Species Description: In apical view (Figures 1, 2) cells are nearly circular, with a small concavity at the ventral surface that corresponds to the sulcus location. In lateral or side view, cells are lenticular and strongly compressed anterior-posteriorly (Figures 3, 4). The epitheca and hypotheca are somewhat unequal in height. The epitheca is conical and occasionally slightly concave. The hypotheca is more rounded and slightly flattened at the antapical end. The cingulum is equatorially located and lacks displacement in its circumference. The thecae are easily separable and, because of their shape, orient themselves in a position where the plates are quite visible (Figures 1, 5, 7). The cells are thecate, with plates numbered in the Kofoidean system. With a few exceptions, a typical plate arrangement is as follows: the epitheca has a pore plate with two curved slits, seven apical plates and 12 precingular plates; 12 cingular plates and eight sulcal plates; the hypotheca has 12 postcingular plates, three posterior intercalary plates and three antapical plates. The abbreviation of this plate formula is (Po, 7’, 0a, 12’’, 13c, 8s, 12’’’, 3p, 3’’’’). There is considerably more variability in posterior intercalary plates than in others. Consequently, authors have identified the plate affinities differently (Matsuoka 1985, Figure 2). For example, the posterior intercalary plates in Figures 5 and 6 are generally interpreted as (12’’’, 1p, 5’’’’) but could be interpreted as plates as (12’’’, 3p, 3’’’’). Matsuoka (1985) also reported that cells in culture were more variable in their plate numbers than those from natural samples. Radially arranged short stripes (hachures) sometimes occur on precingular plates of the epitheca (Figure 7). Most thecate dinoflagellates are fairly conservative in the distribution of hypothecal plates; Pyrophacus is an exception. For the genus as a whole, the plate formula is (Po, 5-8’, 0a, 9-12’’, 9-14c, 8s, 9-14’’’, 1-11p, 3’’’’). The cells of P. steinii are unicellular and photosynthetic, with numerous golden brown chloroplasts. P. steinii has two heterodynamic flagella, and the diameter is reported to be 82-191µm (Wall & Dale 1971). As in other members of the genus, P. steinii is bioluminescent. There have been no reports of toxicity in this species.
Habitat & Regional Occurrence: In part, the distribution of Pyrophacus steinii depends on one’s view of the taxonomy. Taylor (1976) for example, recognized three separate species, with P. steinii the most common and widespread in Indian Ocean samples, and mostly oceanic in distribution down to 27°S. With a distribution closer to landmasses, P. vancampoae is much less common; likewise for P. horologium. Both P. horologium and P. steinii are widespread and common in the North Atlantic. Though P. steinii has a preference for warmer water, it was recorded from 48°N by Gaarder (probably introduced as part of the North Atlantic Drift). There are also many records of P. steinii from the Pacific Ocean, primarily tropical or subtropical locations. The cysts have been found in surface sediments where the surface water temperature was 12.7-29.5°C and the salinity was 16.9-36.6 psu (Zonneveld & Susek 2007). Its presence in the Baltic Sea (Ostsee) suggests a tolerance for brackish conditions, and Faust (1998) suggests that, at least in mangrove environments, P. steinii is mixotrophic. There is some evidence that P. steinii is sensitive to pollution. Inhibition of its bioluminescence by anthropogenic contaminants has been suggested as a novel method to determine lethal and sublethal contaminant levels in environmental risk assessment (Lapota et al. 2007).
Indian River Lagoon Distribution: In the IRL, all observed cells have been vegetative cells of P. steinii: neither horologium cells nor hypnozygotes were seen. Occurrences have been rare, never more than a few cells per liter, and confined to the summer (25°C or higher) and salinities of 30-36 psu. Wherever found, P. steinii is never abundant, though quantitative data are incomplete and cryptic.
Reproduction: Details of the life history of P. steinii have been studied by Faust (1998) and Pholpuntin et al. (1999) and are summarized in Figure 9. Vegetative (asexual) reproduction is by nuclear division binary fission inside the parent cell, with the two (rarely four) daughter cells breaking the parent cell apart and forming new thecae and flagella (Figure 9: 6 & A1). Sexual reproduction is anisogamous and heterothallic (male and female of different size and morphology; Figure 9: 2B,C). Male gametes are smaller and rounder than vegetative cells, with a different plate arrangement, whereas female cells are indistinguishable from vegetative cells. Eight or 16 sperm cells are released from one male cell (Figure 9: 2B). The sperm cell enters the female cell, usually where the cingulum and sulcus meet, and the gametes fuse. This normally takes place in the dark. The resulting cell with two trailing flagella (the planozygote; Figure 9: E) is motile for two or three days, then transforms into an inactive resting cyst, the hypnozygote (Figure 8, Figure 9: 4F). The hypnozygote is the form called Tuberculodinium by micropaleontologists.
In most dinoflagellates, the germination of the hypnozygote (Figure 9: 5) takes place only after a refractory time (obligate dormancy), which varies from weeks to months. The refractory period for P. steinii is unknown, though Faust (1998: 176) implies that it is short. In a series of experiments with cultures grown from cysts isolated from Japanese coastal waters, Zonneveld and Susek (2007) found that cyst production could take place at 16.5-34.8°C, with highest cyst production at 27°C; and at 20-45 psu, with highest cyst production at 35 psu in moderate to strong light conditions.
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Zonneveld, K & E Susek. 2007. Effects of temperature, light, and salinity on cyst production and morphology of Tuberculodinium vancampoae (the resting cyst of Pyrocystis steinii). Rev. Palaeobot. Palyno. 145: 77-88.