Bacterial Paleontology

A.Yu. ROZANOV

Paleontological Institute RAS, Profsoyuznaya 123, Moscow, 117997, Russia

Abstract. Bacterial paleontology is rather young branch of paleontological studies. Bacteria and microbes in general could be perfectly preserved as fossils. The major part of the sedimentary rocks formed in the photic zone of epicontinental basins of the past originated under the influence of microorganisms. Studies have increased the number of minerals known to be formed with the participation of microorganisms to >100. Bacterial-pal eontological data accompanied by the data on the first origin of eukaryotes, metazoans, etc. significantly enrich the knowledge of evolution of the biosphere and reveal a long period of transitional biosphere prior to the appearance of the typical eukaryotic biosphere of the modern type. The bacterial-paleontological data on carbonaceus chondrites make a foundation for the possibility of the presence of extraterrestrial bacterial life.

The important role of bacteria and other microbes in geological processes was suggested long ago. Already, by the end of the 19th century, N.I. Andrusov, in his lectures, spoke about the bacterial role in the origin of some Cenozoic deposits of iron and sulfur. Later, numerous microscopic studies were aimed to prove the presence of fossilized bacteria in rocks. One of the most prominent achievements of that period was A.G. Vologdin's report [1].

However, the images obtained by specialists using relatively low magnification could not convince the scientific community. Moreover, many specialists of that period who claimed the tremendous role of bacteria in the formation of the sedimentary mineral resources, such as bauxite, iron ores, etc., incurred fierce criticism or even derision by the leading scientists of the period.

The significant breakthrough of the post-war science was the discovery of fossil microorganisms in cherts. The pioneering work in this field belongs to Barghoorn and Tyier [2]. Subsequent studies were developed in a variety of countries [3-8]. Vast amounts of material have been collected, permitting the reconstruction of, not only the origin and development of microorganisms on Earth [6,9,10], but also the general pattern of biosphere evolution in the Precambrian. These studies also considered the data obtained by the traditional method of maceration commonly used for extraction of acritarchs, spores, and pollen. In such cases, primarily, forms with organic (acid resistant) shells were extracted.

However, the most significant breakthrough was made after the introduction of electron microscopy to the study of microbial remains. The new technology revealed that almost all sedimentary rocks contain well-preserved fossils of bacteria [11-19]. The perfect preservation could not be ensured without high rates of fossilization [12,13,15,20-23], and this result was proven experimentally.

The intensive study of fossil bacteria, bacterial paleontology [15], was undertaken during the last decade. On one hand, it is tightly connected with the extensive investigation in the field of geomicrobiology, which studies the interrelation of recent microbial biota with rocks, minerals and other materials [24,25]. On the other hand, it is greatly influenced by the study of cyanobacterial mats (CBM), both recent and fossil [13,26,27]. These studies confirmed the extreme antiquity of CBM and their highly important role in the formation of an oxygenic atmosphere and the whole biosphere in general. The significance of the studies of bacteria in extreme environments should be mentioned here. Perhaps, some of those conditions are similar to those present in the Archean.

The main task for bacterial paleontology is to study the fossil microbes. Traditional paleontology deals with remains of ancient organisms. It creates systematics based on the organisms' morphological features considering their phylogenetic relations. The main fields of study that utilize paleontology are evolutionary theory, history of the organic world, and biostratigraphy. Recently, paleontology has yielded priceless data for the study of biosphere evolution. Paleogeography also gains a significant benefit from paleontology.

At first, bacterial paleontology is limited in its input data by the simplicity of the objects' morphology and specificity in systematization of the bacterial material. So, the morphological approach is important, but there is not enough criteria for diagnostic and sys-tematization of the fossil objects. The products of bacterial metabolic activity expressed by the lithology and geochemistry of rocks have special significance, and the study of carbon isotopes and other biomarkers has the highest importance.

The main applications of traditional paleontology and bacterial paleontology differ strikingly. Bacterial paleontology is very important for sedimentology, and consequently for the study of the genesis of sedimentary mineral resources, including oil and gas. Bacterial paleontology also has importance for paleogeography in the study of epicontinental basins. The greatest significance of bacterial paleontological data is plain to see within the study of biosphere evolution, especially in the Precambrian and the Early Paleozoic. Finally, bacterial paleontology is one of the principle aspects for the study of astromaterials.

For a more precise understanding of bacterial paleontology, geomicrobiology [24,25] needs to be addressed. The main task for any geomicrobiologist is to study the interrelation between bacteria or microorganisms on one side and rocks and minerals on the other. Certainly, the main biological objects of such works are recent organisms. Geomicrobiological studies are necessary not only for elucidation of the influence of recent microorganisms on the geological environment but also for the interpretation of fossil objects. Geomicrobi-ological studies have revealed that:

• Microorganisms actively affect all minerals without exception, forming biofilms on their surfaces, which, in turn, are essential features of the weathering process in a broad sense.

• Microorganisms can accumulate various metals.

• Microorganisms assist the formation of a variety of minerals that compose many sedimentary rocks, (including dolomite, layer silicates, and obviously calcium carbonate and phosphates).

Bacterial paleontology and geomicrobiology overlap in some areas; some geomicrobi-ological investigations are bacterial-paleontological and vice versa. These relationships are quite natural between closely related sciences.

The recent electron-microscopy studies of rocks varying in chemical composition and age prove that fossilized microorganisms can be found in almost all sedimentary rock. The ancient phosphorites have become the classic objects for such studies. It is this study that has revealed the excellent state of preservation of diverse cyanobacteria and other microorganisms [14,17] (Fig. 1a and 1b). Parallel investigations treated rocks aged from the Ar-chean to Quaternary, and microorganisms of uncertain nature were found in the Archean [5,28-30]. However, some strongly negative valuations [31] that repudiate some of the

Merchesen Metiorite

Figure 1. (a) Fragments of cyano-bacterial mat from Phosphorites (Khubsugul, Mongolia, Tommotian atage, Lower Cambrian). See differences in preservation (1. cyanobacterial tubes and 2. holes of tubes of cyanobac-teria in purpule bacteries). (b) Cyanobacterial mat and purpul bacteria in phosphorits (Khubsugul, Mongolia, Tommotian stage, Lower Cambrian).

Figure 1. (a) Fragments of cyano-bacterial mat from Phosphorites (Khubsugul, Mongolia, Tommotian atage, Lower Cambrian). See differences in preservation (1. cyanobacterial tubes and 2. holes of tubes of cyanobac-teria in purpule bacteries). (b) Cyanobacterial mat and purpul bacteria in phosphorits (Khubsugul, Mongolia, Tommotian stage, Lower Cambrian).

finds have been proffered. But, it seems, that the critics of the Shopffs work may not have done it in a completely correct manner.

The microorganisms from the Lower Proterozoic were found in the rocks of various types. The most interesting finds were made in jaspilites (BIF) (Fig. 2), shungites, cherty shales, and, certainly, in stromatolites. The presence of cyanobacteria in all of those rocks could almost be considered a proven fact.

Going higher up the stratigraphic column, fossil bacteria have been found in the Vendian of the Russian Platform (Fig. 3(a)), the Lower Cambrian of Australia (Fig. 3(b)), Middle and Late Cambrian of the Siberian Platform (Fig. 3 (c) and (d)) [12], Ordovician Dyctionema Shales of the northwest Russian Platform, Domanik-like rock of the Ural region, and in thin clay layers in the Carboniferous of the Moscow Syneclise.

Unique and somewhat unexpected results were obtained by the bacterial-paleontological study of graphite from the Botogol Deposit of East Sayan (South Siberia).

Figure 2. Coccoids bacteria in BIF (Kursk Magnetic Anomaly, Lower Proterozoic).

Cyanobacteria Characteristics Lecture
Figure 3. (a) Cyanobacterial mat in Vendian of East European Platform, (b) cyanobacteria in Lower Cambrian (South Australia), and (c) and (d) cyanobacterial mats with filaments and coccoids (Lower Cambrian, Synyaya formation, Siberian platform).

For a long time, graphite of this deposit was considered to be magmatogenic, since the graphite bodies are hosted by nepheline-syenite. At first, tubular structures similar in shape and size to cyanobacteria were found in the graphite [19]. But the micron-sized tubes of very simply morphology did not convince the specialists on nepheline-syenites.

Oribatid Mites

Figure 4. (a) Fungi-bacterial assosiation in graphites of Botogol deposites (Devonian, E. Sayan, Siberia), (b)-

(d) fragments of oribatid mites with trichobotrias in fungi-bacterial assosiation (Devonian graphites, Siberia),

(e)-(f) trichobotria of mites (Holocen, Siberia) [32].

Figure 4. (a) Fungi-bacterial assosiation in graphites of Botogol deposites (Devonian, E. Sayan, Siberia), (b)-

(d) fragments of oribatid mites with trichobotrias in fungi-bacterial assosiation (Devonian graphites, Siberia),

(e)-(f) trichobotria of mites (Holocen, Siberia) [32].

The study of this graphite continued [16], and in a short time, numerous fungal-like remains and fragments of loricate ticks with perfectly preserved trachybotria were found (Fig. 4). The finding of well-preserved ticks strongly supported the bacterial-paleonto-

logical results and closed the discussion on the graphite origin. It is now obvious that these graphites were formed as the result of a conversion of highly carbonic sedimentary carbonate rocks.

The deposits of sedimentary mineral resources give another perfect example of the tremendous role of bacterial in geological history. Besides the above mentioned phosphorites, jaspilites (BIF), graphites, and rare metal ores, the following rocks are noteworthy: iron ores, manganese ores (including oceanic manganese concretions), bauxites, sulfides, gold, etc. [13,33-36].

As already noted concerning Andrusov's lectures, the bacterial role in the accumulation of iron (Fe), sulfur, silicon, phosphorous, and manganese (Mn) was undoubtful. Later it was realized that there are few elements in Mendeleev's Periodical System (except for artificial elements and inert gases) that are not connected in some way with bacterial activity.

The most striking example of the latest studies is the Tomtor rare metal deposit (Siberian Platform) [37]. Here cyanobacteria act as a specific filter, increasing the concentration of elements to tremendous levels: e.g., niobium content is 12 percent.

It is clear today, that only a few species of bacteria are element-specific, and they extract a wide spectrum of elements from water. Two decades ago only ~20 minerals were known that were originated by the microbial activity [38]. The number of such minerals has grown significantly and now exceeds 100. Quartz, cristobalite, pyrolusite, silicates (including layered silicates), and feldspars are of special interest [13,22,25,39].

The biomineralization process in modern hot springs and in some terrestrial water basins convincingly demonstrate a rapid (minutes to hours) mineral formation with the participation of bacteria. Clear examples of authigenic bacterial mineralization were shown by Tazaki et al. [22].

Quartz, crystobalite, barite, feldspar, sulfur, buserite, ferrohydrite, hematite, pyrite, jarosite, smectite, kaolinite, calcite, and other minerals were discovered in differing environments of hot springs, lakes, mines, and waterfalls. In contrast to prior fragmentary data on these minerals, the Japanese discoveries are very impressive, and are described in their book [22].

Consider the example derived from the Hiraya hot spring. The spring was cased, and three groups of minerals were formed in a box and a tray where three bacterial communities (white, brown, and green mats) grew (Fig. 5 (a)).

Changes in temperature, pH, and Eh values were measured (Fig. 5b). In the box with sausage-shaped bacteria (the white mat) located immediately at the hot spring, sulfur minerals of rhomboidal and amorphous form and fewer clay minerals, 7 A, were formed. It has long been known that rhombic sulfur is generated with the participation of microorganisms.

Two different mats occurred in the chute. The brown mat of filamentous bacteria and bacilli was in the axial part, where quartz, 3.3 A; feldspar, 2.8 A; cristobalite, 4.0 A; and clay minerals, 7.0 A, were formed. The green mat of filamentous bacteria functioned at the peripheral parts, where calcite, 3.0 A; quartz; and clay minerals appeared.

Thus in this case, it can be confidently stated that the bacteria participated in authigenic formation of the minerals listed—with some values, e.g.: those of pH, being registered.

It should be stressed that diffractograms of mineral structure most frequently show a distinct hallo reflecting the amorphous phase of these minerals. Tazaki and his colleagues unambiguously noted the amorphous phase of the minerals. This makes the successive stages of mineral formation more clear.

Among the many results that are now routinely published [24,25], those obtained by Gorbushina et al. [11] are most impressive. The researchers revealed that forsterite was formed with the participation of microorganisms, such as cyanobacteria, actinomycetes, and lichens.A very high rate (minutes to hours) of mineral formation was confirmed by numerous laboratory tests [13,20]. This can explain the excellent state of preservation of fossil

Figure 5. (a) Illustration and (b) graphic representation of the distribution of different types of mats and minerals in Hiraya Hot Spring, Japan [22].

bacteria. In addition, the high rate of fossilization brought to attention a problem of dating the fossilization events; they could repeat over and over again in geological history.

As already mentioned, the analysis of the x-ray structural data of the mineral phase of bacteria or inside bacterial mats revealed the presence of an amorphous mineral stage in

Figure 6. Coccoidal bacteria (a) in clay and (b) with biofilm in clay (Carboniferous, Moscow basin).

most cases, except for carbonates. The amorphous stage is typical for minerals of the quartz group, Fe and Mn oxides, layered silicates, feldspars, etc. The amorphous stage is almost absent or unrecognizable by the x-ray structural method in calcite and aragonite.

Probably, the preservation of fossilized bacteria is better when the amorphous stage is longer and more abundant. The worst preservation corresponds with carbonates since the

Evaporite Platform
Figure 7a. Lower Cambrian paleogeography of the Siberian platform. The index denotes (1) evaporite basins, (2) biogerms, (3) deep water basins, and (4) the boundary of the Siberian Platform) (after V. Savitsky and V. Astashkin).

cellular shells and membranes are destroyed by the rapid crystallization that occurs with a short amorphous stage. Carbonate micrites are usually believed to be products of microbial activity.

Currently, the most interesting examples of bacterial formation of minerals are obviously referred to those minerals that are widely known as composing of sedimentary rocks, e.g.: clay minerals, feldspars, etc. Laboratory experiments and recent natural examples are already known [22]. However, the verification by data from the geological past is forthcoming.

The first step in proving the idea concerning the authigenic formation of thin clay layers was the investigation by P.B. Kabanov, a specialist in the lithology and stratigraphy of Carboniferous deposits of the Moscow region. Kabanov used scanning electron microscopy to study the samples of thin layers from the Peski section (Fig. 6). The first illustration shows rounded bodies immersed within the unstructured matrix. It is noteworthy that all bodies are 2-5 ^m in size and have a rounded shape. If it were terrigenous debris, the shapes would be obligatorily angular at such sizes. Therefore it must be supposed that they are coccoid bacteria.

The second case is even more convincing. It was found that the rock is composed of spherical or granular bodies of 3-5 |im in size that are connected by thin threads.

This situation is principally similar with the structure of the biofilms. Further study of the clayish rocks will present new evidence favoring the authigenic origin of most of them, especially the thin layers with inconstant spreading. The main problem is how to recognize and calculate ratios between terrigenous (probably more abundant) and authigenic-bacterial compounds of clay.

Figure 7b. Lower Cambrian (Botomian) paleogeography of the Siberian platform [15].

In connection with the studies of clays, the problems of high-carbon rocks, including so-called black shale, become very important. The above-mentioned works on the Cambrian bituminous rocks of Siberia suggest that the earlier interpretation of such rocks being deep-water formations is dubious.

The presence of benthic cyanobacterial remains in the Sinyaya Formation [15,40] forces the reinterpretation of the paleogeography of the Sinsk paleobasin. Instead of a rather deep-water basin opened towards the ocean, a shallow-water, partly closed sea with bottom anoxia (probably within the sediment) (Figs 7a and 7b) can be concluded.

In addition to the biological evidences, the geological data also support this reconstruction.

First, the boundary between the transitional zone and eastern part of the basin is clearly traceable within the middle reaches section of the Lena River. One has no chance to suggest a 400-m depth for the basin without ignorance of the structural geology and stratigraphy. Second, the sections of the Kolyma Masiff (although fragmentary ones) correspond to the transitional zone of the Siberian Platform in faunal and thickness characteristics.

All of those facts induce a description of the general features of the ancient epicontinental basin. This type of the basin is absent from the recent environment. These basins covered a significant part of modern continents, mostly shallow water, and probably with intensive hydrodynamics that formed numerous short-lived and migrant islands. Owing to shallow water conditions and the fact that an entire water column was placed within the photic zone, the water masses were rich in bacteria.

A study of paleogeographic reconstructions (Fig. 8) shows that such gigantic pools were typical for the Proterozoic and Paleozoic. They had nothing in common with the recent seas and oceans, so the character of sedimentation was different and, to a significant extent, was determined by bacterial (especially by cyanobacterial) activity.

Murchison Meteorite

All these data make a serious basis for the study of the biomorphic structures in meteorites. It was bacterial paleontology that gave a possibility to reconsider previous data and get new data on carbonaceous chondrites.

A review of these data is present in several recent publications [13,41-43]. It is of special importance that all these works deal with objects of micrometric but not nannometric

Murchison Meteorite

Transitional biosphere

Eucaryotic biosphere

Procaryotic biosphere

Figure 9. Principle events in the evolution of the Precambrian biosphere (after Rozanov, 1992, supplemented).

Transitional biosphere

Eucaryotic biosphere

Procaryotic biosphere

Figure 9. Principle events in the evolution of the Precambrian biosphere (after Rozanov, 1992, supplemented).

size. In addition, the problem of the recognition of contamination commonly present in meteorites was also discussed.

Bacterial paleontology is a rather young branch of paleontological studies, but the first steps were made long ago. However, during recent years special bacterial-paleontological investigations have made it possible to formulate a series of six statements that significantly changed our conceptions in different areas of knowledge.

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    What is microbial paleotology?
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