DESCRIPTION OF THE PHYLUM RAPHIDIOPHYTA (CHADEFAUD 1950)

EUKARYA> CHROMALVEOLATA> HETEROKONTAE> RAPHIDIOPHYTA

Raphidiophyta (ra-fi-de-O-fa-ta) is derived from two Greek roots that mean sew (ravo -ράβω); and plant (phyto -φυτό). The reference may be to the needle-like trichocysts that are abundant just beneath the cell membrane.

The raphidiophytes, also called the chloromonads, make up a small group of photosynthetic motile unicellular organisms. They are relatively large and sluggish members of protist communities associated with rotting vegetation in fresh water (Figures 1 and 2) or they can become locally abundant in the marine plankton and often cause fish kills in near shore and brackish water environments (Figure 3). They became taxonomic nomands for some time because of the occurrence of a set of characters that seemed to intersect with the dinoflagellates ( trichocysts (text with tooltip) Trichocysts are extrusosomes that also are characteristic of the ciliates and dinoflagellates. They can discharge and serve almost as an organelle equivalent to the nematocyst. However, their particular functions have yet to be defined to my satisfaction. ) and the chrysophytes (a rhizoplast (text with tooltip) A rhizoplast is a striated root system that extends from the basal body tinto the cell, usually to the nuclear envelope. and heterokont motile cell). This phylum is considered to be a class of the Chrysophyta by Margulis and Schwartz (1988 Pr-4 and 1998 Pr-13) and Bold and Wynne (1985). Lee (1980) places the chloromonads in a position of uncertain status (as did Smith 1950). Protozoologists like Kudo (1966), Grell (1976) and Lee et al (1985) treat the chloromonads as an order within the phytoflagellates. Because they have a unique set of cellular characters (e.g. trichocysts?, mucocysts, rhizoplast, kinetochores, many more than 2 chloroplasts per cell), Taylor (1976) puts the chloromonads in an isolated position but near the chrysophytes. Similarly, Heywood (1989) cautiously suggests that the chloromonads might occupy a unique position within the Chromophyta (chlorophyll a and c containing algae). Dodge (1973) shows a similar kind of association. Heywood (1990) argues that the term chloromonad is inappropriate as a root for taxa in the phylum because Chloromonas does not belong to this phylum; thus the basis for the change in designation from Chloromonada to Raphidiophyta. The following system follows the phylogeny of Taylor (1976) and the analysis of Dodge (1973) in which the group is given isolated or phylum status. Sogin and Patterson (Tree of Life Project) also shows them as a natural group. Cavalier-Smith and Chao (1996) in a cladistic analysis suggest that the raphidiophytes are sisters to the eustig-chrysophyte clade. Earlier, Cavalier-Smith (1986) united most of the photosynthetic heterokonts into a single phylum, the Ochrophyta, a taxon adopted by Graham and Wilcox (2000) to include the diatoms, raphidiophytes, chrysophytes, eustigs, silicoflagellates, xanthophytes, and phaeophytes. Although these taxa do seem to be part of the same clade, we interpret them as related within the same subkingdom because of the enormous variation between these groups. Heywood (1990) contends that much more work is required before a valid taxonomy can be created for this group. Since that time, much more confirmation for the association of the raphidiophytes with the other heterokonts has been made (e.g. Cavalier-Smith and Chao 1996 and Tyrrell et al. 1996). More recent analyses (e.g. Andersen 2004; Brown and Sorhannus 2010; Yang et al. 2012) have placed the raphidiophytes at the base of a clade that includes Phaeophytes and Xanthophytes (Figure 4).

FIGURE 1. Gonyostomum DIC image shows the reservoir from which the single anterior flagellum (text with tooltip) An anteriorly-directed flagellum extends in the direction of the motion of the motile cell. The interpretation is that the flagellum functions by pulling the cell. emerges. The radiating lines under the cell membrane are the trichocysts (or mucocysts (text with tooltip) A mucocyst (also called mucus body, muciferous body, mucus trichocyst) is an organelle or inclusion that extrudes mucus. ). Image from Prikko Kokkonen, on a NOAA website.	FIGURE 2. Vacuolaria is similar to Gonyostomum; however, Vacuolaria has two flagella (one is recurrent (text with tooltip) Recurrent flagella bend to the posterior end of the cell. Typically, they are identified as recurrent when anteriorly-directed flagella are present. ) and lacks trichocysts. Image from http://protist.i.hosei.ac.jp/PDB/Images/Heterokontophyta/Vacuolaria/sp_07.html	FIGURE 3. Olisthodiscus, a marine raphidiophyte that causes toxic red tides. Image from http://microscope.mbl.edu/scripts/microscope.php

FIGURE 4. A cladogram showing the relationship of the raphidiophyte 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).

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By Jack R. Holt. Last revised: 02/24/2014