Protists

Key points

• Micropaleontology is a multidisciplinary science, focused on the study of microorganisms or the microscopic parts of larger organisms.

• Prokaryotes, unicellular microbes lacking nuclei and organelles, include the carbonate-producing cyanobacteria, the oldest known organisms; their radiation during the mid-Precambrian promoted an oxygen-rich atmosphere.

• Protists, unicellular organisms with nuclei, include a large variety of organisms with external protective coverings (tests and cysts) assigned to the kingdoms Protozoa and Chromista.

• Fossilized protists can also be split into organisms with organic (acritarchs, dinoflagel-lates, chitinozoans), calcareous (coccolithophores, foraminiferans) or siliceous (diatoms, radiolarians) skeletons.

• Foraminifera, single-celled animal-like protozoans, contain both benthic and planktonic forms with chitinous, agglutinated, but most commonly calcareous (hyaline and porcellaneous), tests occurring throughout the Phanerozoic.

• Radiolarians, animal-like protozoans with siliceous tests, and diatoms, plant-like protozoans with silicic skeletons, are both important rock formers.

• Acritarchs, dinoflagellates and chitinozoans are palynomorphs, most commonly preserved as cysts, with important biostratigraphic applications. The first two are assigned to the protozoans, the third is currently difficult to classify.

• Coccolithophores and diatoms are assigned to the chromistans.

It has long been an axiom of mine that the little things are infinitely the more important.

Arthur Conan Doyle (1891) A Case of Identity

The world of microbes is more bizarre than the most contrived science fiction novel. The Earth is host to creatures that ingest iron and uranium, thrive in environments akin to boiling sulfuric acid or even live within solid rock itself (Box 9.1). These amazing organisms have a huge variety of shapes, belong to a multitude of groups living in many different environments while pursuing a wide range of lifestyles with often apparently alien metabolisms. Microbes such as bacteria and viruses are by far the most abundant life forms on the planet, a situation undoubtedly true of the geological past. Microfossils are the microscopic remains, commonly less than a millimeter in size, of either microorganisms or the disarticulated or reproductive parts of larger organisms. They thus include not only microbes themselves but also the microscopic parts of animals and plants.

In his famous book Small is Beautiful, Schumacher argued for small-scale economics in the world. Among paleontologists, micro-paleontologists are obsessed with microscopic fossils. Until you have screwed up your eyes and peered down a binocular microscope, you can have no idea of the exquisite beauty of microfossils, their tiny shapes showing infinite detail in their sculpture, spines and plate patterns. And they are not only beautiful, but useful too! Micropaleontology has thus attracted the attentions of botanists, zoologists, biochemists and microbiologists together with, of course, paleontologists and geologists. The disparate taxonomic groups included as microfossils are, nonetheless, united by their method of study - all require the use of an optical microscope, although more recently both scanning and transmission electron microscopes have taken microfossil studies to new, amazing levels. The majority of microfossils are indeed small and perfectly formed; but they display often the most complex and intricate of organic morphologies.

Microfossils thus include material derived from most of the major groups of life, Bacteria, Protozoa, Chromista, Fungi, Plants and Animals, although Fungi are rarely found as fossils. The broad classification adopted by most textbooks is both conventional and operational: microfossils are usually divided into the prokaryotes (mainly bacteria), pro-

Box 9.1 Microbes in extreme environments: the extremophiles

We are aware that microbes are everywhere, but are they as widespread as we believe? Yes, and probably more so. Scientists have been investigating a range of microbes, the extremophiles ("lovers of extremes"), that appear to be adapted, with specific enzymes, to some of the most extreme environments on Earth. Thus acidophiles (acid environments), alkaliphiles (alkaline environments), barophiles (high pressure), halophiles (saline environments), mesophiles (moderate temperatures), thermophiles (high temperatures), psychrofiles (cool temperatures) and xerophiles (arid environments) have now been identified. Extremophiles are spread across both the prokaryotes and eukary-otes, although most belong to the Archaea and Bacteria and some scientists have argued they should be included in a separate domain on the basis of their unique metabolic processes. Thus if modern microbes can function in both frozen and geothermal habitats, both acid and alkaline ponds and even deep within the crust, the extreme environments of the Early Precambrian and perhaps even space were probably not a great challenge to evolving life of this type. Moreover such groups of organisms could clearly survive the extreme environments of great extinction events. But it remains a challenge to identify such groups in the fossil record. One group of ingenious algae, the acritarchs (see p. 216), made it through one of the most extreme series of ice ages our planet has experienced. The "snowball Earth" hypothesis (see p. 112) suggests that the planet's oceans froze over during the Late Proterozoic, with life coming to a virtual standstill. Acritarch diversity was maintained through the crises (Corsetti et al. 2006). Have we identified a group of extremophiles, or was the climate not so harsh as suggested by the snowball Earth hypothesis?

tists (unicellular eukaryote organisms with a variety of tests (external shells) and cysts (enclosed resting stages)), microinvertebrates (mainly the ostracodes, see p. 383), microver-tebrates (mainly the conodonts and various other microscopic parts of fishes, see p. 441) and spores and pollen (microscopic reproductive organs of plants, see p. 493). We devote this chapter, however, to the more advanced microbes themselves, represented by the second group. The protists are most probably derived from within the Archaea, splitting from them between 4.2 to 3.5 Ga, but the group is almost certainly polyphyletic. The prokaryotic Archaea and Bacteria are intimately tied to the origin of life and the limited Precambrian fossil record (see pp. 191-4); this is all the evidence of life in rocks over 1 billion years old!

The abundance and durability of many microfossil groups makes them invaluable for biostratigraphic correlation (see p. 25). Sequences of samples can be collected from rock outcrops and even from the very small samples available from drill cores and drilling muds. Consequently they are very widely used in geological exploration by petroleum and mining companies. In addition, many microfossils are produced by planktonic organisms with very wide biogeographic distributions, making them invaluable for reliable long-distance correlation. Microfossils in oceanic sediments also provide a continuous record of environmental change and paleoclimate, and study of changing assemblages and the geochemistry of microfossil shells provide the fundamental data for paleoceanographic research. Moreover, consistent color changes through thermal gradients have made microfossils, particularly conodonts and palyno-morphs, invaluable for assessments of thermal maturation and the prediction of hydrocarbon windows.

Microorganisms have made a phenomenal contribution to the evolution of the planet as a whole. Many, such as the coccolithophores, diatoms, foraminiferans and radiolarians, are rock-forming organisms. The prokaryotic cyanobacteria fundamentally changed the planet's atmosphere from anoxic to aerobic during the Precambrian, and probably continued to mediate atmospheric and hydrosphere systems. For example, recent research suggests that carbonate mudmounds - such as the Late Ordovician mudbanks in central

Ireland, the north of England and Sweden, the Early Carboniferous Waulsortian mounds in Ireland and elsewhere, together with the Early Cretaceous mudmounds in the Urgonian limestones of the Alpine belt - were precipitated by microbes. The influence of microorganisms may also be more subtle. Coccolith-producing organisms, for example Emiliania, can, during blooms, manufacture massive amounts of calcium carbonate; this material is much more readily subducted than shelf carbonates and it is then recycled through volcanoes as carbon dioxide (CO2). The buildup of this greenhouse gas probably maintained warmer climates during the last 200 million years.

The extraction and retrieval of microfossils from rocks and sediments requires a range of preparation techniques, some of which can only be attempted in purpose-built laboratories. For many groups, preparation consists essentially of disaggregation of the rock in water or more potent solvents followed by sieving to remove the clay fraction. The silt-and sand-sized residue is then hand picked under a microscope to collect microfossils such as foraminiferans and ostracods. For other groups such as radiolarians, diatoms and conodonts, acetic or hydrochloric acid is used to remove the carbonate fraction and concentrate the fossils. For palynomorphs, the silicate minerals are removed with hydrofluoric acid, an extremely dangerous chemical that requires special facilities. Finally, microfossils may be concentrated by settling in heavy liquids or by electromagnetic separation. Many groups, such as algae and forami-niferans, may also be studied in thin section.

PROTISTA: INTRODUCTION

The protists are predominantly single-celled organisms with nuclei and organelles, including both autotrophs, organisms that convert inorganic matter such as CO2 and water into food, and heterotrophs, organisms that eat organic debris or other organisms. The Protista is a convenient grouping but it is not well defined. Essentially it consists of all eukary-otes once the multicellular animals, fungi and vascular plants are removed. Consequently it is a paraphyletic collection of rather disparate organisms. Most are microscopic and unicellular but multicellularity has evolved numerous times and the multicellular algae (seaweeds) are conventionally included in the Protista too

Figure 9.1 Protist positions on the tree of life. In this tree, developed by Patrick Keeling, University of British Columbia, the protozoans (foraminiferans and radiolarians) lie within the Cercozoa far divorced from the chromists (diatoms and dinoflagellates) within the Chromalveolates. (From Keeling et al. 2005.)

Figure 9.1 Protist positions on the tree of life. In this tree, developed by Patrick Keeling, University of British Columbia, the protozoans (foraminiferans and radiolarians) lie within the Cercozoa far divorced from the chromists (diatoms and dinoflagellates) within the Chromalveolates. (From Keeling et al. 2005.)

(Fig. 9.1). Subdividing the diversity of protists is equally problematic. The division into auto-trophic protozoans and heterotrophic algae (chromistans) is important ecologically, but phylogenetically almost meaningless as both groups are polyphyletic. The first protists were almost certainly heterotrophs, but chlo-roplasts were acquired separately in at least six lineages, producing heterotrophs, and lost secondarily even more often: for example, the classic protozoan ciliates almost certainly evolved from algae. Protists are also often subdivided according to their means of locomotion, most simply into flagellates and amoebans. Again, however, these are poly-phyletic groups. So simplisitic attempts at classifying protists do not really work and they are perhaps better regarded as a loose grouping of 30 or 40 disparate phyla with diverse combinations of trophic modes, mechanisms of motility, cell coverings and life cycles. Modern molecular genetic and cyto-logic research is slowly making sense of this diversity but this is not the place to go into the rapidly changing details of this research. Instead, we should simply note that groups with microfossil records are widely scattered across the diversity of protists. Here, following Cavalier-Smith (2002) and others, the protists are grouped into protozoans (forami-niferans, radiolarians, acritarchs, dinoflagel-lates and ciliophorans) and chromistans (coccolithophores and diatoms); chitinozoans are difficult to classify in this scheme and are thus treated separately.

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