Revision of the genusTetraselmis (Class Prasinophyceae)

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  • Bot. Mag. Tokyo 93: 317-339, 1980

    Revision of the Genus Tetraselmis (Class Prasinophyceae)

    RICHARD E. ~ORRIS*, TERUMITSU HORI**

    AND MITSUO CHIttARA**

    * Department of Botany and Friday Harbor Laboratories, University of Washington, Friday Harbor, Washington, U.S.A. 98250;

    ** Institute of Biological Sciences, The University of Tsukuba, Sakura.mura, lbaraki, 305

    Information available on tile structure of species belonging to the genera Tetraselmis, Platymonas and Prasinoeladus has been reviewed. Detailed comparison of these data has convinced the authors that species in these genera all belong to the same genus. Stalk development by cast-off thecae, a characteristic used to define Prasinocladus, is variable within different species and is not reliable for separation of a genus. Similarly, the penetration of the pyrenoid by a lobe of the nucleus cannot be held reliable in separation of these genera because it occurs in varying degree in different species. Platymonas G.S. West (1916), Prasinocladus Kuckuck (1894) and Aalacochlamys Margalef (1946) are considered to be synonyms of Tetraselmis Stein (1878). Tetraselmis is redescribed using characters visible with light and electron microscopy as well as life-history characteristics. The description reviews information on many species of Tetraselmis that have been found in the western and eastern Pacific as well as species from Great Britain. It is determined that variations in life-histories may be explained by different environmental factors, whereas structure of vegetative cells, as viewed by electron microscopy, seems to be quite stable and characteristic for each species.

    Key words: Aulacochlamys - - Platymonas - - Prasinocladws - - Prasinophyceae Taxonomy - - Tetraselmis.

    Tetraselmis Stein (1878) is a genus of green flagellates (Prasinophyceae) that

    contains many marine as well as a few freshwater species. Marine species often occur

    in dense populations causing blooms in tide pools or bays, thereby being important

    to an understanding of the dynamics of plankton growth in neritic habitats. Tetraselmis

    may also occur as a symbiont with marine animals (Provasoli et al., 1968).

    Tetraselmis has been recognized as being very close, if not the same genus, to

    Platymonas G.S. West (1916) (Parke and Green, 1976), but this uncertainty was

    recently decided by Melkonian (1979) who studied fine structural characteristics of the

    type species, T. cordiformis, and found that characteristics of this species are the same as

    those of the genus Platymonas. This important discovery corroborates the t reatment

    of Butcher (1959) who transferred several species of Platymonas to Tetraselmis and

    described several new species. Melkonian (1979) reviewed taxonomic problems that

  • 318 R.E. NoRRIs et al.

    have arisen because of the confusion between these two genera. Within Tetraselmis taxonomic decisions are difficult because reliable characteristics are not easily assessed with the light microscope, and also because there has been difficulty in recognizing limits in variability of cell characteristics.

    In the following discussion, the authors present an information and collation of previously published analyses of Tetraselmis, Platymonas and Prasinecladus, adding their own unpublished information on these genera. In this way, the authors hope to establish the generic limits of Tetraselmis and to faciliate delimitation of species within the genus. The species of the genus, however, will be more completely described in future publications.

    Materials and Methods

    Most specimens used in this analysis of the genus Tetraselmis were collected in various parts of Japan, Washington State, U.S.A., and British Columbia, Canada. Several British specimens of Tetraselmis were supplied by the Culture Centre of Algae and Protozoa, Cambridge, England, and were studied. All of the cells figured in this publication are indexed by culture numbers; cultures of the Department of Botany, University of Washington, have three numbers separated by two dashes, those of the University of Tsukuba have numbers preceded by T, and those of the Culture Collec- tion of Algae and Protozoa, Cambridge, have two numbers separated by/ .

    Cultures of Tetraselmis were grown according to the methods and media described by Norris and Pearson (1975) and Tanimoto and Hori (1975). Preparation of speci- mens for electron microscopy was done following the techniques outlined by Norris and Pearson (1975).

    Results and Discussion

    Living cells as observed with the light microscope Tetraselmis has at least three phases in its life-history (Provasoli et al., 1968;

    Kobara and ttori, 1975; Tanoue and Aruga, 1975); a flagellate stage, a non-motile vegetative phase and a third stage in which the cells become converted into an aflagellate cyst with a thick, often ornamented wall (Fig. 35). Such cysts germinate by dividing into four cells. The term 'cyst' should be reserved for this thick-walled phase that divides into four cells, an uncommon phase compared with the occurrence of the other two phases in Tetraselmis' life-history. Species of Tetraselmis often remain in a vegetative, non-motile phase for very long period of time, and undoubtedly it is the dominant phase in the life-history of certain species under some environmental condi- tions. During this non-motile phase new walls develop, old walls being cast-off and accumulating as concentric rings around the cell or becoming densely arranged and polarized on one side of the cell forming a stalk. Prasinocladus has a similar non- motile phase, but in Prasinocladus cast-off walls accumulate to form long acellular stalks subtending single cells or small groups of cells. Kylin (1935) and Proskauer (1950) discussed wall formation and stalk development in Prasinocladus and compare this

  • Revision of Tetraselmis 319

    development with similar wall formation in Tetraselmis (as Platymonas). Proskauer (1950) outlines the following series of events in Prasinocladus ascus: 1) cells settle and throw off the flagella, 2) the protoplast retracts from the anterior part of the wall, 3) the part of the wall that was in contact with the apical depression of the cell becomes everted, 4) if cells are in fresh media a new wall is secreted inside the original wall, 5) the new wall elongates and the outer wall is ruptured at its upper end, 6) the protoplast rounds off and lies at the upper end of the wall; as the wall grows in length a tube is formed behind the protoplast, 7) the protoplast develops a new wall inside the tube, 8) the protoplast may divide, always longitudinally, 9) one daughter cell slides past the other and the two cells lie obliquely in the apical end of the tube within their mother celt wall, 10) one of the daughter cells may rotate 180 ~ within the mother wall, 11) the upper cell may continue the formation of a tube, leaving the lower cell behind, or 12) both daughter cells may repeat the above process, producing two tubes, each breaking through the original wall, or 13) each daughter cell may continue to divide in situ.

    I f cells remain in nutrient depleted media, or are in some other way placed in an unfavorable environment, they may behave in an entirely different way. In this situation the protoplast forms successive walls, each of which is broken in turn without appreciable elongation at each step. This type of development is more typical of the species of Tetraselmis than Prasinocladus, but Tetraselmis species may not have polarized cast-off walls. Instead, the cast-off walls may form a concentric layer of wall material around the cell, the walls being broken in various positions, or, perhaps not broken at all but becoming somewhat gelatinized.

    It is important to note that cells at any of the stages discussed above may develop flagella and become motile. In such cases, therefore, the cast-off walls may be vacated entirely, or only one of a pair of daughter cells may swim from the cast-off walls leaving

    the other in a non-motile condition at the end of the stalk or inside the concentric walls. I t is also very important to note that for the life-history of the type species of

    Tetraselmis, T. cordiformis, the only stage that has been studied (Melkonian, 1979) is the flagellate cell. At the present time the authors do not have information on any non-motile cell in the life-history of that species.

    Hori and Chihara (1974a) showed that Prasinocladus marinus, when grown under favorable conditions, develops a stalk of cast-off walls by contraction of the cell and its subsequent emergence through a pore that develops in the posterior part of the wall, the cell remaining attached to the cast-off wall, forming a new one and repeatedly contracting and casting off old walls. Under unfavorable conditions the same species was shown by Hori and Chihara (1974a) to accumulate cast-off wails concentrically around the cell, presumably the main difference being that the cell did not emerge from the cast-off wall. The demonstration of these two systems of behavior in the same isolate of Prasinocladus, one being more typical of Tetraselmis, conclusively demon- strates that the stalked nature of Prasinocladus colonies cannot be used to separate that genus from Tetraselmis.

  • 320 R.E. NORRIS et al.

    Figs. I-8. Electron micrographs of Tetrasdmis species, showing tim structure and distribution of cell organelles. 1: Longitudinal section of a cell (cult. 15-8-17). Note two mas- sive rhizoplasts (rh) originating from the basal body apparatus. Distal end of left rhizoplast connects to the broad cell surface where there is no special structure. The nucleus (N) lies between the rhizoplasts and beneath it is a small pyrenoid (P) surr