Discostella gabinii Paillès & Sylvestre, 2020

Paillès, Christine, Sylvestre, Florence, Tonetto, Alain, Mazur, Jean-Charles & Conrod, Sandrine, 2020, New fossil genus and new extant species of diatoms (Stephanodiscaceae, Bacillariophyceae) from Pleistocene sediments in the Neotropics (Guatemala, Central America): adaptation to a changing environment?, European Journal of Taxonomy 726, pp. 1-23 : 10-15

Discostella gabinii Paillès & Sylvestre sp. nov.

Figs 41 –58

Etymology

This taxon is named in honor of deceased Gabin Sylvestre, the courageous 7 years old nephew of F. Sylvestre.

Type material

Holotype

Slide PC0608732 and sediment PC0608730 deposited at the Laboratoire de Cryptogamie, Muséum national d’histoire naturelle ( MNHN) Paris, France. Specimen on slide PC0608732 ( Fig. 45 ) represents the holotype designated here.

Isotype

Slide ZU 11/31 and sediment R1285 deposited at the Friedrich Hustedt Diatom Center in Bremerhaven, Germany.

Other material examined

Modern specimens collected from Cenote Juarez and Lake Amatitlan (see Table 1 ).

Type locality

GUATEMALA • Department of Petén, Lake Petén-Itzá ; 16º15′50″ N, 89º15′00″ W; lacustrine sediment in core PI-06; sample GLAD9 - PET06-6 B-18E1- 35.4– 36.4 cm (51.53 m below lake floor) consisting of dark gray clayish sediment; core collected in February 2006 .

Description

Light microscopy ( Figs 41–50 )

Cells quadrangular in connective view. Valvar views circular and flat, 8–18 µm in diameter with a small central area (¼ of the valve radius). Central area with 5 to>30 scattered large areolae, the number being independent of valve size ( Figs 41–50 ). When numerous, the scattered areolae give the impression of a colliculate/granular flat center. The marginal area of the valve face has radial striae numbering from 10 to 14 in 10 µm. The striae are long (¾ of the valve radius) and of equal length. On large specimens, marginal striation is crossed circumferentially by a ring (ʻSchattenlinieʼ = ʻshadow lineʼ) close to the valve center ( Figs 48–50 ).

Scanning electron microscopy (Figs 51–58)

Valves flat to barely concave externally with gently sloping mantle. Central area covered with several scattered punctae separated by knots (colliculate) bearing papillae (Figs 51–52). Radiating striae starting on the mantle as crescents of three to five rows of fine areolae (60–70 areolae/ 10 µm), merging into two rows near the central area and ending with a single large pore (Figs 53–54). The central area is thus bordered by a ring of large areolae. On the valve face, striae are depressed, whereas they are smooth on the mantle. Near the valve margin, every third to fifth striae, pores just below the crescent of fine areolae mark the external openings of marginal fultoportulae (Fig. 54). Interstriae are narrow, domed and granular on the valve face, whereas smooth on the mantle. The mantle is unornamented except for the large round openings of marginal fultoportulae and few papillae. The external opening of the rimoportula was not observed, although it should be positioned at the same level since it is within the ring of marginal fultoportulae.

Interior views of the valve show a flat to slightly concave but smooth central area with none or single areola (Figs 55–56). The internal lamina spread from the valve center to ¾ of the valve radius. The alveoli are thus medium sized, oblong and of unequal length, those bearing marginal fultoportulae being longer (Fig. 56). Marginal fultoportulae with two laterally positioned satellite pores surrounding a short tubulus are located at the distal extremity of every third to fourth alveoli (Fig. 57). One nearly sessile rimoportula with vertically orientated lips located between two costae at the edge of an alveolus and within the ring of marginal fultoportulae (Fig. 58). Girdle bands present, an open valvocopula with two copulae (Fig. 56); a row of fine pores is noted on the interior of the girdle band (Fig. 58).

Time range

Present since at least 84 ka in the geological record, present in Lake Amatitlan and Cenote Juarez (20°48′09.6″ N, 87°27′23.8″ W) in March 2008.

Remarks

With marginal fultoportulae and rimoportula being located between costae on the marginal side of the alveolus, Discostella gabinii sp. nov. belongs undoubtedly to the genus Discostella . Amongst the 15 species of Discostella described so far ( Kociolek et al. 2018), D. gabinii sp. nov. showed some resemblance to D. areolata (Hust.) Houk & Klee. However, in LM they look somewhat different, the unique holotype of D. areolata having coarser striation (6–9 striae in 10 µm) and a large colliculate central area ( Houk et al. 2010: table 330, figs 1–7). A reexamination of the original material of D. areolata from Hustedt by Tagliaventi & Cavinaci (2002) provided unambiguous SEM images of external views but only ambiguous internal views since D. areolata was rare and mixed with D. stelligera (Cleve & Grunow) Houk & Klee var. robusta (Hust.) Houk & Klee in the original material. The central area of D. areolata is concave or convex, smooth or consisting of alternating impressions and protrusions of various size with small punctae being mainly located in the depressions. Sometimes domed radiating striae resembling a poorly defined rosette are present in the central area. In D. gabinii sp. nov., the central area is always flat, indeed colliculate but with large punctae inserted in the depressions. Moreover, in D. areolata , striae are depressed and costae elevated on their entire length, whereas in D. gabinii sp. nov., this feature is restricted to the valve face, the mantle being smooth. Internally, two types of central area could be attributed to D. areolata : smooth with no central fultoportula or smooth with a punctum. These variations are also visible in D. gabinii sp. nov. Despite uncertainties related to the species described as D. areolata , D. stelligera var. robusta and D. stelligera var. hyalina (Hust.) Houk & Klee , the structure of marginal costae and the position of marginal fultoportulae and rimoportula are quite different compared to that of D. gabinii sp. nov. Marginal costae can be forked or not. Furthermore, marginal fultoportulae (composed of one tube and two satellite pores placed horizontally) and rimoportula (vertically orientated slit) are inserted within the alveolar chamber.

Another somewhat similar species is D. elentarii (Alfinito & Tagliaventi) Houk & Klee with flat valves, although it has a large central area with radiate rows of granules and scattered punctae, coarsely striated (9–10 striae in 10µm) and reduced marginal area, and a marginal row of small spinae. Internally, it has similar smooth central area (sometimes with a faint stellate pattern) and similar structure and position of mfp and rm. The only difference is that, internally, in D. elantarii costae are broadening toward the valve margin with a punctum in the middle giving the impression of forked costae. After reexamination of D. elantarii by Knapp et al. (2006), it appears that the correct striae density is 8–14 and that each collared marginal fultoportula and the single rimoportula are surrounded by satellite pores covered by a cribum. Although we did not use a field emission variable pressure SEM, such structures are absent in D. gabinii sp. nov. Interestingly, it is the only morphological feature used to differentiate D. elantarii from D. stelligera in SEM ( Knapp et al. 2006). The presence of pores in the girdle band is also a subtle character shared by D. elentarii and D. gabinii sp. nov. that requires further investigation. Despite morphological similarities with D. areolata and D. elentarii , D. gabinii sp. nov. possesses distinctive characteristics that are sufficient to define a new species. Stelligeroid species of Cyclotella have been transferred to the genus Discostella on the basis of the unique position of strutted and labiate processes ( Houk et al. 2010). However, difficulties arise because these species are often heterovalvate and size and morphological variations exist. As reported by Tagliaventi & Cavinaci (2002), Alfinito & Tagliaventi (2002) and Knapp et al. (2006), only minute distinctive features allow one to differentiate D. areolata , D. stelligera , D. stelligera var. robusta , D. stelligera var hyalina and D. elantarii . This latter species is endemic to New Zealand and coexists with D. stelligera in two lakes. Knapp et al. (2006) suggest that considering the difficulty in differentiating them, they could be sibling species and D. elantarii may descend from D. stelligera .

Stratigraphic diatom succession

The base of the section (84 ka) is characterized by an assemblage dominated (58–90%) by Aulacoseira granulata (Ehrenb.) Simonsen and A. ambigua (Grunow) Simonsen ( Fig. 59 ). Cyclotella meneghiniana and Discostella stelligera occurred punctually (<20%) between 82.3 and 80 ka. The Aulacoseira dominated assemblage persists up to 70 ka then greatly recedes (<20%) up to 1.5 ka. From around 60 ka, we observed successive occurrences of D. stelligera , C. meneghiniana , Discostella gabinii sp. nov. and Cyclotella caspia Grunow. At 45 ka, Cyclotella petenensis takes over the assemblage (73–97%) then declines abruptly at 31.5 ka. Prior to the collapse of C. petenensis , Discostella gabinii sp. nov. returned with fluctuating percentages for about 5 ka. Then, Cyclocostis rolfii gen et sp. nov. emerges at 26.9 ka, develops with fluctuating abundances with Nitzschia amphibioides Hust. , Mastogloia smithii Thwaites , M. elliptica (C.Agardh) Cleve and Navicula seminuloides Hust. At 22.2 ka, Cyclocostis rolfii gen et sp. nov. disappears definitely while Cyclotella petenensis reoccurs. At first, C. petenensis coexists with Discostella gabinii sp. nov. (10–60%) then it takes over when D. gabinii sp. nov. declines. The dominant C. petenensis persists until 16.1 ka and does not reoccur thereafter in the sequence.

Ecology and associated diatom flora

In the modern dataset ( Pérez et al. 2013), Discostella gabinii sp. nov. was initially identified as “ Cyclotella sp22 ” (code CP22) and another species was identified as “ Discostella aff. pseudostelligera ” (CYAP). When analyzing the fossil flora and diagnosing D. gabinii sp. nov., we re-examined modern samples and observed that the two species were similar. “ Discostella aff. pseudostelligera ” and “ Cyclotella sp22 ” were therefore combined together under the name Discostella gabinii sp. nov.

Conductivity, which is related to the precipitation gradient and marine influence on the Yucatan Peninsula, is the main variable that structures diatom, ostracod and cladoceran communities ( Pérez et al. 2013). Discostella gabinii sp. nov. occurs in 11 water bodies of the Yucatan Peninsula ( Table 1 ). In the Guatemalan highlands, its maximum occurrence (13%) was in Lake Amatitlan, a hypereutrophic alkaline lake spreading over 15.2 km 2 at 1200 m a.s.l. altitude. Water was calcium-bicarbonate rich, warm (22.6°C), with electrical conductivity of 630 µS/cm and high dissolved oxygen content (17.8 mg /L). Subdominant species were Aulacoseira granulata , Cyclotella meneghiniana and Nitzschia pseudofonticola Hust. In Cenotes Juarez , D. gabinii sp. nov. reached 4.8% in an assemblage dominated by Achnanthidium exiguum (Grunow) Czarn. and A. lineare W.Sm. Water was warm (27.9°C) and alkaline with relatively high dissolved oxygen content (8.7 mg /L). Conductivity was 643 µS/cm. Water analyses determined Ca 2+ (68.3 mg /L) and Mg 2+ (23 mg /L) as the dominant cations and HCO 3 - (292.7 mg /L) as important anion. Overall, D. gabinii sp. nov. seems to tolerate varying conductivities but is most abundant in alkaline, low conductivities (600–650 µS/cm) and calcium-bicarbonated waters.

Cyclotella petenensis , although considered to be fossil at the time of description ( Paillès et al. 2018), was identified as C. meneghiniana (CYMG) in the modern dataset mainly due to the fact that specimens were small in size, tangentially undulated with <5 valve face fultoportulae on the raised part. Once diagnosed as a new species in the sedimentary record, a re-examination of modern samples revealed that C. petenensis was present in low percentages (<4%) in five water bodies. Only in Lake Yalahau (Yucatan lowlands), C. petenensis reached 17.8% ( Table 1 ). Of all water bodies investigated, Lake Yalahau had the highest diatom species richness. In this lake, water is shallow, warm (28.8°C) and alkaline (pH 8.9) with a high dissolved oxygen content (8.7 mg /L). Electrical conductivity is high 2350 µS/cm. Water was magnesium (136.8 mg /L) and bicarbonate (707.4 mg /L) rich. Its diatom population was composed of 33% of C. meneghiniana accompanied by Brachysira australofollis Lange-Bert. & Gerd Moser , B. neoexilis Lange-Bert. , Encyonema densistriata Novelo, Tavera & Ibarra and Fragilaria famelica (Kütz.) Lange-Bert. In coastal Lake Progreso where C. petenensis represents 4% of the flora, water conductivity was 2040 µS/cm. In the modern samples, it appears that C. petenensis seems to favor waters with conductivities close to 2000 µS/cm.

Figures

Figs 41–50. Discostella gabinii Paillès & Sylvestre sp. nov., Lake Petén-Itzá (Guatemala); LM valve views. 41–42. Modern specimens of D. gabinii sp. nov. from Cenote Juarez. 43–44. Modern specimens of D. gabinii sp. nov. from Lake Amatitlan. 45–50. Type material of fossil lacustrine diatom D. gabinii sp. nov. 45. Holotype (MNHN, slide PC060873). 48–50. A shadow line is visible in large specimens. Scale bar = 10 µm.

Fig. 59. Diagram showing the succession of Stephanodiscaceae Glezer & Makarova in Pleistocene sediments (0–84 ka) from Lake Petén-Itzá (Guatemala).