Heterotrophic metabolism

T. roseopersicina needs so many distinct hydrogenases. Our working hypothesis links this abundance of various NiFe hydrogenases to the fact that this bacterium should be able to perform various metabolic activities (photoautotrophic, photoheterotrophic, heterotrophic metabolism) in order to survive in its natural habitat [Imhoff, 2001]). Having numerous hydrogenases at hand increases the chances of survival for the bacterium and increases our chances to understand basic phenomena of hydrogenase catalysis. 

A variety of molecular indicators of the freshness (and perhaps nutritional quality) of organic matter exists. For example, certain labile phytoplankton constituents, such as polyunsaturated fatty acids, are readily degraded in the environment or in herbivore guts, and are thus depleted in more degraded particles (de Baar et ak, 1983 Wakeham and Canuel, 1988). Preferential loss of labile algal fatty acids resulting in the enrichment of more stable components in the products of heterotrophic metabolism has been observed in both field studies and laboratory feeding experiments (Prahl et ak, 1985 Wakeham and Canuel, 1988 Harvey et ak,... 

The most common anaerobic heterotrophic metabolisms found in the water column are denitrification and sulfete reduction. Their stoichiometries are given in Eqs. 8.11... 

Common heterotrophs depend on preformed organic compounds for all three primary needs. Although some carbon dioxide is fixed in heterotrophic metabolism, the het-erotrophic cell thrives at the expense of compounds formed by other cells, and is not capable of the net conversion (fixation) of carbon dioxide into organic compounds. Some bacteria referred to as photoheterotrophs are able to regenerate ATP photochemically but cannot use photochemical reactions to supply electrons to NADP+. Such organisms are like other heterotrophs in their dependence on preformed organic compounds. But because of their photochemical... 

Because of the high area of solid surfaces covered with biofilms, these biofilms dominate the heterotrophic metabolism in many aquatic ecosystems. In streams, rivers, and shallow lakes, bacterial activity in epilithic and epiphytic biofilms may be several times higher on an areal basis than the activity of free living bacteria. By the differential use of specific DOM fractions, biofilm bacteria influence the biogeochemical composition of DOM in these ecosystems. Biofilms thus can control biogeochemical fluxes of DOM and are important sinks of organic matter. 

Romani, A. M., and S. Sabater. 1999. Effect of primary producers on the heterotrophic metabolism of a stream biofilm. Freshwater Biology 41 729—736. 

In freshwater ecosystems, particularly streams and wetlands, biofilms account for a large portion of heterotrophic metabolism, as well as primary production (Edwards etal., 1990 see Chapter 12), acting as both sources and sinks for DOM. As the depth of the overlying water in the system increases, attached communities account for a declining share of system metabolism. 

Osmotrophs are organotrophic organisms that acquire nutrients from their environment one molecule at a time. Osmotrophs include most members of the bacteria and archaea domains, as well as fungi and several protistan groups. Except in association with coarse organic particles where eukaryotic fungi and oomycetes are prominent, prokaryotes dominate heterotrophic metabolism, so are the focus of the text that follows. We use the term bacteria loosely, recognizing that heterotrophic archaea may well be major consitituents of the microbial loop of many systems. 

Availability is not the only constraint on substrate consumption. The value of a substrate is also related to the resources needed to convert the molecule into an intermediary metabolite. Thus in aerobic environments, monosaccharides and amino acids are readily consumed under anaerobic conditions, heterotrophic metabolism is largely fueled by small organic acids. Some taxa consume other types of substrates as long as they are reasonably abundant. Important examples include P-proteobacteria that consume phenols, a significant component of DOM inputs originating from plant material, and methylotrophs that consume single carbon compounds produced by anaerobic metabolism, photochemical reactions, and oxidation of methyl and methoxy substituents (Giovannoni and Rappe, 2000). 

There are a number of other important physiological events that must occur if the iron-oxidizing bacteria are to survive and perform in metal transformations. These include CO2 fixation, generation of reducing power (NADH and NADPH), heterotrophic metabolism and reduced sulfur oxidation. 

Tabita, R. and Lundgren, D.G., 1971a. Heterotrophic metabolism of the chemolitho-trophic Thiobacillus ferrooxidans. J. Bacteriol., 108 334—342. 

Some 90 per cent or more of the total metabolism in aquatic ecosystems is microbial, accomplished by heterotrophic metabolism of bacteria, fungi, and many protists, all of a size less than 100 m [1,31]. Therefore, the material and energy fluxes of aquatic ecosystems is totally dominated by metabolism of particulate detritus (non-living) and especially DOM from autochthonous and allochthonous sources. The pelagic open water is but a portion of the whole lake or river ecosystem. In relation to loading and fluxes of DOM, allochthonous and littoral sources are critical because of their chemical differences from that produced by algal photosynthesis. 

The aerobic and anaerobic heterotrophic microbes present at vents often have complex nutrient requirements (e.g. organic acids, carbohydrates, elemental sulphur) some are believed to be mixotrophic, using both autotrophic and heterotrophic metabolisms (Karl, 1995 Kelley etal., 2002). Organic matter in sulphide mineral chimneys may come from other dead and/or pyrolysed microbes and invertebrates (e.g. worm tubes and muccus) during growth of chimneys. At most hydrothermal sites, thermal environments at moderate temperatures (50-90°C) are... 

Santruckova, H., Bird, M. L, and Lloyd, 1. (in press). "C fractionation associated with heterotrophic metabolism in gras.sland soils. Funct. Ecol. 

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