Photosynthesis and the Earth system

It is now clear that the chloroplasts in plant cells were originally derived from prokaryotes (Margulis, 1971; Margulis and Dolen, 2002), therefore questions about the early history of photosynthesis on Earth are really questions about the origin of photosynthesis in bacteria. There has been a general assumption by many workers that anoxygenic photosynthesis predates oxygenic photosynthesis (Fenchel and Finley, 1995), conclusions which have been supported by more recent molecular analysis (Xiong et al., 2000).

The evolution of anoxygenic photosynthesis on a planet is ecologically important as it provides a source of energy for organisms which does not just rely on pre-existing organic chemicals. Circumstantial evidence for anoxygenic photosynthesis has been described from 3,416 million year old rock in South Africa (Tice and Lowe, 2004). However, the rise of oxygenic photosynthesis is probably an even more important major transition in the history of the Earth's environment as it led to a greatly increased supply of free energy for life (Lenton et al., 2004). Aerobic food chains can potentially support many more trophic levels than anaerobic ones (Fig. 7.2). Since size is a major factor in structuring many food chains, longer food chains make possible the abundance of macroscopic animals which feature so prominently in many ecology textbooks. In a detailed study of published food webs Cohen et al. (1993) found that about 90% of the feeding links between animal species with known sizes had a larger animal consuming a smaller prey. Obvious exceptions to this general rule are most parasites, also many whales and several species of shark are surprisingly larger than one might expect given the fact that they eat plankton! But in general, multicellular predation may require an oxygen-rich atmosphere for its evolution, for both the physiological and ecological reasons outlined above. These arguments suggest that oxygenic photosynthesis may be intimately connected with a planet's biomass and hence the likelihood of the merging of organismal and ecological physiology (Chapter 6). Anoxygenic photosynthesis will increase the system's energy source thereby potentially increasing the biomass; however,

Trophic level

Fig. 7.2: Oxygen should allow longer food chains on a planet. In extant anaerobic fermentation on Earth the energy yield is about 25% that of aerobes. Assuming a growth efficiency of 10% for anaerobic food chains and 40% for aerobic ones, then a similar amount of energy will support a much longer food chain in an aerobic system (from Fenchel and Finlay, 1995).

Trophic level

Fig. 7.2: Oxygen should allow longer food chains on a planet. In extant anaerobic fermentation on Earth the energy yield is about 25% that of aerobes. Assuming a growth efficiency of 10% for anaerobic food chains and 40% for aerobic ones, then a similar amount of energy will support a much longer food chain in an aerobic system (from Fenchel and Finlay, 1995).

oxygenic photosynthesis—if it leads to an oxygen-rich environment—sets the stage for a far larger planetary biomass.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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