Photosynthetic and Nitrogen-Fixing Prokaryotes

It is likely that the earth was once a completely anaerobic place containing water, ammonia, methane, formaldehyde, and more complicated organic compounds. Perhaps the first forms of life, which may have originated about 3.5 x 109 years ago, resembled present-day anaerobic bacteria. The purple and green photosynthetic bacteria may be related to organisms that developed at a second stage of evolution: those able to capture energy from sunlight. Most of these gram-negative photosynthetic bacteria are strict anaerobes. None can make oxygen as do higher plants. Rather, the hydrogen needed to carry out the reduction of carbon dioxide in the photosynthetic process is obtained by the splitting of inorganic compounds, such as H2S, thiosulfate, or H2, or is taken from organic compounds. Today, photosynthetic bacteria are found principally in sulfur springs and in deep lakes, but at one time they were probably far more abundant and the only photosynthetic organisms on earth. Before organisms could produce oxygen a second complete photosynthetic system, which could cleave H2O to O2, had to be developed. The simplest oxygenproducing creatures existing today are the cyanobacteria, 34 also known as blue-green algae. Many
cyanobacteria are unicellular, but others such as Oscilatoria, a slimy “plant” that often coats the inside walls of household aquaria, consist of long filaments about 6 m in diameter (see Fig. 1-11). All cyanobacteria contain two groups of pigments not found in other prokaryotes:
chlorophyll a and -carotene, pigments that are also found in the chloroplasts of true algae and in higher plants. A recently discovered group of bacteria, the prochlorophytes, are even closer to chloroplasts in their pigment composition. In addition to pigmented cells, some cyanobacteria contain paler cells known as heterocysts. They have a specialized function of fixing molecular nitrogen. The development of the ability to convert N2 into organic nitrogen compounds represents another important evolutionary step. Because they can both fix nitrogen and carry out photosynthesis, the blue-green algae have the simplest nutritional requirements of any organisms. They need only N2, CO2, water, light, and minerals for growth. Evolution of the photosynthetic cleavage of water to oxygen was doubtless a major event with farreaching consequences. Biologists generally believe that as oxygen accumulated in the earth’s atmosphere, the obligate anaerobes, which are poisoned by oxygen, became limited to strictly anaerobic environments. Meanwhile, a new group of bacteria, the aerobes, appeared with mechanisms for detoxifying oxygen and for using oxygen to oxidize complex organic compounds to obtain energy.

Abbreviations:

BM, basement membrane, ER, rough endoplasmic reticulum, (with ribosomes attached; smooth, ER is depicted nearer the nucleus and on the right side of the cell.), DI, deep indentation of plasma membrane, GI, glycogen granules Gap, space ~10-20 nm thickbetween adjacent cells, M, mitochondrion, Mb, microbody, L, lysosome, D, desmosome, TJ, tight junction, Mv, microvilli, C, cillium, SG, secretion granule, V, vacuole, Nu, nucleolus, G, Golgi apparatus, CW, cell wall (of a plant), Ct, centrioles, P, plasmodesmata, N, nucleus, Cp, chloroplast, St, starch granule.

The “average” eukaryotic cell. This composite drawing shows the principal organelles of both animal and plant
cells approximately to the correct scale. (Adapted from a drawing by Michael Metzler.)