Читать книгу Ecology - Michael Begon - Страница 82

Oxygen is a resource for both animals and plants as the final electron acceptor in the process of aerobic respiration that provides the energy that drives their metabolism. In above‐ground terrestrial environments it is rarely in limited supply, but its diffusibility and solubility in water are very low and so it can become limiting much more readily in aquatic and waterlogged environments. Because oxygen diffuses so slowly in water, aquatic animals must either maintain a continual flow of water over their respiratory surfaces (e.g. the gills of fish), which often have very large surface areas relative to body volume, or they may have specialised respiratory pigments (e.g. diving mammals and birds, see Mirceta et al. (2013)), or may continually return to the surface to breathe. The roots of many higher plants fail to grow into waterlogged soil, or die if the water table rises after they have penetrated deeply. Even if roots do not die when starved of oxygen, they may cease to absorb mineral nutrients so that the plants suffer from mineral deficiencies.

However, it would be wrong to adopt a higher‐organism centred point of view in which respiration is predictably aerobic, reliant on oxygen as a resource that is equally predictably available. On the contrary, there are environments where oxygen is simply absent – often described as ‘extreme’, such as hot springs or deep in the ocean – and many others in which oxygen levels are depleted by biological activity at rates that cannot be counteracted by diffusion or by the activities of photoautotrophs. This is the case, for example, when organic matter decomposes in aquatic environments, and microbial respiration makes a demand for oxygen that exceeds the immediate supply. It is true, too, in water bodies that suffer eutrophication (see Section 21.1.3) when they are overly enriched with nutrients, particularly nitrates and phosphates, often as pollutants, inducing excessive growth of plants and algae that may again deplete oxygen faster than it can be replaced. Many microorganisms living in all these types of environment respire anaerobically, using alternative resources to oxygen as the final electron acceptor in the respiratory process: nitrates, sulphates, CO₂, ferric iron and many others. Of course, where oxygen is absent altogether, all those respiring actively must do so anaerobically.

anaerobic respiration: widespread and varied

Anaerobic respiration is generally far less efficient (produces much less energy) than aerobic respiration. Hence, when oxygen is in ready supply, aerobes thrive and anaerobes are little in evidence. However, the balance within ecological communities can change rapidly. One reason is that many microbes are facultatively anaerobic – capable of both aerobic and anaerobic respiration. We see an example in Figure 3.27a, where pitcher plants (carnivorous plants that trap their prey in pitcher‐shaped modified leaves) contain a digestive liquid that supports a community of microbes. Natural communities of pitchers of the northern pitcher plant, Sarracenia purpurea, from Vermont, USA, were compared with pitchers that had been enriched through the repeated addition of finely ground insects (without a microbial community of their own) similar to the plants’ natural prey. Such excess loading of organic material leads to an increase in microbial activity and hypoxic (low oxygen) conditions within the experimental pitchers, as can happen naturally in pitcher plants, and as it does in much large water bodies such as ponds and lakes. The microbial activity within the communities of the control and experimental pitchers were very different, as judged by the peptides within them, which could be extracted, identified and assigned to the types of microbes producing them (Figure 3.27a). When oxygen was readily available as a resource, most of the peptides were contributed by aerobic bacteria. But when oxygen was in very short supply, most came from facultative anaerobes that could rapidly switch their metabolism from aerobic to anaerobic respiration. It is also notable, therefore, that in neither case was there a major contribution from bacteria that were obligatory anaerobes.

Figure 3.27 Enrichment commonly leads to a switch from oxygen to (anaerobic) alternatives as a resource for respiration. (a) The proportion of microbial peptides in communities occupying the pitchers of Sarracenia purpurea that were either controls or enriched, originating from microbes with different respiratory modes. (b) The percentage of taxa that were dormant (metabolically inactive) in control and nitrogen‐enriched plots in saltmarshes over four years. Bold lines are medians, boxes represent 25–75 percentiles and whiskers show ranges, with outliers also shown.

Source: (a) After Northrop et al. (2017). (b) After Kearns et al. (2016).

Similarly, but on a larger scale, enrichment of salt marshes in Massachusetts, USA, led to soil microbial communities in which a much increased proportion of the taxa was dormant, that is, metabolically inactive (Figure 3.27b), but among those that were active, there was a major shift from aerobic to (in this case) obligatorily anaerobic taxa, many using sulphate or sulphur rather than oxygen as their respiratory resource. Clearly the prevalence of dormancy and the presence of facultative anaerobes mean that communities can switch rapidly from a widespread reliance on oxygen to the use of alternative resources for respiration.