No other group of microbes participates in as many symbioses as cyanobacteria, including extra- or intracellular relationships with plants,fungi, and animals. This phenomenon, coupled with the plantlike photosynthesis of cyanobacteria, suggests that cyanobacteria were the progenitors of chloroplasts . Endosymbiotic theory holds that ancestral eukaryotic cells engulfed the ancient cyanobacteria that evolved into modern plastids. Better candidates may be prochlorophytes, oxygenic photosynthetic bacteria that contain chlorophyll and and form an evolutionarily related group with cyanobacteria and plastids.
Ever since I was thrust into an soon after graduating from college, I became a student of wealth, poverty, and humanity’s problems. My of changing humanity’s energy paradigm have had a lifelong impact. It took me many years to gain a comprehensive understanding of how energy literally runs the world and always has. A good demonstration of that fact is to consider the average day of an average American professional, who is a member of and lives in Earth’s most industrialized nation. A typical day in my life during the winter before I wrote this essay can serve as an example.
Cyanobacteria are a morphologically diverse group of photosynthetic prokaryotic microorganisms that form a closely related phylogenetic lineage of eubacteria. Historically, cyanobacteria were classified with plants and called blue-green algae, although true algae are eukaryotic . Cyanobacteria appear early in the fossil record with some examples approximately 3.5 billion years old. Stromatolites are large, often fossilized colonies of cyanobacteria that build up layer upon layer. Cyanobacteria contributed to the conversion of 's atmosphere from an anoxic -reducing environment to one rich in oxygen. Commonly studied genera include , and
Marine and freshwater aquatic environments (including aquaria) are rich in cyanobacteria, either free-living, in biofilms, or in mats. Cyanobacterial species ( or ) that produce compounds (e.g., micro-cystins) toxic to humans and animals are sometimes associated with large-scale blooms in aquatic systems. Curling crusts on soils are often due to cyanobacteria. Pioneer communities on bare rock surfaces often include cyanobacteria or lichens, the latter existing as symbiotic associations of cyanobacteria and fungi. Cyanobacteria are found in extreme environments, including hot springs, desert sands, hypersaline ponds, and within the rocks of dry Antarctic valleys. Urban cyanobacteria are found as biofilms on concrete, brick buildings, and wooden fences.
bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, . Bacteria fall into one of two groups, (ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or Archaea, and the Eubacteria, or Bacteria, as major groupings (sometimes called domains) above the kingdom level.
Bacteria were the only form of life on earth for 2 billion years. They were first observed by Antony van in the 17th cent.; bacteriology as an applied science began to develop in the late 19th cent. as a result of research in medicine and in fermentation processes, especially by Louis and Robert .
Bacteria are remarkably adaptable to diverse environmental conditions: they are found in the bodies of all living organisms and on all parts of the earth—in land terrains and ocean depths, in arctic ice and glaciers, in hot springs, and even in the stratosphere. Our understanding of bacteria and their metabolic processes has been expanded by the discovery of species that can live only deep below the earth's surface and by species that thrive without sunlight in the high temperature and pressure near on the ocean floor. There are more bacteria, as separate individuals, than any other type of organism; there can be as many as 2.5 billion bacteria in one gram of fertile soil.
Bacteria are grouped in a number of different ways. Most bacteria are of one of three typical shapes—rod-shaped (bacillus), round (coccus, e.g., streptococcus), and spiral (spirillum). An additional group, vibrios, appear as incomplete spirals. The cytoplasm and plasma membrane of most bacterial cells are surrounded by a cell wall; further classification of bacteria is based on cell wall characteristics (see ). They can also be characterized by their patterns of growth, such as the chains formed by streptococci. Many bacteria, chiefly the bacillus and spirillum forms, are motile, swimming about by whiplike movements of flagella; other bacteria have rigid rodlike protuberances called pili that serve as tethers.
Some bacteria (those known as aerobic forms) can function metabolically only in the presence of free or atmospheric oxygen; others (anaerobic bacteria) cannot grow in the presence of free oxygen but obtain oxygen from compounds. Facultative anaerobes can grow with or without free oxygen; obligate anaerobes are poisoned by oxygen.
In bacteria the genetic material is organized in a continuous strand of DNA. This circle of DNA is localized in an area called the nucleoid, but there is no membrane surrounding a defined nucleus as there is in the eukaryotic cells of protists, fungi, plants, and animals (see ). In addition to the nucleoid, the bacterial cell may include one or more plasmids, separate circular strands of DNA that can replicate independently, and that are not responsible for the reproduction of the organism. Drug resistance is often conveyed via plasmid genes.
Reproduction is chiefly by binary fission, cell division yielding identical daughter cells. Some bacteria reproduce by budding or fragmentation. Despite the fact that these processes should produce identical generations, the rapid rate of mutation possible in bacteria makes them very adaptable. Some bacteria are capable of specialized types of genetic , which involves the transfer of nucleic acid by individual contact (conjugation), by exposure to nucleic acid remnants of dead bacteria (transformation), by exchange of plasmid genes, or by a viral agent, the (transduction). Under unfavorable conditions some bacteria form highly resistant spores with thickened coverings, within which the living material remains dormant in altered form until conditions improve. Others, such as the radioactivity-resistant can withstand serious damage by repairing their own DNA.
Most bacteria are heterotrophic, living off other organisms. Most of these are saprobes, bacteria that live off dead organic matter. The bacteria that cause disease are heterotrophic parasites. There are also many non-disease-causing bacterial parasites, many of which are helpful to their hosts. These include the "normal flora" of the human body.
Autotrophic bacteria manufacture their own food by the processes of and (see ). The photosynthetic bacteria include the green and purple bacteria and the . Many of the thermophilic archaebacteria are chemosynthetic autotrophs.
Harmless and beneficial bacteria far outnumber harmful varieties. Thousands of bacterial species live commensally in humans, and many provide health benefits to humans, aiding in digestion, for example, or helping to prevent the establishment of colonies of pathogenic bacteria. Because they are capable of producing so many enzymes necessary for the building up and breaking down of organic compounds, bacteria are employed extensively by humans—for soil enrichment with leguminous crops (see ), for preservation by pickling, for fermentation (as in the manufacture of alcoholic beverages, vinegar, and certain cheeses), for decomposition of organic wastes (in septic tanks, in some sewage disposal plants, and in agriculture for soil enrichment) and toxic wastes, and for curing tobacco, retting flax, and many other specialized processes. Bacteria frequently make good objects for genetic study: large populations grown in a short period of time facilitate detection of , or rare variations.
Bacterial parasites that cause disease are called pathogens. Among bacterial plant diseases are leaf spot, fire , and wilts; animal diseases caused by bacteria include , , , , and . Some bacteria attack the tissues directly; others produce poisonous substances called toxins. Natural defense against harmful bacteria is provided by antibodies (see ). Certain bacterial diseases, e.g., tetanus, can be prevented by injection of or of serum containing antibodies against specific bacterial antigens; immunity to some can be induced by ; and certain specific bacterial parasites are killed by .
New strains of more virulent bacterial pathogens, many of them resistant to antibiotics, have emerged in recent years. Many believe this to be due to the overuse of antibiotics, both in prescriptions for minor, self-limiting ailments and as growth enhancers in livestock; such overuse increases the likelihood of bacterial mutations. For example, a variant of the normally harmless has caused serious illness and death in victims of . See also .
See P. Singleton, (1992); W. Biddle, (1995).
A distinguishing feature of cyanobacteria is their photosynthetic pigment content. In addition to chlorophyll , cyanobacterial thylakoids include phycobilin-protein complexes (phycobilisomes) containing mixtures of phycocyanin, phycoerythrin, and allophycocyanin, which give cyanobacteria their characteristic blue-green coloration. Phycobilisomes harvest light at wavelengths (500 to 650 nanometers ) not absorbed by chlorophylls. Mostcyanobacteria perform oxygenic photosynthesis like higher plants. A few species perform anoxygenic photosynthesis, removing electrons from hydrogen sulfide (H2 S) instead of water (H2 O). There is a general dependence on carbon dioxide as a carbon source, although some cyanobacteria can live heterotrophically by absorbing organic molecules. The reductive pentose phosphate pathway predominates for carbon assimilation, as cyanobacteria have an incomplete tricarboxylic acid (Krebs) cycle.
Regardless of the debate, bacteria have been around since the dawn of life on Earth, and they have continued to evolve. A major problem facing the medical community today is the ability of disease-causing bacteria to develop a resistance to antibiotics and other antibacterial drugs. These types of bacteria have been able to change their forms or have even been able to secrete enzymes that destroy the antibiotics. Since the
Many species of cyanobacteria fix atmospheric dinitrogen (N2) into ammonia (NH3) using nitrogenase, an enzyme that is particularly sensitive to the presence of oxygen. In filamentous cyanobacteria, such as and , certain cells differentiate into heterocysts (thick-walled cells that do not photosynthesize), in which nitrogen fixation occurs under reduced oxygen concentrations. Cyanobacterial nitrogen fixation produces bioavailable nitrogen compounds that are important in nitrogen-limited aquatic ecosystems and plays an important role in global nitrogen cycling.
There is , but it is currently thought that life on Earth today descended from organism, a creature known today as the Last Universal Common Ancestor (“”). The reasoning is partly that all life has a preference for using certain types of molecules. Many molecules with the same atomic structure can form mirror images of themselves. That mirror-image phenomenon is called . In nature, such mirror images occur randomly, but life prefers one mirror image over the other. In all life on Earth, proteins are virtually without exception left-handed, while sugars are right-handed. If there was more than one line of descent, life with different “handedness” would be expected, but it has never been found, which has led scientists to think that LUCA is the only survivor that spawned all life on Earth today. All other lineages died out (the likely answer, and there was probably hundreds of millions of years of evolution on Earth before LUCA lived), or they may have all descended from the same original organism. As we will see, this is far from the only instance when such seminal events are considered to have probably happened only . Also, the unique structure of DNA and many enzymes are common to all life, and they did not have to form the way that they did. That they came through different ancestral lines is unlikely.
In October 2000, a team of biologists claimed to have revived a bacterium that existed 250 million years ago, well before the age of the dinosaurs. They found the bacterium in a drop of fluid trapped in a crystal of rock salt that had been excavated from an air duct supplying a radioactive waste dump 1,850 feet (564 meters) below Earth's surface near Carlsbad, . When the biologists drilled into the pocket of fluid in the crystal and mixed nutrients with the fluid, bacteria soon appeared. However, other scientists quickly suggested that the bacteria that grew was simply modern bacteria that had infected the crystal sample. The questioning scientists also pointed out that it would be impossible for the bacterium's DNA (a complex molecule that stores and transmits genetic information) to have survived more than a few thousand years, at best.
The diagrams used in this chapter are only intended to provide a glimpse of the incredible complexity of structure and chemistry that takes place at the microscopic level in organisms, and people can be forgiven for doubting that it is all a miraculous accident. I doubt it, too, as . Prokaryotes do not have organelles such as mitochondria, chloroplasts, and nuclei, but even the simplest cell is a marvel of complexity. If we could shrink ourselves so that we could stand inside an average bacterium, we would be astounded at its complexity, as molecules move here and there, are brought inside the bacterium’s membrane, used to generate energy and build structures, and waste products are ejected from the organism. Cellular division would be an amazing sight.