Hustedt [7] (p. 414), N. communis Rabenhorst [8] (p. 949), N. pusilla Grunow [9] (p. 579), N. hantzschiana Rabenhorst [10] (p. 40), N. thermalis (Kutzing) Auerswald in Rabenhorst [11] (p. 1064a), and N. vasta Hustedt [12] (p. 660) can be prominent species in waters with pH < 3.5 [5]. Nitzschia tubicola sensu lato has been identified at mining waste sites, although this taxon sensu stricto is a marine species (high pH and highly buffered waters) [13]. It is, therefore, no surprise that a closer look at the diatom species in an extreme acidic environment is warranted, with a look at species selectivity and resilience.

The objective of this study is to document the acidophilic diatom community, more specifically acidophilic Nitzschia species in anthropogenic disturbed aquatic waters related to mining activity in Türkiye.

2. Materials and Methods

2.1. Study Area

The study area is located in the Çan District of Canakkale Province in Biga Peninsula (Western Anatolia). In this region, there are ecologically similar types of ponds that were created through commercial mining activities and subsequently abandoned (not returned to pre-mining conditions). These artificial mining ponds are sustained primarily through groundwater sources, with some surface water runoff. The main geological formations of the study area are metamorphic rocks, ophiolites, Neogene sediments, and magmatic rocks. Also, the products of the acidic magmatism active in different stages of processing are mostly of granite, granodiorite, and diorite compositions [4]. Further, the poorly developed soils are mainly acidic, contributing to acidic surface water runoff. The samples were collected from three mining ponds (Figure 1).

2.2. Sampling

The samples were collected in June, September, and November 2020; October 2021; and September 2022 from epilithic and epipelic substrata. Epilithic samples were collected with a toothbrush, and epipelic samples were collected with a pipette [14]. At the time of sampling, water temperature (°C), dissolved oxygen (mg·L⁻¹), oxygen saturation (%), pH, electrical conductance (µS·cm⁻¹), and salinity (%) were measured in situ using a Lange Hach 40d multi-parameter meter (Hach Lange CmbH Headquarter, Düsseldorf, Germany).

For typification of the new species, we followed article 8.2 of the International Code for Botanical Nomenclature [15], selecting the entire microscopic slide as the holotype. Type materials and the holotype slide are deposited at the Canadian Museum of Nature (CANA), Ottawa, Ontario, Canada, and isotype slides are deposited at Kütahya Dumlupınar University, Türkiye, and in the collection of Andrzej Witkowski at Szczecin University.

2.3. Sample Preparation

The collected samples were prepared by treatment with 10% HCl and 30% H₂O₂ to remove organic material [16]. The acidic residue after treatment was removed through a series of five deionized water dilutions. Permanent microscopic slides were made using cleaned material dried on cover glass and subsequently mounted on a microscope slide using Naphrax with a refractive index of 1.65. The slides were analyzed using a Nikon Ci light microscope with a Nikon Eclipse Ci and a DS-Fi2 camera (Nikon Instruments Inc., Tokyo, Japan) in the Diatom Laboratory at Kütahya Dumlupınar University. Observations were completed at 1000× magnification with a 100× Plan Apochromat oil immersion objective (NA = 1.4).

The ultrastructure morphological observations were performed using SEM. For that purpose, part of the cleaned material was filtered through a polycarbonate membrane filter with a pore diameter of 5 μm. These membrane filters were fixed on aluminum stubs with double-sited cardon tape after air-drying. Stubs were sputter-coated with a gold layer reaching a thickness of ~20 nm and studied using a FEI Quanta FEG450 field emission scanning electron microscope at the University of Sakarya, Türkiye. SEM examinations were conducted using secondary electrons, at 5 KV and working distances of 5–9 mm.

3. Results

Division: Bacillariophyta.

Class: Bacillariophyceae Haeckel, 1878 (emend. D.G. Mann, in Round et al. [17]).

Subclass: Bacillariophycidae D.G. Mann, in Round et al., 1990.

Order: Bacillariales D.G. Mann, in Round et al., 1990.

Family: Bacillariaceae Kützing 1844.

Genus: Nitzschia Hassal, 1845.

Nitzschia efeiana T. Sevindik, P.B. Hamilton and C.N. Solak sp. nov.

Description (Figure 2, Figure 3 and Figure 4). Valves linear and linear-lanceolate with capitate rounded ends. Length 20.0–43.0 µm, width 3.5–4.0 µm (n = 250, Table 1). Valve face transapically arched, with keel on one side and elevated ridge on opposite side. Keel rounded, wing-shaped and central area slightly reduced and indented; no areolae on keel, with areolae along edges of keel. At apex, keel merges onto valve face. Externally, raphe filiform with adjacent ridge towards face. Proximal raphe endings widely space within a small central area on mantle side. Terminal fissures extend from keel onto apex mantle and bend away on mantle. Transapical striae uniseriate, visible in LM, parallel throughout, 22–28 in 10 µm. Areolae round to elliptical, positioned in external surface depressions and evenly spaced, with 40–50 in 10 µm. Random virgae with additional external ridges. Internally, valve face flat with perpendicular mantle. Proximal raphe endings and central nodule visible. Distal raphe endings in slightly distinct helictoglossae. Fibulae variable in width, irregularly spaced, merging onto 1–6 virgae; some merge to form broad fibula. Internally, areolae weakly recessed, round to elliptic, and occluded by hymenes. On mantle, 4–6 areolae present per stria. Copulae 4+, open bands with 2 rows of occluded rounded pores on the pars exterior.

Type. Turkey, Çanakkale, Çan, Keçiağılı District (GPS 39°58′06″ N, 26°51′54″ E), 164 m. a.s.l., collector Cüneyt Nadir Solak 17.06.2020. Holotype: slide number CANA_129570 (plate images representing specimens from the holotype slide) in the Canada Nature Museum.

Isotype. Slide number TR_ÇNK_Keciagili_Lake_1_EPL_June2020 deposited at Kütahya Dumlupınar University (Turkey) and slide number 27,356 deposited in Andrzej Witkowski Collection at the Szczecin University.

Etymology. The species is dedicated to Efe SEVİNDİK, son of Dr. Tuğba ONGUN SEVİNDİK.

Differential diagnosis. The primary characters identifying N. efeiana are shape, variable fibulae widths, thickened virgae on external valve face and elevated ridge on valve margin opposite the keel. Nitzschia bremensis Hustedt [18] (p. 284), N. capitellata Hustedt [7] (p. 414), N. homburgiensis Lange-Bertalot [19] (p. 650), N. hybrida Grunow in Cleve and Grunow [20] (p.79), N. palustris Hustedt [21] (p. 395) and N. pellucida Grunow in Cleve and Grunow [20] (p. 80) are similar in valve outline. N. palustris is more similar considering dimensions; however, N. efeiana has broader and variable fibulae (broad and narrow), and the diminution series is smaller than N. palustris (20–43 µm vs. 35–60 µm length and 3.5–4.0 µm vs. 4.0–7.0 µm width, respectively). N. bremensis is larger (60–90 µm length and 6.0–9.0 µm width) and with a higher striae density (26–32 in 10 µm). N. homburgiensis is broader (5.0–6.0 µm width). Regarding the striae density, N. homburgensis (34–40 in 10 µm) and N. capitellata (10–18 in 10 µm) have higher densities than N. efeiana. Among them, N. hybrida and N. pellucida have a strongly concave structure in the middle of the valve.

Distribution. Observed only in the type locality.

Ecology. The water temperature ranged from 24.3–27.4 °C, pH was 2.4–2.7, dissolved oxygen (DO) was 6.4–8.0 ppm, and electrolyte conductivity was 2.1–2.3 µS.cm⁻¹.

Associated diatom flora. Nitzschia efeiana was the second dominant taxon (17.4 %) in the sample with Nitzschia tubicola s.l. (35.9 %) and Eunotia exigua (10.3 %).

Nitzschia szaboiana P.B. Hamilton, C.N. Solak & T. Sevindik sp. nov.

Description (Figure 5). Valves linear-elliptic outline with broadly rounded ends. Length 14.5–21.0 µm, width 3.0–4.0 µm (n = 10, Table 1). Keel rounded, slightly constricted at mid-valve, with no areolae. Keel terminates on valve face close to valve end. Canal raphe continuous, externally distal raphe endings curved in same direction. Fibulae 14–16 in 10 µm, central nodule not observed. Transapical striae uniseriate, invisible in LM, parallel throughout valve, irregularly distributed at end, 47–50 in 10 µm, composed of small rounded areolae, with 55–60 in 10 µm. Single row of areolae along each side of keel margin. Silica-thickened ridge present opposite side to keel. Internally, valve face flat with perpendicular valve mantle. Distal raphe endings with helictoglossae form on face/mantle junction. Fibulae regularly spaced, connecting with 2–3 virgae. Areolae small, rounded and occluded by hymenes.

Type. Türkiye, Çanakkale, Çan, Keçiağılı District (GPS 39°57′46″ N, 26°51′31″ E), 166 m. a.s.l., collector Cüneyt Nadir Solak 17.06.2020. Holotype: slide number CANA_129571 (plate images representing the holotype) in the Canada Nature Museum.

Isotype. Slide number TR_ÇNK_Keciagili_Lake_3_EPL_June2020, deposited at Kütahya Dumlupınar University (Türkiye) and slide number 27,359, deposited in the Andrzej Witkowski Collection at Szczecin University.

Etymology. The species is dedicated to Gábor Szabó, father of our colleague Prof. Dr. Éva Ács.

Differential diagnosis. Nitzschia szaboiana is identified by small size, dense areolae, and thickened marginal ridge opposite side to the keel. Nitzschia anatoliensis Górecka, Gastineau and Solak in Solak et al. [22] (p. 4), N. perspicua Cholnoky [23] (p. 262), and N. pusilla Grunow [9] (p. 579) are similar taxa. Regarding striae density, N. pusilla and N. anatoliensis have higher densities than N. szaboiana (14–19 in 10 µm and 20–23 in 10 µm, vs. 14–16 in 10 µm, respectively).

Distribution. Observed only in the type locality.

Ecology. The water temperature ranged from 25.3 to 27.8 °C, pH was 2.1–2.4, dissolved oxygen (DO) was 5.2–7.4 ppm, and electrolyte conductivity was 2.4–2.7 µS·cm⁻¹.

Associated diatom flora. The taxon was rare, found in the sample with Eunotia exigua (29.4 %), Nitzschia tubicola s.l. (18.7 %) and Pinnularia acidophila (8.3 %).

Nitzschia arslaniana C.N. Solak, P.B. Hamilton and T. Sevindik sp. nov.

Description (Figure 6). Valves linear with narrowing, rounded ends. Length 12.0–14.0 µm, width 2.0–2.5 µm (n = 87, Table 1). Keel flat to weakly rounded, mid-valve area slightly constricted. Keel terminates at apex along valve margin. Canal raphe, marginal, on mantle, central raphe endings indistinct, central area small central nodule not observed. Externally, proximal raphe endings slightly depressed, distal raphe endings curve in same direction, and terminal fissures hooked. Fibulae 13–14 in 10 µm. Transapical striae uniseriate, visible in LM, parallel throughout, with 27–29 in 10 µm, composed of small round to elliptic areolae between slightly thickened virgae, 40–50 in 10 µm. Single row of large areolae on keel. Internally, distal raphe ends in small, distinct helictolossae. Fibulae regularly spaced, with branches extending onto 2–3 virgae. Internally, areolae rectangular–rounded and occluded by hymenes. Two areolae extend down mantle. Valvocopula open bands wide, one row of small pores on pars exterior; open copula band narrow, with ligula.

Type. Türkiye, Çanakkale, Çan, Keçiağılı District (GPS 39°57′46″ N, 26°51′31″ E), 166 m. a.s.l., collector Cüneyt Nadir Solak 17.06.2020. Holotype: slide number CANA_129572 (plate images represent the holotype) in the Canada Nature Museum.

Isotype. Slide number TR_ÇNK_Keciagili_Lake_3_EPL_June 2020 deposited at Kütahya Dumlupınar University (Türkiye) and slide number 27,359, deposited in Andrzej Witkowski Collection at Szczecin University.

Etymology. The species was dedicated to Tolga ARSLAN, friend of our colleague Dr. Cüneyt Nadir SOLAK.

Differential diagnosis. The primary identifying characteristics of N. arslaniana are formation of the bent terminal raphe fissures, the single areola aligned with each stria on the keel, and marginally smaller areolae. Nitzschia acidoclinata Lange-Bertalot (1976: 277), N. frustulum (Kützing) Grunow in Cleve and Grunow (1880: 98) [including N. frustulum var. subsalina Hustedt (1925: 415)], N. inconspicua Grunow (1862: 579) and N. perminuta Grunow (1881: pl.69), are similar taxa. Among them, N. inconspicua is the most similar, considering outline in LM and morphology in SEM. However, N. inconspicua is broader, with a lower fibulae density than N. arslanana (2.5–3.5 µm width and 8–13 fibulae vs. 2.0–2.5 µm width and 13–15 in 10 µm, respectively) [24]. The terminal raphe ending of N. inconspicia are crooked, bending dorsally and ventrally, not curved. Nitzschia frustulum and N. perminuta are wider than N. arslaniana, with subrostrate to capitate apices. Nitzschia acidoclinata is similar in outline, but with rostrate apices, 2–4 small areolae/stria on the keel, not one, as in N. arslaniana, and valves are wider.

Table 1. Morphological characteristics of Nitzschia arslaniana, N. szaboiana, and N. efeiana sp. nov., with other Nitzschia taxa sharing similar morphological features (--: no information).

	Valve Length (µm)	Valve Width (µm)	Striae (in 10 µm)	Fibulae (in 10 µm)	Valve Shape	Apex Shape	Keel Shape	Copulae	Reference
N. arslaniana	12.0–14.0	2.0–2.5	27–29	13–15	linear	broadly rounded, cuneate	elevated with conopeum, recessed at center	open bands, 4+, 2 rows of large pores	this study
N. szaboiana	13.0–17.0	3.0–4.0	47–50	14–16	linear-elliptic	broadly rounded, cuneate	round, elevated, terminates at end on valve face	--	this study
N. efeiana	20.0–43.0	3.5–4.0	23–27	6–10	linear, linear-lanceolate	capitate	flat, marginal, terminates at apex	Open bands, 4+, one row of small round pores	this study
N. acidoclinata	8.0–45.0	2.5–3.0	27–34	10–16	linear-lanceolate to narrowly linear	shortly subrostrate	--	--	[25]
N. anatoliana	7.8–16.1	2.7–3.7	48–52	20–23	linear-elliptic	broadly rounded	marginal	--	[26]
N. bremensis	60.0–90.0	6.0–9.0	26–32	5–9	linear	rounded to flat truncated	Submarginal, broad, on valve face, elevated, with conopeum?	-	[27]
N. capitellata	20.0–70.0	3.5–6.5	23–40	10–18	linear, linear-lanceolate	rounded, wedge shaped	marginal, small	--	[27]
N. homburgiensis	32.0–52.0	5.0–6.0	34–40	9–15	linear, strongly concave in the middle	rounded, wedge shaped	submarginal	--	[27]
N. hybrida	34–125	5–9	22–26	9–12	linear, strongly concave in the middle	wedge shaped	marginal, round, elevated, recessed	--	[26]
N. inconspicua	6.0–11.5	2.6–3.1	24–29	11–17	linear to lanceolate, elliptic-lanceolate	slightly protracted, narrow	marginal, round, not elevated, interrupted at center	copulae 4+, open bands, one row of pores	[24]
N. frustulum	10.8–34	3.0–3.9	27–30	13–15	linear-lanceolate to lanceolate, elliptic-lanceolate	slightly protracted, narrow	marginal, round, not elevated, terminating along margin at apex	open copulae, one row of pores	[24]
N. palustris	35.0–60.0	4.0–7.0	22–28	6–10	linear	rounded to flat truncated	marginal, interrupted at center	--	[28]
N. pellucida	--	--	30–40	12–18	linear-lanceolate	boat-shaped	submarginal, round, elevated, terminating on valve face at end	--	[27,29]
N. perminuta	8.0–45.0	2.5–3.0	26–36	10–16	linear-lanceolate to narrowly linear	subrostrate to subcapitate	marginal	--	[25]
N. perspicua	8.0–55.0	1.5–4.0	44–45	14–19	rhombic-lanceolate to narrow linear-lanceolate	gradually narrowing rounded	marginal	--	[27]
N. pusilla	8.0–33.0	2.5–5.0	43–55	14–20	linear-lanceolate to linear	broadly rounded, slightly protruding in larger forms	marginal	--	[27]

Table 1. Morphological characteristics of Nitzschia arslaniana, N. szaboiana, and N. efeiana sp. nov., with other Nitzschia taxa sharing similar morphological features (--: no information).

	Valve Length (µm)	Valve Width (µm)	Striae (in 10 µm)	Fibulae (in 10 µm)	Valve Shape	Apex Shape	Keel Shape	Copulae	Reference
N. arslaniana	12.0–14.0	2.0–2.5	27–29	13–15	linear	broadly rounded, cuneate	elevated with conopeum, recessed at center	open bands, 4+, 2 rows of large pores	this study
N. szaboiana	13.0–17.0	3.0–4.0	47–50	14–16	linear-elliptic	broadly rounded, cuneate	round, elevated, terminates at end on valve face	--	this study
N. efeiana	20.0–43.0	3.5–4.0	23–27	6–10	linear, linear-lanceolate	capitate	flat, marginal, terminates at apex	Open bands, 4+, one row of small round pores	this study
N. acidoclinata	8.0–45.0	2.5–3.0	27–34	10–16	linear-lanceolate to narrowly linear	shortly subrostrate	--	--	[25]
N. anatoliana	7.8–16.1	2.7–3.7	48–52	20–23	linear-elliptic	broadly rounded	marginal	--	[26]
N. bremensis	60.0–90.0	6.0–9.0	26–32	5–9	linear	rounded to flat truncated	Submarginal, broad, on valve face, elevated, with conopeum?	-	[27]
N. capitellata	20.0–70.0	3.5–6.5	23–40	10–18	linear, linear-lanceolate	rounded, wedge shaped	marginal, small	--	[27]
N. homburgiensis	32.0–52.0	5.0–6.0	34–40	9–15	linear, strongly concave in the middle	rounded, wedge shaped	submarginal	--	[27]
N. hybrida	34–125	5–9	22–26	9–12	linear, strongly concave in the middle	wedge shaped	marginal, round, elevated, recessed	--	[26]
N. inconspicua	6.0–11.5	2.6–3.1	24–29	11–17	linear to lanceolate, elliptic-lanceolate	slightly protracted, narrow	marginal, round, not elevated, interrupted at center	copulae 4+, open bands, one row of pores	[24]
N. frustulum	10.8–34	3.0–3.9	27–30	13–15	linear-lanceolate to lanceolate, elliptic-lanceolate	slightly protracted, narrow	marginal, round, not elevated, terminating along margin at apex	open copulae, one row of pores	[24]
N. palustris	35.0–60.0	4.0–7.0	22–28	6–10	linear	rounded to flat truncated	marginal, interrupted at center	--	[28]
N. pellucida	--	--	30–40	12–18	linear-lanceolate	boat-shaped	submarginal, round, elevated, terminating on valve face at end	--	[27,29]
N. perminuta	8.0–45.0	2.5–3.0	26–36	10–16	linear-lanceolate to narrowly linear	subrostrate to subcapitate	marginal	--	[25]
N. perspicua	8.0–55.0	1.5–4.0	44–45	14–19	rhombic-lanceolate to narrow linear-lanceolate	gradually narrowing rounded	marginal	--	[27]
N. pusilla	8.0–33.0	2.5–5.0	43–55	14–20	linear-lanceolate to linear	broadly rounded, slightly protruding in larger forms	marginal	--	[27]

Distribution. Observed only in the type locality.

Ecology. The water temperature ranged from 25.3 to 27.8 °C, pH was 2.1–2.4, dissolved oxygen (DO) was 5.2–7.4 ppm, and electrolyte conductivity was 2.4–2.7 µS·cm⁻¹.

Associated diatom flora. The taxon was rare, found in the sample with Eunotia exigua (29.4 %), Nitzschia tubicola s.l. (18.7 %), and Pinnularia acidophila (8.3 %) dominating the community.

4. Discussion

Nitzschia species observed in this study align with documented species, but with finer examination, they have distinct morphological differences that can be discerned using LM and SEM. However, the addition of genetic data in the future will further delineate the relationship between these species (populations) of Nitzschia and other taxa (populations) within clades 6B, 8A, and 8B of [5]. As one reviewer of this manuscript noted, severe environments, like acidity, are not selected in specific genetic clades (based on 4-genes) but extend across clades [5]. Future work with a deeper integrative assessment (environment and genetics) may reveal interesting environmental drivers within genetically identified taxonomic clades.

Nitzschia efeiana sp. nov. has a similar valve morphology to brackish and marine species like N. homburgiensis, N. hybrida, N. pellucida, and N. capitellata, while also being comparable in valve form to circumneutral species like N. palustris and slightly acidic species like N. bremensis. Similar valve morphologies across the broad pH/alkalinity gradient in taxa from the subgenera Dubiae/Bilobatae (even from the subgenus Lanceolatae) highlight the difficulties in taxonomic identifications. From the six prominent Nitzschia reported from extreme acidic environments, four (N. capitellata, N. communis, N. thermalis, N. vasta) are considered brackish marine taxa, and one (N. pusilla) has an uncertain autecology [4,27]. A closer look at these reported lobate species from acidic environments should reveal new species or morphs that are likely not taxa associated with brackish marine or highly alkaline environments. Based on both environmental and morphological characteristics, this taxon is likely positioned within the Lanceolatae Clade 6B [5].

Nitzschia efeiana displays an asymmetric, linear shape in larger forms and linear-lanceolate in smaller valves, which have a similar shape to many Nitzschia taxa. Nitzschia bremensis, N. homburgiensis, N. pellucida, and N. palustris are easy examples of a similar valve form. N. bremensis was described from Bremen by Hustedt [18] and prefers slightly acidic electrolyte-poor waters [27]. The taxon is significantly larger than N. efeiana (60–90 µm length and 6.0–9.0 µm width vs. 20–43 µm length and 3.5–4.0 µm width, respectively) and has a higher striae density (26–32 in 10 µm vs. 23–27 in 10 µm, respectively). N. homburgiensis Lange-Bertalot also prefers electrolyte-poor waters, which are not dystrophic; this taxon has a similar size but a higher stria density. N. palustris was described from Germany by Hustedt [21], prefering circumneutral, electrolyte-rich waters [27]. It is of a similar length to, but wider than, N. efeiana (4.0–7.0 µm vs. 3.5–4.0 µm width, respectively). In contrast, N. pellucida is large and narrow, with indistinct striae, and found in brackish waters along seacoasts.

Nitzschia szaboiana has a linear-elliptic outline with broadly rounded apices. Nitzschia anatoliensis Górecka, Gastineau and Solak, N. perspicua Cholnoky, and N. pusilla Grunow are similar taxa. N. anatoliensis was described by Solak et al. [22] from Van Lake in Türkiye. The taxon prefers highly alkaline waters. It has a higher fibulae density compared to N. szaboiana (14–16 in 10 µm vs. 20–23 in 10 µm, respectively). N. perspicua was described by Cholnoky [23] from Swartkops Lagoon in Port Elizabeth, South Africa. The taxon has a lower striae density than N. szaboiana (44–45 in 10 µm vs. 47–50 in 10 µm, respectively). Another similar taxon is N. pusilla. The taxon was described by Grunow [9] in Austrian diatomaceous deposits. Regarding the outline, the taxon is quite similar to N. szaboiana. However, N. szaboiana has a more elliptic outline and lower striae density than N. pusilla (14–16 in 10 µm vs. 14–20 in 10 µm, respectively). Additionally, N. pusilla has larger valves (Table 1). Like N. efeiana, based on ecology and morphological characters, this taxon is likely positioned within the Lanceolatae Clade 6B [5].

Nitzschia arslaniana has linear valves, slightly constricted at mid-valve, with cuneate, wedge-shaped endings. N. inconspicua, N. frustulum, N. acidoclinata, and N. perminuta are similar in outline. Nitzschia inconspicua was described by Grunow [9] from Vienna in a warm-water pond and generally observed in freshwaters with medium to moderately high electrolyte content compared to an extreme acidic environment for N. arslaniana. N. inconspicua is slightly broader than N. arslaniana, and the later taxon has fewer fibulae (>13 fibulae in 10 µm). The type material of N. inconspicua together with N. frustulum in the Grunow Collection in the Naturhistoriches Museum Wien was studied by Trobajo et al. [24]. Regarding ultrastructure under SEM, N. inconspicua has larger areolae than N. arslaniana (0.10 versus 0.12 μm diameter). Externally, N. inconspicua has strongly hooked distal raphe endings, while N. arslaniana has “L”-shaped endings, and internally, the keels are flat, with long keels in N. inconspicua, while N. arslaniana has thick, bridge-like, and rarely long keels. N. frustulum and N. sp. [cf. N. perminuta] (arctic form, Antoniades et al. [30]), like N. inconspicua, prefer higher pH conditions and low to moderately high electrolyte content. N. frustulum was described by Kützing [31] as Synedra frustulum, and then combined by Grunow in Cleve and Grunow [20]. It is linear-to-lanceolate, with slightly protracted endings; a small canal nodule exists in the central area, and externally, the proximal raphe endings are slightly depressed, while the distal raphe endings are hooked. N. arslaniana has a linear outline, and the raphe is continuous (no central nodule). Moreover, striae density is lower in N. frustulum (23–26 in 10 µm) than in N. arslaniana (27–29 in 10 µm). Considering the size, N. arslaniana is smaller than N. frustulum (12.0–14.0 µm length and 2.0–2.5 µm width vs. 20.0–70.0 µm length and 3.5–6.5 µm width, respectively). N. perminuta, as described by Grunow in Van Heurck [32], is linear-lanceolate to narrowly linear, with subrostrate to subcapitate endings, and is wider than N. arslaniana (2.5–3.0 µm width in N. perminuta). N. acidoclinata was described by Lange-Bertalot [33], preferring electrolyte-poor waters. The taxon is linear-lanceolate to narrow-linear, with shortly subrostrate endings, and has higher stria and fibula counts. Based on both environmental and morphological characters, this taxon is likely positioned within the Lanceolatae genetic Clade 6B [5].

In this study, the described species are found in highly acidic lakes in Çanakkale, Western Anatolia. The diatom community is generically similar to that of other acidophilic systems [4,6]. The acidophiles Eunotia exigua, Nitzschia sp. [cf. N. tubicola], and Pinnularia acidophila are dominant taxa in the samples. Eunotia exigua is classified as an acidobionthic taxon [25,34] and considered an “acid rain phenomenon” taxon [35]. DeNicola [4] reported E. exigua from lakes in the lignite area in Germany at pH 2–3 [36], as also observed in highly acidic springs in Yellowstone National Park, U.S.A., by Whitton and Diaz [37]. Pinnularia acidophila was found in an opencast mine lake with a low pH value by Hofmann and Krammer in Krammer [26]. The similarities in taxonomic compositional structure from extremely acidic anthropogenically disturbed sites around the world hint at deterministic processes in taxonomic community structure, with the autecological preferences of selected genera (e.g., Eunotia, Frustulia) impacting colonization. In contrast, broad-spectrum-selecting genera, like Nitzschia and Pinnularia, may have other drivers for selectivity of valve shapes and morphology.