Rhizosolenia setigera Brightwell
R. setigera is an elongate, unicellular, photosynthetic diatom
The general shape is tubular, with a length (pervalvar axis) to width (diameter) ratio often in excess of 20:1. Each valve terminates in a spine (the external part of the rimoportula) of variable length and morphology.
Each cell contains many irregularly elliptical chloroplasts, distributed parietally throughout the cell. The nucleus is also parietal, and usually located approximately mid-length. Cytological events during valve morphogenesis in cell division have been documented by Van De Meene and Pickett-Heaps (2004). These authors found that after cytokinesis the rimoportula (labiate process) rotates 5-8 turns in one direction, pauses, then rotates similarly in the opposite direction. Similar events have been seen in no other diatom.
Cells are straight or slightly curved, tubular, and solitary except in rapidly growing populations, where several cells may be clustered in parallel. The valves are conical and end in long, slightly oblique spines. Depending on the variety, the external part of the rimoportula (spine) may be slightly wider at the base for some distance, then tapering gradually to a fine, hairlike tip (var. setigera). In var. pungens, the spine is initially narrow, widens more or less abruptly for a portion of its length, then narrows to a hairlike tip. The interior part of the spine is a rimoportula (labiate process). In contrast to most other Rhizosolenia species, the valves lack otaria; claspers are rudimentary or lacking. (These are features which aid in linking adjacent cells together after cell division). The copulae (girdle bands) are in two dorsiventral columns, each band is approximately trapezoidal in shape and perforated by many tiny poroid areolae. In lateral view, bands appear as a zig-zag line. Occasionally, after cell division, the imprint of the long spine is visible in several of the copulae in a pervalvar direction (see Sunesen and Sar, 2007). Resting spores are formed in var. setigera but have not been confirmed in var. pungens. Spores are formed in pairs, torpedo-shaped and smooth, with or without a small proboscis on the spore epitheca. Details of spore structure are found in Hargraves (1976).
A number of nucleotide and protein sequences exist for R. setigera (www.ncbi.nlm.nih.gov) but there is some question as to whether accurate identification preceded the sequencing, or whether the name includes a complex of cryptic species.
In publications where var. setigera and var. pungens are differentiated, either as varieties or as separate species, the size distinction involves a smaller (pungens) and larger (setigera) separation. In practice, there is a complete size overlap between the two varieties. According to literature reports, variety setigera varies in width (diameter) from 2-50µm, with a length (apical axis) of ~100-725µm, without spine, and up to >1 mm including spine. Variety pungens varies from 4-20µm in width, with a length of 116-450µm, without spine, and a total length of 370-760µm
Ecology and Distribution
R. setigera has been classified as a species with north temperate distribution. However, it has been found throughout the world's oceans, from cold and warm north temperate to tropical to cold and warm south temperate, apparently excluded only from polar seas. It is primarily a species of coastal and estuarine environments, but is occasionally found in the open ocean. The variety pungens appears to prefer lower salinity than variety setigera.
Based on culture experiments and distribution records, R. setigera is eurythermal and euryhaline. It has been recorded in the temperature range of 2-34C, and salinities of 1.5-37 PSU. In culture experiments, Baars (1988) found that a North Sea strain grew best at 6-12C over a range of -1.5 to 26C, but did not grow beyond 20C. On the other hand, in the subtropical Indian River Lagoon, Florida, where the annual temperature range is about 15-32C, Phlips et al. (2009) found R. setigera at concentrations exceeding 4 million cells per liter. Likewise, Guillard and Kilham (1977) found substantial differences in growth rates over a temperature gradient, between tropical and temperate strains of R. setigera. These disparities suggest substantial variability in ecological preferences among geographically separated strains.
Rhizosolenia setigera var. setigera cells produce resting spore pairs in nature and in culture, probably due to nutrient depletion, especially nitrogen (Hargraves and French, 1984). Rhizosolenia setigera var. pungens apparently does not produce resting spores. For this species, resting spore formation is an asexual process.
Sexual reproduction is oogamous with uniflagellate sperm (up to 32 spermatozoids per cell). The zygote forms an auxospore that restores the maximum size dimension of the species. Details of sexual reproduction in diatoms can be found in Round et al (1990).
Evolution and Systematics
There are reports of R. setigera reliably occurring in the late Pleistocene (Jouse and Mukhina, 1978; Ryu et al., 2005) and from 2-3 MYA (Koizumi, 1992). It is unclear whether these reports are based on the presence of the theca & spine, or whether the spore (as Pyxilla baltica) was identifed. If these reports are accurate, the species already had a global distribution about two million years ago.
Rhizosolenia setigera was described by T. Brightwell in 1858. Generally accepted synonyms are Rhizosolenia japonica Castracane 1886 and Rhizosolenia hensenii Schuett 1900. Rhizosolenia pungens was described by Cleve-Euler in 1937. Rhizosolenia crassispina Schroeder shares many features with both var. setigera and var. pungens, and may also be synonymous. The resting spores were originally described as Pyxilla baltica Grunow in Van Heurck 1881. According to Hustedt (1930), R. setigera var. kariana Henckel 1925 is also subsumed into var. setigera.
Sundstrom (1986) postulated that R. setigera is not a "true" Rhizosolenia species, a conclusion supported by Hernandez-Becerril (1995), who also maintains separate species identities for R. setigera, R. pungens, and R. crassispina. The inclusion of R. pungens into R. setigera was first mde by Brunel (1970) as Rhizosolenia setigera Brightwell forma pungens (Cleve-Euler) Brunel.
Rhizosolenia setigera and R. pungens have alternately been considered separate species, varieties, or forms. The primary distinctions are the shape of the spine (gradually tapering in setigera; basally thickened, then abruptly hairlike distally in pungens) and the presence of resting spores (setigera only). These differences are clear, but when many cells are present, e.g., bloom conditions, one may easily find cells that are indeterminate between R. setigera and R. pungens, and sometimes cells with setigera spine structure on one theca, and pungens on the other theca can be seen. One may thus question separation at the species level. Whether the ternary names are appropriately 'subspecies', variety' or 'form' becomes a matter of personal conviction. In this discussion, 'variety' is used for convenience. Certainly the ability to form resting spores in some but not others implies a genetic differentiation at some level, as yet to be defined.
Rhizosolenia setigera is subject to pathogenicity by fungi (Johnson, 1967), viruses (Nagasaki et al., 2004) and protistan flagellates of uncertain affinity (Kuehn et al., 1996).
There are scattered reports of marine mortalities associated with natural populations dominated by R. setigera; and the species is known to produce monocyclic alkenes (Masse et al., 2004), but it is likely that any marine mortality is due to oxygen depletion upon bloom decay.