DESCRIPTION OF THE PHYLUM EUSTIGMATOPHYTA (HIBBERD & LEEDALE 1970)

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Eustigmatophyta (u-stig-ma-TA-fa-ta) is derived from three Greeks roots that mean true or good (eu – ευ); spot (stigma – στίγμα); and plant (phyto -φυτό). The reference is to the eyespot (also called a stigma) in the zoospores. Hibberd and Leedale (1970) described the class, Eustigmatophyceae. They raised it to phylum status (Eustimatophyta) in 1990.

INTRODUCTION TO THE EUSTIGMATOPHYTA The eustigs are few in number and make up a relatively new group of organisms. One, Nannochloropsis, is a member of the marine picoplankton, but most tend to be coccoid algae in soil or freshwater (Figures 1 and 2). Hibberd and Leedale (1970) studied the ultrastructure of the zoospores of certain coccoid cells and found that they made a stigma or eyespot (text with tooltip) An eyespot is a light-sensitive structure that does not form an image. This can be part of an organelle as in the chloroplast of certain microbial eukaryotes. It can be an elaborate structure that involves a light-sensitive swelling at the base of a flagellum (as in the euglenoids) or it can be a multicellular structure as in planarians. outside of the plastid but near a swelling (text with tooltip) A flagellar swelling is a portion of the base of a flagellum that usually is light sensitive (as in the eyespot apparatus of euglenoids). at the base of the flagellum (Figure 3). This arrangement is similar to that of the euglenoids rather than the chrysophytes or xanthophytes, whose eyespots are contained within the membranes of one of the chloroplasts. In fact, this character found in the zoospore served to separate the eustigs from the xanthophytes, with which they had once been classified. Prior to the recognition by Hibberd and Leedale (1970) of the unique stucture of the eustig motile cell, members of the Eustigmatophyta were placed in the xanthophytes. Now many sources (e.g. Bold and Wynne 1985; Sze 1986; and Lee 1980, 1995, 1999) consider the eustigs to be a class within the chrysophytes. Graham and Wilcox (2000) refer to the photosynthetic heterokonts as Ochrophytes and use Cavalier-Smith and Chao (1996) to support their association with the chrysophytes, raphidiophytes, and the synurophytes. Margulis and Schwartz (1988, Pr-10), Sleigh et al. (1984), and Hibberd (1990b) consider the differences between the eustigs, xanthophytes and chrysophytes sufficient to place them into separate phyla. The phylogeny of Taylor (1976) shows the eustigs emerging from a line near the chloromonads (now called raphidiophytes) and prior to the chrysophyte complex. The analysis of Dodge (1973) shows the eustigs occupying a link between the euglenoids and xanthophytes (or chrysophyte complex). Andersen (2004), Brown and Sorhannus (2010), and Yang et al. (2012) have generally resolved the eustig taxa in a clade with the chrysophytes (see Figure 4).

FIGURE 1. Coccoid cells of Nannochloropsis, a tiny marine plankter. Image from http://microscope.mbl.edu/scripts/microscope.php?func=imgDetail&imageID=2613	FIGURE 2. Chlorobotrys (formerly known as Chlorococcum) is a common plankter in acid bogs and fens. In greenhouses it is found growing on the outsides of clay pots. Image from NOAA, in the Public Domain	FIGURE 3. A TEM micrograph of a eustig zoospore (text with tooltip) A zoospore is an asexual spore that is motile. Zoo- (pronoumced zo-o) is a prefix that means moving. . It shows the characteristic eustgma (an eyespot that is outside of the chloroplast, labeled E). * = expanded region of the flagellum, N = nucleus, M = mitochondrion; P = plastid. Image from Graham and Wilcox (2000)

FIGURE 4. A cladogram showing the relationship of the eustig taxa (in shaded box) within the heterokonts (taxa in bold). Oc is the Ochrophyta clade, the photosynthetic taxa. The topology of this cladogram was informed by Andersen (2004), Brown and Sorhannus (2010), and Yang et al. (2012).

LITERATURE CITED Andersen, R. A. 2004a. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany. 91(10): 1508-1508. Bold, H. C. and M. J. Wynne. 1985. Introduction to the Algae. 2nd Edition. Prentice-Hall, Inc. Englewood Cliffs. NJ. Brown, J. W. and U. Sorhannus. 2010. A molecular genetic timescale for the diversification of autotrophic stramenopiles (Ochrophyta): substantive underestimation of putative fossil ages. PLoS ONE 5(9): e12759. doi:10.1371/journal.pone.0012759. Cavalier-Smith, T. and E. E. Chao. 1996. 18S rRNA sequence of Heterosigma carterae (Raphidiophyceae) and the phylogeny of heterokont algae (Ochrophyta). Phycologia. 35: 500-510. Dodge, J. D. 1973. The fine structure of algal cells. Academic Press. New York. Graham, L. E., and L. W. Wilcox. 2000, Algae: Prentice Hall, Upper Saddle River, NJ. Hibberd, D. J. 1990b. Eustigmatophyta. In: Margulis, L., J. O. Corliss, M. Melkonian, and D. J. Chapman, eds. 1990. Handbook of the Protoctista; the structure, cultivation, habits and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. Jones and Bartlett Publishers. Boston. pp. 326-333. Hibberd, D. J. and G. F. Leedale. 1970. Eustigmatophyceae – a new algal class with unique organization of the motile cell. Nature. 225: 759-760. Lee, R. E. 1980. Phycology. Cambridge University Press. Cambridge. Lee, R. E. 1995. Phycology. 2nd Edition. Cambridge University Press. Cambridge. Lee, R. E. 1999, Phycology: 3rd ed., CambridgeUniversity Press, Cambridge, UK. Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2nd Edition. W.H. Freeman and Co. New York. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Sleigh, M.A., J.D. Dodge and D.J. Patterson. 1984. Kingdom Protista. In: Barnes, R.K.S., ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc. Sunderland, Mass. Sze, P. 1986. A Biology of the Algae. Wm. C. Brown Publishers. Dubuque, Iowa. Taylor, F. J. R. 1976. Flagellate Phylogeny: A Study in Conflicts. J. Protozool. 23: 28-40. Van Den Hoek, C., D. G. Mann, and H. M. Jahns. 1995. Algae, an introduction to phycology. Cambridge University Press. Cambridge. Yang, E. C., G. H. Boo, H. J. Kim, S. M. Cho, S. M. Boo, R. A. Andersen, and H. S. Yoon. 2012. Supermatrix data highlight the phylogenetic relationships of photosynthetic stramenopiles. Protist. 163: 217-231.

By Jack R. Holt. Last revised: 02/19/2013