The phylum Haptophyta is divided into two subclasses, the Prymnesiophycidae and Pavlovophycidae, that differ significantly from one another. The phylum is usually classified within the kingdom Chromista with other algae containing chlorophyll a and c (excluding dinoflagellates), but their exact relationship to the heterokont algae remains unclear.
Although a few freshwater species are known, most are marine, and species diversity within the phylum is greatest in nutrient-poor waters of the tropical and subtropical open oceans. Most are motile biflagellate single cells or nonmotile coccoid (walled unicells) cells. Colonial, filamentous, and palmelloid forms are also known.
Prymnesiophyte cells are typically eukaryotic. The nucleus is positioned centrally or nearer the posterior end of the cell and lies between the chloroplasts. Most cells probably possess a single highly branched mitochondrion, although it is possible that multiple mitochondria may be present in some species. An exceptionally large Golgi body (dictyosome) with many cisternae is located anteriorly, just beneath the flagellar basal region.
The motile cells of most prymnesiophytes bear two equal or slightly subequal heterodynamic flagella that lack mastigonemes (hairlike appendages) and arise from a shallow apical depression.
However, the longer flagellum in members of the Pavlovophycidae is adorned with fibrous hairs and knobscales. Further differences between the Prymnesiophycidae and Pavlovophycidae are evident in the flagellar and mitotic apparatuses.
In the Prymnesiophycidae the flagellar root system originating near the two basal bodies consists of three to four microtubular roots, the nuclear envelope breaks down prior to division, the mitotic spindle axis is straight, and a fibrous microtubular organizing center (MTOC) is absent.
In contrast, the Pavlovophycidae flagellar root system includes only two roots, the nuclear envelope remains intact during division, the mitotic spindle is V-shaped, and a fibrous MTOC is present.
A haptonema—a thicadlike structure that, along with two flagella, extends from the cell wall—is found in many, but not all, species and gives this phylum its name. However, the haptonema is unique to the Prymnesiophyta.
In many coccolithophorids the haptonema is reduced (vestigial) or absent. The haptonema, which may have evolved by duplication of a portion of the flagellar apparatus, arises between the flagellar basal bodies and at its base is composed of three to eight singlet microtubules.
The microtubules are arranged in an arclike or crescentlike fashion and are surrounded by a layer of the endoplasmic reticulum. The movement of the haptonema (which does not beat) and flagella are highly coordinated.
When present, the haptonema may be long or short and may or may not be capable of coiling. Coiling is induced by a rapid uptake of calcium from the environment. The haptonema can be used for attachment (in fact, the Greek word haptein means “to fasten”) and, at least in some species, is used to capture prey (such as bacteria and smaller eukaryotes).
In phagotrophic species a single membrane bound food vacuole is posteriorly located, and the haptonema delivers captured prey to this organelle. The haptonema is also involved in tactile responses; for example, upon contact with an obstacle it may stimulate a change in the direction of swimming.
Chloroplasts and Nutrition
Most prymnesiophytes possess two chloroplasts that are surrounded by four membranes; in most species investigated, the outer membrane is studded with ribosomes and is confluent with the nuclear envelope. A girdle lamella, a thylakoid encircling the periphery of the plastid that is found in some other algae containing chlorophyll a and c, is absent.
In members of the Prymnesiophycidae, eyespots are absent. Eyespots are present beneath an invagination of the plasmalemma in members of the Pavlovophycidae but usually are not associated with a flagellar swelling as in many other algal taxa.
The photosynthetic pigments of prymnesiophytes are diverse. All contain chlorophylls a, c1, c2, beta carotene, diadinoxanthin, and diatoxanthin, but chlorophyll c3, fucoxanthin, 19′-hexanoyloxy-fucoxanthin and 19′-butanoyloxyfucoxanthin are present or absent in different combinations in other species.
Pyrenoids are present and may be immersed (embedded within the chloroplast) or bulging, in which case they protrude from the periphery of the chloroplast and into the surrounding cytosol. Pyrenoids may be traversed by one or a few thylakoid membranes.
Most prymnesiophytes are photoautotrophs and, in addition to using photosynthesis for nutrition, probably are also capable of directly obtaining inorganic or organic nutrients dissolved in the surrounding water.
A number of species, particularly those possessing a relatively long and flexible haptonema, are phagotrophic and ingest bacteria and smaller eukaryotes. Thus, mixotrophic species (those that combine photoautotrophic and heterotrophic means of obtaining food) are common among prymnesiophytes.
Members of the Pavolophycidae lack body scales. However, cells of species placed in the Prymnesiophycidae are covered by organic base plate scales, mineralized calcium carbonate scales, or a combination of both that are external to the plasmalemma.
Nearly 70 percent of all known species of prymnesiophytes are known as coccolithophorids. The external covering of these cells is composed of mineralized calcium carbonate (calcite) scales termed coccoliths. Individual coccoliths are intricately arranged around the cell to forma coccosphere.
Both organic scales and coccoliths are produced within the Golgi complex and are released onto the cell surface near the point of flagellar insertion. An organic base plate scale serves as a matrix for coccolith calcification that may occur inside or outside the cell; thus, organic scales and mineralized scales are homologous structures.
Organic base plate scales are microfibrillar and composed of proteins, celluloselike and pectinlike substances. Themicrofibrils on the proximal side of the scale are arranged radially, whereas those on the distal surface are arranged spirally. In species bearing only organic scales (such as Chrysochromulina, Phaeocystis, and Prymnesium), one or more layers of organic scales may be present.
In coccolithophores the coccoliths are external to the organic scales (when present). Pleurochrysis is an example of a coccolithophorid that possesses organic scales as well as coccoliths, whereas the widely distributed coccolithophorids Emiliana and Gephyrocapsa bear mineralized scales only.
There is tremendous diversity in coccolith morphology, ranging from those that are plate like to highly ornamented forms with rims and spines. The taxonomy of species within the group is based primarily upon structural differences among coccoliths.
However, some species may bear coccospheres composed of morphologically different coccoliths, and transitions between coccolith types are now known to be associated with different life history phases. An accurate account of the biodiversity of prymnesiophytes species remains uncertain because different life history phases bearing different scales are often considered separate species.
The function of coccoliths is not known with certainty. It is likely that they serve multiple functional roles in some species, whereas in other species more specific functions may be attributed to morphologically different coccoliths. Coccoliths are effective at deterring only smaller grazers (such as protozoans), and coccolithophorids are readily eaten by other organisms.
It has been suggested that coccoliths protect the delicate plasmalemma from osmotic, chemical, and physical disruption or invasive bacteria and viruses. Coccoliths, which may be shed or produced when needed, may also play a role in buoyancy control. The long spines on the coccoliths of Rhabdosphaera and other coccolithophores also reduce sinking rates.
Calcification and photosynthesis in most coccolithophorids appears to be physiologically linked. It is possible that the carbon dioxide released during calcification may be used in the dark reactions of photosynthesis and that coccoliths increase the amount of surface area available for light capture.
There are more than one thousand different types of fossilized coccoliths, and these are among the most commonly used microfossils for stratigraphic analyses in the petroleum industry.
In addition, because some coccolithophorids are restricted to water masses defined by a particular temperature range, fossil coccoliths are frequently used as paleoclimatic indicators. Because their calcium carbonate scales are birefringent, satellite imagery can be used to deduce the relative abundance and position of prymnesiophyte blooms.
Most prymnesiophytes are biflagellate motile single cells that reproduce asexually via binary fission. Life history stages may include transitions between nonmotile and motile forms and also the production of different scales (organic versus mineralized) and the production of mineralized scales of different morphologies.
Thus, during their life histories flagellate cellsmay bemorphologically transformed into amoeboid, coccoid, colonial, palmelloid (walled cells embedded in a mucilaginous envelope), or filamentous life forms. For example, Pleurochrysis possesses a nonmotile benthic colonial or filamentous haploid stage that alternates with a diploid motile coccolith bearing stage.
Some life histories include alternations between two motile cell stages that bear completely different types of coccoliths. In Hymenomonas and Ochrosphaera, diploid coccolith-bearing cells alternate with cells possessing only organic scales.
Phylogeny and Fossil Record
Although they may have originated earlier, the coccoliths of prymnesiophytes first appear in the fossil of the Late Triassic, approximately 220 million years ago (mya). The abundance of coccolith fossils reached its peak during the Late Cretaceous (95-63 mya).
Fossil records indicate that perhaps 80 percent of all coccolithophorids went extinct during the Cretaceous-Tertiary (K-T) event at the end of the Cretaceous period. Today’s prymnesiophytes represent a radiation of the minority of species that survived the K-T extinction event, which also brought about the demise of dinosaurs.
According to most scholars, the most primitive prymnesiophytes are those lacking body scales and possessing a haptonema. Flagellates bearing organic scales are believed to have diverged next. The absence of a haptonema and the presence of coccoliths are considered derived features for the group, and these characteristics are found in most coccolithophorids.
Prymnesiophytes, particularly coccolithophorids, play important roles in coastal and open ocean environments. For example, they are integral contributors in global carbon and sulfur cycles.
Coccoliths that are shed, derived from dead cells, are ingested and expelled are transported to the sea floor. In some areas the accumulation of coccoliths has led to the formation of enormous deposits of chalk or limestone, a notable example being the White Cliffs of Dover in England. By this sedimentary process, calcium and carbon are cycled from the oceans back into the lithosphere.
The ocean is the largest long-term sink of inorganic carbon on the planet, and carbonate deposits cover one-half of the sea floor, an area equal to one-third the surface of the earth. Coccoliths account for approximately 25 percent of the total yearly vertical transport of carbon to the deep sea.
Blooms of coccolithophorids (such as Emiliania, Pheocystis)release dimethylsulfide that becomes aerosolized and subsequently acts as a nucleating agent for water droplets in the atmosphere, ultimately producing acid rain. Because coccoliths reflect light, large blooms may also have a cooling effect on the local climate.
Other species of prymnesiophytes, including some species of Chrysochromulina, Prymnesium, and Phaeocystis, are known to form blooms that are toxic to other marine organisms or that interfere with marine fisheries. On the other hand, Pavolova and Isochrysis are widely used as food in the aquaculture industry.