Chromista

The chromistans are probably a paraphyletic group of eukaryotes that usually contains chloroplasts with chlorophyll c, which is absent from all known plant groups. The group includes various algae, the coccolitho-phores and the diatoms and the majority are primary producers, functioning as part of the phytoplankton.

Nannoplankton, are defined as plankton less than 63 |m across, the smallest standard mesh size for sieves. Although the nanno-plankton includes organic-walled and siliceous forms, the calcareous groups are most prominent in living floras and dominate the fossil record. Coccolithophores are the dominant members of the fossil calcareous nanno-plankton, and the calcareous plates they produce, coccoliths, dominate nannofossil assemblages. Many calcareous nannofossils lack obvious shared characters with cocco-liths and so are excluded from the coccolitho-phores and instead are termed nannoliths. These nannoliths may be related to coccolith-bearing organisms, but in view of their diversity in form, the group may contain calcareous structures produced by quite unrelated microbes. As a whole, calcareous nanno-plankton first appeared during the Late Trias-sic, increased in abundance and diversity through the Jurassic and Cretaceous, reaching an acme of diversity in the Late Cretaceous. They were severely affected by the KT mass extinction, but subsequently radiated in the Early Paleogene and remained a major component of the calcifying plankton throughout the Cenozoic. They are extremely abundant in the surface waters of modern oceans.

Morphology and classification

Coccolithophores are unicellular algae, predominantly autotrophic in dietary mode, usually ranging in size from 5 to 50 |m, and globular, fusiform or pyriform in shape. The group constitutes the Phylum Haptophyta, within the Kingdom Chromista, together with various closely related non-calcifying algae; they have golden-brown photosynthetic pigments and, in motile phases, two smooth flagella together with a third flagellum-like structure, the haptonema. Coccolithophores are almost exclusively marine (there is just one, rather rare, freshwater species), usually open marine, occupying the photic zone where they photosynthesize. The group today is most diverse and has its highest relative abundances in the tropics although coccolitho-phores occur at all latitudes. The shell is composed of distinctive calcitic platelets or coccoliths. These are produced intracellularly;

Box 9.10 Atomic force microscopy of coccolithophores

Coccolithophores, despite their small size, are attractive and sophisticated organisms. A number of plate morphs, emphasizing the diversity of form within the group, have been described (Fig. 9.18c): asterolith, star-shaped plates; cyclolith, open rings; lopadolith, vase-shaped morphs with elevated edges; placolith, two disks fused by the median tube; stetolith, column-shaped plates; zygolith, elliptical ring with arches applied to holococcoliths. Apart from the term placolith most are not in routine use. Additionally, helioliths, composed of a large number of small radially arranged crystals, and ortholiths, with only a few crystals, have been recognized.

The coccolithophore is precipitated within the cell from the coccolith vesicle or Golgi body with tightly regulated crystal growth, allowing the crystals to integrate as the complex and exquisite networks that comprise a complete skeleton. Karen Henriksen, a former graduate student at the University of Copenhagen, applied atomic force microscopy (AFM) to the surface of three coccolith species, a technique that allows investigation at higher orders of magnitude than even scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipment. Henriksen and colleagues (2004) established key differences among these taxa suggesting that subtle changes in the mechanisms of biomineralization can drive significant changes in morphology that have knock-on effects for the adaptability, lifestyle and distribution of the coccolith species. The large morphological disparity seen in this remarkable group is thus a function of the mode and orientation of crystal growth at the atomic level and where the organism ultimately lived depended on the whims of a crystal lattice.

they then migrate to the cell surface and are expelled to form a composite exoskeleton, the coccosphere. Commonly the coccosphere consists of 10-30 discrete coccoliths, although some forms have many more (Box 9.10). Many taxa produce coccospheres formed of only one type of coccolith, but others show a variety of coccolith morphologies (Fig. 9.18); in particular there are often specialized coc-coliths around the flagellar pole of the cell. There are two fundamentally different types of coccoliths: heterococcoliths have a radial array of relatively few (typically 20-50) complex-shaped crystal units, whereas holo-coccoliths are formed of planar arrays of hundreds of minute uniform-sized (typically c. 0.1 ^m) rhombohedral crystallites.

Haptophyte life cycles were very poorly known until recently; research has now shown that cocolithophores, and possibly most hap-tophtes, typically have alternating haploid and diploid stages that are both capable of asexual reproduction. Coccolithophores usually have life cycles consisting of two main phases producing radically different cocco-liths that were often described initially as two different species. The haploid phase (with half the complement of chromosomes) is always flagellate, and is usually coated by minute holococcoliths; the diploid phase (with full complement of chromosomes) is usually non-flagellate, and is coated by heterococcoliths. Both phases are capable of indefinite asexual reproduction and it appears likely that the two-phase life cycle is an adaptation allowing coccolithophores to survive challenging ecological conditions. The haploid (holoccolith-producing) phase is thought to be adapted to oligotrophic conditions (when nutrients are scarce) whilst the diploid (heterococcolith-producing) phase is thought to be adapted to more eutrophic conditions (when nutrients are abundant).

The classification of extant coccolitho-phores is based largely on coccosphere morphology and coccolith structure because the intricate and distinctive form of coccoliths makes them ideal for morphological classification. Cell characters can only be studied with transmission electron microscopy and have generally proved rather invariant. Data from cytology and molecular genetics have strongly supported the classification based on morphological criteria. The reliance on coc-coliths in the extant classification also means that there are relatively few problems in align-

Figure 9.18 Some coccolith morphotypes: (a) coccospheres of the living Emiliana huxleyi, currently the most common coccolithophore (x6500), and (b) Late Jurassic coccolith limestone (x2000). (c) Coccolith plate styles: 1 and 2, Coccolithus pelagus; 4 and 5, Oolithus fragilis; 5 and 6, Helicosphera carteri. In C. pelagus and H. carteri growth was upwards and outwards with the addition of layer upon layer of calcite; in O. fragilis growth was different with curved elements, in non-parallel to crystal cleavage directions. (a, b, courtesy of Jeremy Young; c, courtesy of Karen Henriksen.)

Figure 9.18 Some coccolith morphotypes: (a) coccospheres of the living Emiliana huxleyi, currently the most common coccolithophore (x6500), and (b) Late Jurassic coccolith limestone (x2000). (c) Coccolith plate styles: 1 and 2, Coccolithus pelagus; 4 and 5, Oolithus fragilis; 5 and 6, Helicosphera carteri. In C. pelagus and H. carteri growth was upwards and outwards with the addition of layer upon layer of calcite; in O. fragilis growth was different with curved elements, in non-parallel to crystal cleavage directions. (a, b, courtesy of Jeremy Young; c, courtesy of Karen Henriksen.)

ing modern and fossil taxonomies. Some modern coccolithophores are polymorphic, producing several different types of cocco-

liths, but these are mostly in taxa with a limited fossil record. More interesting problems are posed by the alternation of holococ-colith-bearing and heterococcolith-bearing phases in the life cycle of a single species. In modern coccolithophores the taxonomy is being adjusted to reflect this as data become available.

Together with diatoms, dinoflagellates and picoplankton (tiny, single-celled plankton 0.2-2.0 ^m in size), coccolithophores are the most abundant phytoplankton in modern oceans. The greatest diversity is developed in the tropics. Dependence on sunlight for photosynthesis restricts the group to the photic zone, with a depth range of 0 m to about 150 m. Within wave-mixed surface waters there is normally only a slight vertical stratification of assemblages, but a quite different assemblage is often developed beneath the thermocline.

Evolution and geological history

Rare coccoliths first appeared in the Late Tri-assic and increased in numbers during the Jurassic and Cretaceous; the group peaked in the Late Cretaceous, and chalk from that interval is almost entirely composed of these nannofossils. Only a few species survived the end-Cretaceous extinction event but they radiated again during the Cenozoic, recovering their numbers and abundance. However, in the last 4-5 myr there has been a marked decline in the abundance of larger coccoliths and, as a result, they have become less abundant in oceanic sediments, typically forming only 10-30% of modern calcareous oozes. Biostratigraphic zonal schemes using cocco-lithophores have been established from the Jurassic to the present day, and these are widely applied because they are reliable and operate over great distances. Moreover, basic biostratigraphic analyses of coccolithophore samples can be carried out rapidly, typically requiring less than an hour per sample. This is because nannofossils are abundant enough to be studied in simple strew mount preparations and can be reliably identified in cross-polarized light. Nannofossils only occur in low-energy marine sediments and are easily destroyed by diagenesis, but when they are present they provide an ideal means of rapidly dating sediments.

epivalve + epicingulum = epitheca epicingulum hypocingulum

V girdle hypo valve hypovalve + hypocingulum = hypotheca epitheca + hypotheca = frustule

Figure 9.19 Descriptive morphology of the diatoms.

Diatoms_

Diatoms are unicellular autotrophs that are included among the chrysophyte algae; they are characterized by large green-brown chlo-roplasts. Both individuals and loosely integrated colonies of diatoms occur in a range of aquatic environments from saline to freshwater and across a range of temperatures, being particularly common in the Antarctic plankton. Both benthic and planktonic life modes occur, although within the plankton one group - the Centrales - prefer marine environments; the Pennales, on the other hand, are more common in freshwater lakes (Box 9.11).

Morphology and classification

The diatom cell is contained within a siliceous skeleton or frustule comprising two unequally-sized valves or thecae (Fig. 9.19). The smaller hypotheca fits into the larger epitheca; the valve plates and congula of both valves interface with the congulum of the epitheca covering that of the hypotheca to form a connective seal.

During reproductive fission, both the parent valves are used as the epitheca by the offspring, which then constructs its own hypotheca. This process occurs a number of times each day, progressively reducing the size of the fustule. A stage of sexual reproduction kicks in to restore the growth momentum of the individual.

Classification of the group is based on shell morphology (Box 9.11).

Evolution and geological history

Both diatom frustules and, more commonly, endospores are preserved in the fossil record. A Late Jurassic assemblage from western Siberia that includes Stephanopyxis may be the oldest known diatom flora. The first diverse floras appeared during the mid-Cretaceous with almost 10 families recorded from Aptian rocks; the group further diversified after the Turonian. Nearly 100 genera of centric diatoms are recorded from the Upper Cretaceous. Some of the first pennate diatoms appeared during the Paleogene, colonizing freshwater environments for the first time; the group reached an acme during the Miocene.

Remarkably, diatom frustules can accumulate as thick deposits of diatomite (sometimes up to 500 m thick), which is a very porous sediment, often with 80% as spaces, and permeable with a density of about 0.5 g cm-1. These diatomites, also termed kieselguhrs and tripolis, are widely used as purifiers for filtering drinks, medicines and water. Over 2 million tons are extracted each year for commercial use. Modern sedimentation rates suggest that 4-5 mm of diatomaceous ooze is deposited over 1000 years; such an ooze currently occupies over 10% of the ocean floor today. Major commercial deposits occur in the Miocene of the Ardeche, France and in the Pliocene and Pleistocene of Cantal, France are some of the main suppliers, although other deposits occur in Spain, Germany and Russia. The Miocene Monterey Formation in California is particularly widespread, occurring in both onshore and offshore basins; this diato-maceous mudstone is also the source and reservoir rock for most of California's petroleum.

Chitinozoans

Chitinozoans are most common in finegrained sediments, usually those deposited in anoxic environments, and are associated with pelagic macrofauna such as graptolites and nautiloids together with acritarchs. In some lithologies, such as black slates, chitinozoans are the only fossils preserved. These associations, together with their widespread geographic range, suggest that chitinozoans were at least pelagic. The group has proved extremely useful for both regional and global

Box 9.11 Classification of siliceous-walled groups: diatom classification

Two main divisions are recognized based on their shell morphologies: the Centrales, as the name suggests, have round valves with pores radiating in concentric rows from the valve center; the Pennales have more elliptical valves with the pores arranged in pairs (Fig. 9.20). The latter are usually characterized by a median gash or raphe.

Order CENTRALES

Suborder COSCINODISCINEAE

• Valves with a ring of marginal processes

Suborder RHIZOSOLENIINEAE

• Valves are unipolar Suborder BIDDULPHIINEAE

• Valves are bipolar

Order PENNALES

Suborder ARAPHIDINEAE

• Valves without a raphe Suborder RAPHIDINEAE

Diatomite Structure Valves Girdle
Figure 9.20 Some diatom morphotypes: Coscinoconus (x250), Asterolampra (x400), Cocconeis (x360), Achnanthes (x150), Surirella (x200) and Eunotia (x400).

aperture aperture

Chitinozoans Morphology

Figure 9.21 Descriptive morphology of the chitinozoans: (a) Operculatifera (simplexoperculate), Lagenochitina, and (b) Prosomatifera (complexoperculate), Ancyrochitina.

correlations and is a key part of global biostratigraphic schemes for the Ordovician and Silurian systems.

Morphology and classification

Chitinozoans are small (between 50 and 2000 ^m), flask- to vase-shaped, hollow vesicles with smooth or ornamented surfaces (Fig. 9.21). The vesicles were thought to have consisted of a protein called pseudochitin similar in composition to the graptolite rhabdosome, but recent research suggests that they are actually composed of networks of kerogen, and chitin is in fact absent from the pyroly-sates vaporized from the vesicle (Jacob et al. 2007). The vesicle encloses a chamber that ranges in shape from spherical through ovoid to cylindrical and conical forms. The chamber opens through an aperture at the oral end, either directly or at the end of a neck with a collar. The aperture is closed by an operculum that may be supported by the prosome. The base of the vesicle may be flat or extended as a variety of structures, for example a copula (long hollow tube), mucron (short hollow tube), siphon (bulb-like process) or peduncle (solid process). There are nearly 60 genera of chitinozoans.

The precise affinity of the group remains uncertain (Box 9.12). Chitinozoan vesicles were probably tightly sealed, and they occur as chains and clusters that suggest they may have been eggs or egg capsules or even dormant cysts. Chitinozoans have, in fact, been interpreted in the past as egg cases of a huge range of invertebrates, such as annelids, echinoderms, gastropods and graptolites, but they were probably the products of some soft-bodied, worm-like animal during a pelagic life stage.

Two main groups of chitinozoans have been established based on the way the vesicle is sealed, and are further subdivided according to the outline or silhouette of the vesicle together with modifications of the neck. The Operculatifera have a relatively simple operculum and they lack a neck (including the Desmochitinidae with small subspherical vesicles), whereas the Prosomatifera have a more complex opercula with a prosoma and a well-developed neck (including the Conochitinida and the Lagenochitinidae, the second with a recessed operculum) (Fig. 9.23).

Evolution and distribution

Possible chitinozoans, in the form of Desmo-chitina-like sacs, have been reported from the Upper Proterozoic of Arizona, but the first true chitinozoans appeared during the Tremadocian (Early Ordovician) and subsequently diversified rapidly during the Early Ordovician, evolving hundreds of different species spread across at least 50 genera. This diversity continued through the Silurian with all the three main groups represented. They declined during the Devonian, disappearing finally at the top of the Famennian, when the last remaining lagenochitinid went extinct. Through time the group developed smaller, self-contained chambers with an increased complexity of ornament and a greater degree of apparent coloniality (Fig. 9.23).

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