Explicaciones de los parámetros frecuentes

The three elements needed for making carbohydrates—carbon, oxygen and hydrogen—were virtually unlimited and freely accessible to the early plants, so it would seem that there was no obstacle to their productivity. But there is one more element required in large amounts to make many of the other molecules needed for life: nitrogen. Nitrogen is an essential ingredient in proteins, DNA, and many other vital molecules, yet it is often very limited, at least in forms that can be used by organisms. Farmers don’t need to fertilize with carbon, oxygen, or hydrogen, but they must constantly replenish the nitrogen in the soil.

This element is extremely abundant in the atmosphere as nitrogen gas (N₂), but ironically relatively few living organisms can use it in that form. The triple bonds that hold the two nitrogen atoms together are difficult to break, and energy is required to do so. Cyanobacteria are one of the major groups of organisms that have the enzyme (nitrogenase) necessary to break that bond and convert atmo- spheric nitrogen into ammonia. It was the final piece of metabolic machinery they needed to become the most abundant organisms on the planet, and to dominate, virtually alone, for over a billion years.

ATMOSPHERE

Nitrogen gas

Animals ingest protein from plants

Waste products, decomposed animal bodies Ammonia produced in nodules, used by plant to make

amino acids _{Denitrification}

Nitrogen fixation by Rhizobium bacteria in root nodules Nitrogen recycled, absorbed by plants

SOIL

Figure 1.7 Nitrogen gas (N₂) is abundant in the Earth’s atmosphere, but in a stable form unavailable to most living organisms. Certain bacteria convert the nitrogen to ammonia, which can be absorbed by plants or converted to other forms by various soil bacteria. Animals consume plants, and decomposers consume dead animals, animal waste, and dead plant material. This returns nitrogenous waste to the soil where some of it is recycled by plants. More of it is broken down by bacteria in the soil, eventually returning it to the

Plants can absorb and use nitrogen if it is either oxidized to nitrate (NO₃-_{) or}

reduced to ammonia (NH₃), and animals can obtain it only by eating plants or by eating animals that have eaten plants. Ammonia and nitrate that find their way into the soil, from mineral sources or through decomposition of dead plants and animals, disappear quickly. Plants reabsorb some of it, but a host of microorgan- isms compete for it as well, and when they’re done with it, it goes back into the atmosphere as nitrogen gas (N₂). Yet life continues in great abundance. How, then, is nitrogen made available to the living world?

While a significant quantity of nitrogen gas is oxidized to nitrate by lightning in the air, thus becoming available to plants, at least half of the nitrogen needed by the living world is provided by bacteria through biological nitrogen fixation. It is still uncertain where or when this important process evolved. It has variously been proposed to have been present in the common ancestor of life (LUCA), or to have evolved in one group, and then been passed on to others through horizontal gene transfer (Raymond et al. 2004). It is widespread, in several different forms, among bacteria, and in some clades of archaea,4 _{but never in eukaryotes. It is com-}

mon in cyanobacteria and closely related clades, but absent in some of the most ancient lineages of bacteria. The most ancient branch of bacteria is considered to be the Aquificales, of which some practice nitrogen fixation and others do not. In those that do, there appears to have been a later retrofitting (Boyd et al. 2013). The origin of nitrogen fixation thus remains obscure, and its widespread occurrence is most likely due to horizontal gene transfer.

It may have required a specific “nitrogen crisis” to jumpstart the process—a time when increasing abundance of life forms, buildup of oxygen, and decreased availability from rocks made nitrogen a limiting factor for growth. This may have been as late as 2.2 to 1.5 billion years ago (see Boyd et al. 2013, for a good review of this topic). If the latter is true, cyanobacteria acquired it relatively late in their two billion year reign as supreme photosynthesizers.

To make things more difficult, nitrogenases are highly sensitive to oxygen, and so nitrogen fixation must be shielded from exposure. Some nitrogen-fixing bacteria are strictly anaerobic, but for aerobic organisms, this is a problem. Cyanobacteria of course generate oxygen as part of photosynthesis. In some, nitrogen fixation is isolated in specialized nonphotosynthetic cells (see Fig. 1.2D). In others, nitro- gen fixation takes place at night, when there is no photosynthesis. Nitrogen fixa- tion in the terrestrial environment is carried out primarily by bacteria of the genus Rhizobium, which live symbiotically in special nodules on the roots of legumes (family Fabaceae) (Fig. 1.7).

As one more addendum, recent phylogenetic studies (see Hemp and Pace 2010) of the molecular complexes involved with oxygen metabolism in a wide variety of organisms indicate that the genes required for aerobic respiration were transferred from cyanobacteria to a number of different bacteria and archaea via horizontal gene transfer. So cyanobacteria ultimately gave us both sides of the great energy cycle, and possibly nitrogen fixation as well.

captured chloroplasts within its tissues and lives off of their productivity. Its story

parallels an important phase in the evolution of plants. Photographed in the laboratory of

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