Sinerima marigela Nesterovich & Caissie, 2018

Nesterovich, Anna & Caissie, Beth E., 2018, Taxonomy and ultrastructure of Sinerima, a new genus of diatoms (Bacillariophyta), with a description of a new species, S. marigela, Phytotaxa 351 (3), pp. 197-209 : 200-203

Sinerima marigela Nesterovich & Caissie sp. nov.

Description

The valves are circular, 21–75 μm in diameter (n = 50), very delicate, concave, with a shallow mantle. Fine anastomosing ribs (costae sensu Hasle 1973) radiate from the centre and pores penetrate the basal layer between the costae; they are arranged as individual cribra or in continuous rows. Around the circumference only, anastomosing ribs are extended externally, producing an areolate structure represented by pseudoloculi. Pseudoloculi on the margin of the valve face are visible in LM, around 10–22 in 10 μm; they cover less than a third of the diameter. Toward the centre pseudoloculi gradually become flush with the surface until only cribra arranged in uniserial rows between the bifurcate radial ribs are visible, 9 to 16 rows of cribra in 10 μm. The central area of the valve is structureless or has siliceous thickenings. Around the circumference numerous fultoportulae are arranged in several ranks; 4–5 in 10 μm in the outmost ring, positioned one or two pseudoloculi away from the valve edge. A few fultoportulae also occur scattered on the valve face. The processes are not visible in LM. The fultoportulae lack developed external tubes, presenting as chambers with short tubes flush with the valve surface (marginal zone) or raised slightly above it (central zone). Internally the fultoportulae are small, with four, rarely three, satellite pores. No rimoportula is found.

Etymology

The word marigela is derived from the Latin words marituma, which means related to the sea, and gelo, which means frozen, and refers to the presence of the species in the North Pacific and its apparent association with sea ice.

Type locality

Chukchi Sea (71.599° N, 166.0053° W).

Holotype

Specimen with coordinates 17.2:11 ( Sterrenburg et al. 2012) on slide ANSP GC92687 ( Academy of Natural Sciences of Philadelphia, Fig. 6 shows the holotype)

Isotypes

Slides BM 101 881 and BM 101 882 (Natural History Museum, London), CANA 127977 (Canadian Museum of Nature, Ottawa), MSL-ISC 453227 (Marine Sediments Laboratory, Iowa State University, Ames).

Type material

ANSP sample GCM85002 (Academy of Natural Sciences of Philadelphia) collected as sample LB 6.5 by Dr. Caissie in 2008 in the Chukchi Sea.

Habitat

The species is described from sediment samples and thus the environmental preferences of the species are very hard to discern. However, since all the samples containing S. marigela were collected in a seasonally ice covered region, it seems reasonable to assume that the species is cold-water. The relative abundance of the species in our samples shows a positive relationship with sea ice concentrations, with a probable optimum at about 90% sea ice during the spring season ( Fig. 2 ).

LM observations

The valves appear flat or crumpled when flattened with a clearly distinct pseudoloculate marginal zone and the valve face with radial ribs and rows of perforations. No processes are visible in LM ( Figs 3–8 ).

SEM and TEM observations

External surface ( Figs 9–17 ). The marginal zone is covered by pseudoloculi formed by expansion of the distal parts of reticulate ribs ( Figs 9–11 ). Often, there are also “extra ribs” that don’t reach as high and seem to split a cribrum into sections ( Fig. 12 ). Marginal fultoportulae are placed between areolae, positioned at the junctions of ribs. The distal parts of the fultoportulae are wide at the base, the size of an areola, and narrow towards the end. The distal tubes don’t rise above the pseudoloculi ( Fig. 13 ).

The central part doesn’t have outward expansions of the ribs; the surface is level with the exception of slightly sunken cribra ( Fig. 14 ). The transition between the marginal and the central parts is gradual; the walls of pseudoloculi become lower and disappear over a distance of two to three areolae ( Fig. 15 ). The fultoportulae in the central part are smaller; the chamber is star-shaped with radii corresponding to satellite pores, as wide at the base as an areola, but soon narrowing into a small opening that doesn’t extend into a tube ( Figs 16–17 ).

Internal surface ( Figs 18–23 ). In most specimens, areolae are visible as individual cribra (sometimes they are merged to form a continuous cribrum), which are flat, but appear raised because each pore in a cribrum consists of a small tube and looks rimmed ( Figs 19–20 ). On the mantle of some specimens the pores are not rimmed and the cribra are flush with the surface ( Fig. 21 ). There are 10–22 cribral pores in 1 μm. On the mantle the areolae are arranged in a honeycomb structure, between the reticulate ribs ( Figs 20–21 ). The marginal ring of fultoportulae is one to three areolae from the edge. Each fultoportula has four, rarely three, satellite pores ( Fig. 22 ). The proximal tubes are short, the covers are triangular flaps of silica, and there is no cowling. On the valve the cribra are arranged in rows toward the centre. Several fultoportulae are scattered on the valve face, sometimes forming a second, less regular ring three to four areolae from the first ( Fig. 18 ). The structure of the valve face fultoportulae is the same as that of marginal fultoportulae ( Figs 20–22 ). The centre of the valve is either hyaline, has an amorphous or spiral siliceous knot formed by meeting of the ribs ( Fig. 23 ), or has several plaques ( Fig. 24 ); there is never a process. The areolation may be less dense toward the centre.

Differentiating diagnosis

The main feature uniting the genus Thalassiosira is the ability to form colonies held together by long threads, however, we don’t know what type of colonies, if any, Sinerima marigela makes. The new genus differs from Thalassiosira in lacking the characteristic areolae with cribra on the inside and foramina on the outside, since there are no loculate areolae in the species. It also has fultoportulae with short distal tubes, though this character appears in some Thalassiosira species, especially in the group with eccentric rows of areolae, and lacks rimoportulae, which all species in the Thalassiosira genus possess. It should be noted that some of the Thalassiosira species don’t possess the loculate areolae and have only the poroid basal layer with ribs on the valve face, obligatory as in Thalassiosira oceanica Hasle ( Hasle 1983: 220) or occasionally as was documented for T. eccentrica ( Ehrenberg 1841: 146) Cleve (1904: 216) ( Fryxell & Hasle 1972), T. antarctica Comber (1896: 491) , and many others ( Makarova 1988).The latest diagnosis of the genus Thalassiosira probably should be either emended or split to properly accommodate those species. Sinerima still differs from all those species by not having a process in the centre, which most of Thalassiosira species have, having short proximal and distal fultoportulae tubes, and no rimoportula.

Though Sinerima may at first glance look similar to Lauderia Cleve (1873a: 8) , because both genera have radiating ribs on the valve face with poroid basal layer between them, a pseudoloculate structure around the circumference, and fultoportulae in rings on the circumference and scattered on the valve face, there are significant differences that don’t allow Sinerima to be placed into the family Lauderiaceae (Schütt) Lemmermann. The description of the family Lauderiaceae includes fultoportulae with long external tubes and occluded processes ( Round et al. 1990) or, in the very least, a central annulus ( Kaczmarska et al. 2005), all of which Sinerima lacks. Besides, all members of the family Lauderiaceae so far possess a fultoportula.

There is also a certain similarity in areolation with Bacterosira . The valve of a Bacterosira vegetative cell consists of the poroid basal layer with ribs that become reticulate and extend to form a pseudoloculate structure around the circumference. However, both species of Bacterosira have a cluster of fultoportulae in the centre surrounded by a hyaline area and no fultoportulae scattered on the valve face. Sinerima , on the other hand, never has processes in the centre, only the scattered fultoportulae. Moreover, the structure of the valve face fultoportulae is different: in Bacterosira they open externally as rimmed pores or short tubes without visible chambers (see Figs 23 –27 and 40 in Park et al. (2016)), while Sinerima ’s valve face fultoportulae have a developed chamber that opens as a short tube ( Fig. 16 ).

Figures

FIGURE 2. Relative abundance of Sinerima marigela plotted against sea ice concentration for each sample with a local regression line fitted to illustrate the trend. Sea ice concentration is the average proportion of each 25 km2 area covered by sea ice during the spring.

FIGURE 24. TEM micrograph of Sinerima marigela sp. nov. Note the lack of a rimoportula and siliceous thickenings around the center. Scale bar = 5 μm.

FIGURES 3–8. LM micrographs of Sinerima marigela sp. nov. showing variations in valve shape and size. Figs 3–5. Specimens from slide BM 101 881. Fig. 6. Holotype specimen, slide ANSP GC92687. Figs 7–8. Specimens from slide CANA 127977. Scale bar = 10 μm.

FIGURES 9–17. SEM micrographs of Sinerima marigela sp. nov., external valve view. Figs 9–10. Whole valve. Figs 11–13. Valve margin. Note the “extra ribs” separating some of the cribra and marginal fultoportulae. Fig. 14. Central part of a valve. Note the sunken cribra and a hyaline area in the center. Fig. 15. Marginal fultoportula at the inner edge of the pseudoloculate zone with its external tube extending above the smaller reticulate ribs. Fig. 16. Valve face fultoportula with a star-shaped chamber. Fig. 17. Eroded valve face fultoportula showing the openings of the satellite pores. Scale bar = 20 μm (Figs 9–10), 2 μm (Figs 11, 14), 3 μm (Fig. 12), 1 μm (Fig. 13), 500 nm (Figs 15–17).

FIGURES 18–23. SEM micrographs of Sinerima marigela sp. nov., internal valve view. Fig. 18. Whole valve. Fig. 19. Central part of a valve. Note the raised cribra with rimmed pores and a hyaline area in the center. Figs 20–21. Valve margin. Note the marginal fultoportulae with four satellite pores. Fig. 22. Valve face fultoportula. Note the raised cribra with rimmed pores. Fig. 23. Siliceous knot in the center of a valve. Scale bar = 10 μm (Fig. 18), 2 μm (Fig. 19), 1 μm (Figs 20–21, 23), 500 nm (Fig. 22).