CHROMALVEOLATA

SYSTEMATIC BIOLOGY					THE EUKARYOMONADAE
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PHYLUM HAPTOMONADA

INTRODUCTION TO THE HAPTOMONADA

Haptomonada (hap-to-mo-NA-da) is made of two Greek roots that mean hold (hapto -ἅπτω); and unit (monada -μονάδα). The reference is to a cell (a unit) with a holding structure, which is the haptonema.

The haptophytes, coccolithophorids, or prymnesiophytes are important marine phytoplankters that, in the motile form, have a unicell with two whiplash flagella and a covering of delicate calcium carbonate scales called coccoliths (Figures A&B). Another defining feature is the presence of a haptonema (Figure C), a flagellum-like structure (thus the name of the group). Pavlova differs from the other haptomonads in that it has unequal, heterodynamic flagella, a flagellar swelling, and an eyespot.

Some members of this group have a life cycle that alternates between a prostrate haploid filament (gametophyte) and a globose motile diploid sporophyte. The gametophyte produces isogametes that fuse to make the sporophyte. Likely most have such a life cycle, but the gametophyte is cryptic or named something else.

Members of this phylum are not restricted to marine environments. Chrysochromulina, a tiny freshwater form is quite common in certain ponds and lakes. In one sample, I collected a concentration of around 35,000 cells per milliliter in an acid-sensitive pond in central Pennsylvania.

Lovelock (1991) argues that the haptophytes may be responsible for the capture and removal of significant amounts of atmospheric carbon by the deposition of their scales in the ocean. He also argues that haptophytes release sulfur compounds into the air that induce the formation of clouds and increase the albedo of the planet.

A. Cells of Isochrysis showing the variable form of the cell, the anteriorly-directed equal flagella, and the two chromoplasts.

B. An SEM micrograph of the diploid phase (sporophyte) of a fossil Coccolithus showing the scale-like coccoliths.

C. Chrysochromulina displaying the paired flagella and short haptonema between them.

D. Pavlova, unlike other haptomonads, has two unequal heterodynamic flagella.

Images taken from:
A: http://www.nhm.ac.uk/hosted_sites/ina/CODENET/galleries/DICimages/source/iso.htm
B: http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
C: http://www.biol.tsukuba.ac.jp/~inouye/ino/h/prym.gif
D: http://www.reed-mariculture.com/microalgae/pav-100X.jpg

SYNOPTIC DESCRIPTION OF THE HAPTOMONADA

The descriptions of the phylum come from Grell (1976), Kudo (1966) and Lee et al. (1985), Green et al. (1990), Margulis and Schwartz (1988, 1998), Sleigh et al (1985), Bold and Wynne (1985), Sze (1986), and Lee (1980).

I. SYNONYMS: Coccolithophorids, haptomonads, haptophyta, prymnesiophytes.

II. NUMBER: >450 species.

III. PHYLUM CHARACTERISTICS:

A. Structure and Physiology

Cell Form: Flagellated unicells or coccoid.

Flagella: Two flagella, both whiplash and anteriorly directed or 2 unequal heterodynamic flagella; the haptonema, a flagellum-like structure with a core of 6-7 single microtubules, is often present and coiled between the flagella.

Basal Bodies: Basal bodies perpendicular.

Cell Covering: Covered by organic or calcareous scales.

Chloroplasts: Usually 2 yellow chromoplasts with chlorophylls a and c₁ and c₂; also with B-carotene, fucoxanthin and other xanthophylls; 3 thylakoids; pyrenoid; eyespot (present in Pavlovologia) situated within the chloroplast.

Food Reserves: Chrysolaminarin or paramylon; some with lipid globules; some phagotrophic.

Mitochondria: Tubular cristae.

Golgi: ` Present; polarized from edge to edge and heterogeneous from face to face; often fan-shaped beneath the basal bodies.

Nucleus: Outer membrane confluent with outer membrane of one of the chloroplasts.

Centrioles: Basal bodies function as centrioles.

Inclusions and Ejectile Organelles:

Not present in most, but some have muciferous bodies.

B. Mitosis, Meiosis and Life History

Mitosis: Nuclear membrane breaks down (open); spindle poles associated with basal bodies (as centrioles).

Meiosis: Present.

Sexual Reproduction and Life History:

Haploid and diploid phases alternate in some species; the haploid form produces flagellated isogametes.

C. Ecology: Mainly marine organisms.

SYSTEMATICS OF THE HAPTOMONADA

This system is based on Green et al. (1990), Margulis and Schwartz (1988, Pr-5 and 1998, Pr-10) and Sleigh et al (1985) in which the haptophytes ( or prymnesiophytes) are given phylum-level status. Bold and Wynne (1985), Sze (1986), and Lee (1980) consider the haptophytes to occupy a class within the chrysophyte complex. Grell (1976), Kudo (1966) and Lee et al. (1985) treat the haptophytes as a separate order associated with the chrysophytes and within the phytomonads. Taylor (1976) shows the haptophytes as a group which is very closely related to the chrysophytes. The analyses of Dodge (1973) are inconclusive; one shows the haptophytes within the chrysophyte complex while another analysis shows the haptophytes related to the chlorophytes. Hibberd (1980) and Lee et al. (1985) beg the question of haptophyte taxonomy. Because of the unique set of characters (e.g. haptonema, calcium carbonate scales, most with a pair of anteriorly-directed whiplash flagella, and a unique golgi organization), I have separated the haptophytes into their own phylum, Haptotista, which has 2 classes. Patterson (1999) recognizes the haptophytes as a natural group with no known affinities. Baldauf (2003) presents a consensus view of recent molecular-ultrastructural data in which the haptophytes occur in a clade with the cryptophytes and are associated with the heterokont clade. Until I am convinced that they are part of the heterokont group, I will keep them in the provisional kingdom of the Eukaryomonadae.

HIERARCHICAL CLASSIFICATION OF THE HAPTOMONADA

This system is based on Green et al. (1990), Margulis and Schwartz (1988, Pr-5 and 1998, Pr-10) and Sleigh et al (1985).

CLASS PRYMNESIOMONADEA

Cells within this class have equal flagella which are whiplash. This class has 3 orders.

ORDER ISOCHRYSIALES

Haptonema rudimentary or very small; cell naked, lacks scales.

Isochrysis, Emiliania, Ochrosphaera, Imantonia, Dicriteria, Gephryocapsa.

ORDER COCCOLITHALES (COCCOSPHAERALES)

Haptonema much reduced; cells normally covered with scales, some delicate and organic, others calcareous (coccoliths); some with very complex life history involving benthic phases.

Coccolithus (Crystallolithus), Syracosphaera, Calciosolenia, Hymenomonas, Pontospharea, Discosphaera.

ORDER PRYMNESIALES

Haptonema distinct, may be covered with elaborate organic scales.

Prymnesium, Chrysochromulina.

CLASS PAVLOVOLOMONADEA

Flagella unequal, ventrally or subapically inserted; one or both with hairs; eyespot within the chloroplast, some have a flagellar swelling associated with the eyespot; paramylon.

ORDER PAVLOVIALES

Pavlova.

This page is maintained by Jack R. Holt. Last revised 02/15/2008.