Gambierdiscus caribaeus, Vandersea, Litaker, M.A.Faust, Kibler, W.C.Holland & P.A.Tester

Mussai, Prakash, Larsen, Jacob & Jeewon, Rajesh, 2023, Molecular characterization and morpho-taxonomy of Gambierdiscus caribaeus Vandersea, Litaker, M. A. Faust, Kibler, W. C. Holland & P. A. Tester (Dinophyceae) from Mauritius Island, South-West Indian Ocean, Cryptogamie, Algologie 20 (1), pp. 1-11 : 5-7

MORPHOLOGY OF MAURITIAN STRAIN OF GAMBIERDISCUS CARIBAEUS

The Gambierdiscus cell (strain ISOG4) isolated from Mauritius Island was observed using LM and SEM. The Mauritian strain was initially identified as Gambierdiscus caribaeus based on the observed morphological characteristics. Morphological details of the tested strain and those of other strains of G. caribaeus reported by others are summarised in Table 1 .

The Mauritian strain was photosynthetic with golden granular chloroplasts, round to slightly ellipsoid in apical view, compressed in dorsal-ventral view and lenticular in the lateral view ( Figs 2 A-C; 3C). The length, depth and width of SEM treated cells were 43.64-64.57 µm, 45.18-75.45 µm and 51.43-86.36 µm. The ratios of length to width and depth to width were 0.57-1.20 and 0.72-1.18, respectively ( Table 1 ). The epitheca and hypotheca surface were smooth and covered with many more or less round pores ( Fig. 2 B-C).

The Po plate touched three apical plates: 2’, 3’ and 4’ ( Fig. 2B ). The rectangular 2’ plate was the largest apical plate. The number of pores in 4’ plate varied from 72 to105 pores. The 4’’ plate was quadrangular and the 5’’ was pentagonal. The apical pore plate was elliptical, and the apical pore was fishhook-shaped ( Fig. 3A, B ). Close to the cingulum edge the observed precingular plates were smooth ( Fig. 3C ). The level of the right side and left side of the cingulum was nearly the same. Nevertheless, in the direction of the sulcus, the right part of the cingulum gradually ascended and then sharply descended near the sulcus.

The hypotheca consisted of eight plates: S.p., 5’’’ and 2’’’’. The quadrangular 2’’’ and 4’’’ plates were the largest of the postcingular plates. Also, the 1’’’, 3’’’ and 5’’’ plates were quadrangular. The 2’’’’ was broad, pentagonal, 37.4-48.81 µm long and 31.85-41.73 µm wide. The ratio of 2’’’’ plate length to width was 1.05-1.21 ( Table 1 ). The 2’’’’ plate contacts the postcingular plates 1’’’, 2’’’, 3’’’ and 4’’’ and the quadrangular hypothecal plates S.p. and 1’’’’.

PHYLOGENETICS OF MAURITIAN STRAIN OF GAMBIERDISCUS CARIBAEUS

The final LSU data matrix (841 bp in length) consisted of the Gambierdiscus strain isolated during this study, 47 Gambierdiscus strains and seven Fukuyoa strains. When properly aligned the DNA sequences of the Mauritian strain was similar to G. caribaeus LSU identifiers, namely EF202934 ( Belize strain),EF202929 ( Belize strain), KY448444 (Canary Islands strain), MN999499 (Bahía Concha strain),KY448432 (Canary Islands strain), KY448442 (Canary Islands strain), KR230005 (Hainan Island strain),KR230004 (Hainan Island strain), EF202932 ( Republic of Palau strain) and EF202930 (Mataiva, Tahiti strain), differing by 2 to 11 bp (similarities from 99.2 to 99.7%). However, wider nucleotide differences from 40 to 56 bases, were observed relative to G. jejuensis represented with Korean strain (HE775087) together with Japan strains AB605010 and AB604973 (similarities from 89.7 to 92.2%).

Bayesian inference (BI) and Maximum likelihood (ML) based on LSU sequences generated similar trees. DNA based phylogeny in Figure 4 shows that the Mauritian isolate is part of a clade containing G. caribaeus strains. The Gambierdiscus caribaeus clade was closely related to the clade consisting of G. jejuensis (strains HE775087, AB605010 and AB604973) and to the clade G. carpenteri Kibler, Litaker, M.A.Faust, W.C. Holland, Vandersea & P.A.Tester (strains EF202938 and EF202939). Gambierdiscus caribaeus , G. jejuensis and G. carpenteri grouped together with maximal support (Bayesian posterior probabilities: 1.00; ML bootstrap support values: 100), and they were a sister to G. excentricus S.Fraga (strains JF303063 and JF303065). Gambierdiscus caribaeus (11 seqs.) showed a rate of intraspecific evolutionary divergence at 0.01 while for G. jejuensis (3 seqs.) and G. carpenteri (2 seqs.) values were not obtained. The highest interspecific divergence was between G. jejuensis and G. carpenteri (0.094) and the lowest interspecific divergence was between G. caribaeus and G. jejuensis (0.056) ( Table 2 ).

Figures

FIG. 2. — Light and scanning electron micrographs of Gambierdiscus caribaeus Vandersea, Litaker, M.A.Faust, Kibler, W.C.Holland & P.A.Tester (ISOG4): A, light micrograph of Gambierdiscus caribaeus Mauritian strain; B, scanning electron micrograph of apical view; C, scanning electron micrograph of antapical view; D, scanning electron micrograph of inside top view of thecal plate pores. Scale bars: A, 50 µm; B, C, 10 µm; D, 100 nm.

FIG. 3. — Scanning electron micrographs of Gambierdiscus caribaeus Vandersea,Litaker,M.A.Faust, Kibler,W.C.Holland & P.A.Tester (ISOG4):A, elliptical Po plate with pores and a large fishhook-shaped opening; B, inside top view of broken cell showing the elliptical Po plate with pores and a large fishhook shaped open- ing; C, ventral view showing the gradually ascend and then sharp descend of the right part of the cingulum near the sulcus; D, thecal pores on the smooth cell surface. Scale bars: A, B, 1 µm; C, D, 10 µm.

FIG. 4. — Phylogenetic analysis of the Gambierdiscus caribaeus Vandersea, Litaker, M.A.Faust, Kibler, W.C.Holland & P.A.Tester (ISOG4), showing alignment of D1-D2 LSU rDNA sequences using Bayesian inference and Maximum likelihood analyses. Values at nodes represent Bayesian posterior probability support and Bootstrap support. - represents unsupported value. Scale bar is substitution per site.