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PHYLUM XANTHOPHYTA

INTRODUCTION TO THE XANTHOPHYTA

Xanthophyta (zan-THA-fa-ta) is derived from two Greek roots that mean yellow or blonde (xanthia -ξανθιά); and plant (phyto -φυτό).  The reference is to the distinctive yellowish appearance due to the characteristic secondary photosynthetic pigments.

Though relatively small in number of taxa (diversity), the xanthophytes exhibit a large range in form (disparity) from amoeboid and coccoid unicells (Figure A) to filaments (Figures B&C).  Life cycles are as varied with isogamy, anisogamy, and oogamy employed within the phylum.  The common name for the group is the yellow-green algae, but they range in color from grass-green to brown-green depending on the relative abundance of accessory pigments, particularly the xanthophylls.  

Van den Hoek et al. (1995) claim that the taxa of this phylum rarely grow anywhere in abundance.  However, I frequently see the siphonaceous filament, Vaucheria, (also called water felt) in great abundance near seeps and springs.  Vaucheria grows as a mat of dark green filaments on wet soil and on mud in the water.  Bold et al. (1988) declare that the uncharacteristic appearance caused some early phycologists to place Vaucheria with the green algae (they bear a remarkable resemblance to some of the oogamous chlorophytes; they also have an uncanny resemblance to the oomycetes like Saprolegnia); however, Vaucheria does produce the typical heterokont motile cell, eyespot, chlorophyll c, and storage products.  The vegetative filaments are siphonaceous and branched.  Zoospores are produced at the ends of certain filaments, and they have many heterokont pairs of flagella on each zoospore.  The gametangia are distinctive, a large oogonium (or oogonial cluster) surrounded by two or more short antheridial branches.  The antheridia produce many small biflagellate heterokont sperm that can fertilize an egg, which produces a zygote.  Usually, the zygospore is a resting stage.

Tribonema, though it it is nonmotile in the vegetative state, forms short filaments that can be abundant in the cool months as floating light green mats and loosely-attached clumps on submerged vegetation.  The filaments are unbranched and the cell walls are made of overlapping H-pieces (see Figure C), which is reminiscent of the overlapping wall structure of the diatoms.  Their life history has vegetative fragmentation, asexual heterokont zoospores, and isogamous sexual reproduction.

A. A vegetative cell of Ophiocytium, which means "adder cell".  Image shows the characteristic point at one end and rounded tip at the other.	B. Filaments of Tribonema whose cell walls are constructed of overlapping H-pieces.	C. Senescing filaments of Vaucheria showing zygospores developing within old oogonia.  Note siphonaceous nature of the filaments.
Images taken from: A: http://microscope.mbl.edu/scripts/microscope.php?func=imgDetail&imageID=565 B&C: Taken from the Systematic Biology Biodiversity Collection D: http://www2.trincoll.edu/~cschneid/vaucheria.html

SYNOPTIC DESCRIPTION OF THE XANTHOPHYTA

SYSTEMATICS OF THE XANTHOPHYTA

This taxonomy is modified from that of Hibberd (1990) and roughly based on that of Margulis and Schwartz (1988, Pr-9 and 1998, Pr-14) and Sleigh et al. (1984) in which the xanthophytes are given phylum-status. Scagel et al. (1982), Bold and Wynne (1985), Lee (1980) and Sze (1986) consider the xanthophytes to be a class of the chrysophytes. Protozoologists like Grell (1976), Kudo (1966) and Lee et al. (1985) consider only the motile forms like Olisthodiscus (which likely is a Raphidiophyte).

The phylogeny of Taylor (1976) and analysis of Dodge (1973) place the xanthophytes as a distinct group which is associated with the chrysophyte complex (chromophytes). In addition, the xanthophytes seem clearly related to the Oomycota although Beakes (1989) gives a non-committal review of the evidence which links them to the Oomycota (as well as the Eustigmatophyta).  Sogin and Patterson (Tree of Life Project) indicate that they form a natural group and are sisters to the Phaeophyta.  More recent treatments of the heterokonts (e.g. Baldauf 2003) show the heterokonts as a natural group associated with the cryptomonad-haptomonad group and a sister to the alveolates.

HIERARCHICAL CLASSIFICATION OF THE XANTHOPHYTA

This page is maintained by Jack R. Holt.  Last revised: 03/17/2008.