Microbial carbonates are long-ranging, essentially bacterial, aquatic sediments. Their calcification is dependent on ambient water chemistry and their growth is influenced by competition with other organisms, such as metazoans. In this paper, these relationships are examined by comparing the geological record of microbial carbonates with metazoan history and secular variations in CaCO 3 saturation state of seawater. Marine abundance data show that microbial carbonates episodically declined during the Phanerozoic Eon (past 545 Myr) from a peak 500 Myr ago. This abundance trend is generally inverse to that of marine metazoan taxonomic diversity, supporting the view that metazoan competition has progressively limited the formation of microbial carbonates. Lack of empirical values concerning variables such as seawater ionic composition, atmospheric partial pressure of CO 2 , and pH currently restricts calculation of CaCO 3 saturation state for the Phanerozoic as a whole to the use of modeled values. These data, together with palaeotemperature data from oxygen isotope analyses, allow calculation of seawater CaCO 3 saturation trends. Microbial carbonate abundance shows broad positive correspondence with calculated seawater saturation state for CaCO 3 minerals during the interval 150–545 Myr ago, consistent with the likelihood that seawater chemistry has influenced the calcification and therefore accretion and preservation of microbial carbonates. These comparisons suggest that both metazoan influence and seawater saturation state have combined to determine the broad pattern of marine microbial carbonate abundance throughout much of the Phanerozoic. In contrast, for the major part of the Precambrian it would seem reasonable to expect that seawater saturation state, together with microbial evolution, was the principal factor determining microbial carbonate development. Interrelationships such as these, with feedbacks influencing organisms, sediments, and the environment, are central to geobiology.

Geobiology attempts to understand the interactions between Earth and the life which has evolved on it. It is an all-encompassing field that embraces both living systems on Earth as well as an understanding of the history of these systems since life first evolved. The fossil record and associated geological and geochemical data provide a major avenue towards understanding the evolution of geobiological systems. Among the many new and exciting directions of research for geobiology, a particularly fertile field is represented by studies of the various interactions between eukaryotes and microbes that can be detected through examination of the fossil record. For example, such interactions include the various roles which microbes play in taphonomy and preservation of eukaryotes as fossils, particularly cases of exceptional fossil preservation. Similarly, recognition of microbially induced sedimentary structures (MISS) in siliciclastic sediments has been a subject of significant recent interest. Documentation of MISS has led to a greater understanding of the role of microbial mats in providing a distinctive structure to marine subtidal seafloors, which has fostered studies of benthic eukaryote adaptations to such mat-dominated environments. The continued evolution of bioturbation in the Cambrian led to elimination of microbial mats on shallow subtidal seafloors, causing evolutionary and ecological changes in eukaryotes adapted to living on and in seafloors structured by such mats. Studies of these changes, termed the “Cambrian substrate revolution”, have been documented for a variety of echinoderms, molluscs, and trace fossils, and are some of the first to illuminate ecological interactions between eukaryotes and microbes through study of the fossil record. This new awareness which geobiology represents has begun to produce a whole host of approaches that causes researchers in the laboratory of the molecular or microbiologist to interact with those from the laboratory of the geochemist as they are led on a field trip by the palaeontologist exploring some fundamental issue in the history of life. Such interactions are not only important for considerations of life on Earth, but provide a framework for the search for life that may have once existed on other planetary bodies, such as Mars. Data from deep time and hence the fossil record plays a central role in such research, and thus geobiology opens up vast new research opportunities for palaeontology.

The temperature and salinity histories of the oceans are major environmental variables relevant to the course of microbial evolution in the Precambrian, the “age of microbes”. Oxygen isotope data for early diagenetic cherts indicate surface temperatures on the order of 55–85 °C throughout the Archean, so early thermophilic microbes (as deduced from the rRNA tree) could have been global and not just huddled around hydrothermal vents as often assumed. Initial salinity of the oceans was 1.5–2× the modern value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean. Because O 2 solubility decreases strongly with increasing temperature and salinity, the Archean ocean was anoxic and dominated by anaerobic microbes even if atmospheric O 2 were somehow as high as 70% of the modern level. Temperatures declined dramatically in the Paleoproterozoic as long-lived continental cratons developed. Values similar to those of the Phanerozoic were reached by 1.2 Ga. The first great lowering of oceanic salinity probably occurred in latest Precambrian when enormous amounts of salt and brine were sequestered in giant Neoproterozoic evaporite basins. The lowering of salinity at this time, together with major cooling associated with the Neoproterozoic glaciations, allowed dissolved O 2 in the ocean for the first time. This terminated a vast habitat for anaerobes and produced threshold levels of O 2 required for metazoan respiration. Non-marine environments could have been oxygenated earlier, so the possibility arises that metazoans developed in such environments and moved into a calcite and silica saturated sea to produce the Cambrian explosion of shelled organisms that ended exclusive microbial occupation of the ocean. Inasmuch as chlorine is a common element throughout the galaxy and follows the water during atmospheric outgassing, it is likely that early oceans on other worlds are also probably so saline that evolution beyond the microbial stage is inhibited unless long-lived continental cratons develop.

Biogeochemistry can be defined as the mutual interactions (two-way) between the biology and chemistry of the Earth system, and as such is clearly an important component of the broader discipline of geobiology. It is a well-developed field, having a dedicated journal and textbook, and many hundreds of publications appearing in the scientific literature each year. Perhaps the best example of biogeochemistry is the balance between photosynthesis and respiration, autotrophy and heterotrophy, on Earth, the so-called Redfield equation. The modern practice of biogeochemical research has been focused on present-day processes such as those in the Redfield equation, but new tools and approaches are beginning to explore the biogeochemistry of the ancient Earth; this is the challenge for biogeochemists in the future.

More than 10 years after the 1991 Gulf War oil spill on the Saudi-Arabian coast of the Arabian Gulf, natural remediation has only been partially successful. This fact demonstrates the importance of studying the preconditions for, and the process of, hydrocarbon degradation as well as the competing processes which prevent or slow them down. This paper deals with the preconditions of biodegradation: the presence of water, the availability of oxygen, the influence of temperature and the presence and type of degrading microorganisms. This paper discusses abiotic transformation as well as aerobic and anaerobic biodegradation. The importance of emulsification and hydrocarbon uptake into the degrading microorganisms is underlined. Competing processes include the conversion of liquid oil to viscous and finally solid material, the formation of solid sediment–oil mixtures and the clogging of sediment pores preventing oxygen from entering the pores. At first glance, the competing processes seem more “geological” while emulsification and hydrocarbon uptake seem more “biological”. However, since the necessary energy input (e.g., waves and turbulence) is “geo“(physical), it becomes clear, on the one hand, that the biological process requires geophysical energy input and, on the other hand, is inhibited by geological competition. Oil pollution as well as remediation progress affect the impacted environment since they leave undegradable residues and intermediates as well as microbial biomass and its conversion products. Sediments are enriched with organic matter, whose properties and behaviour are altered. Even inorganic matter may be formed during hydrocarbon biodegradation. The final part of this paper consists of case studies of the accident of the tanker BRAER (1993), the oil spill in the Arabian Gulf (1991), and the continual seeping of the wreck PRESTIGE (2002/03). These three cases demonstrate the importance of oil type, energy input, climatic conditions, as well as human interest in the use of the impacted coast, in determining speed and success of remediation and possible restoration measures.