Liebig’s Law of the Minimum and the barrel analogy. 

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... ecosystems through plants is either to be rapidly degraded into heat or buried into organic matter recalcitrant to decomposition. Energy flow within ecosystems is thus relatively simple. Consequently, its study led to straightforward, successful theories and concepts, such as Eltonian pyramids (Elton 1927), ecological efficiencies and food webs (Lindeman 1942). On the other hand, the nutrients flow in ecosystems is a cycle. For a given element, autotrophic organisms are those that incorporate in their biomass the element in its mineral form from the environment. Generally, they are also primary producers, but many heterotrophic microorganisms show this ability, too. Mineral elements in ecosystems come partly from external outputs, but the bulk comes from the decomposition of carcasses and wastes from the organisms themselves. On very long timescales, because the Earth is virtually a closed system, most available elements should cycle at least once through living organisms, short of those spurted from the depths of the planet by volcanoes. This cycling adds a level of complexity that can hardly translate into simple, general laws, as is the case for energy (Loreau 2010). Faced with this complexity, few theoreticians attempted to look for generalities about the flows of matter in food webs and ecosystems, despite Lotka’s (1925) longstanding call (DeAngelis 1992). Admittedly, there have been some fields in which approaches based on elemental composition were applied. Moreover, there have been some sporadic attempts at taking into account the repercussions of the organisms elemental composition on their interactions or on the availability of nutrients in ecosystems. The next section is dedicated to a presentation of these fields that considered the role of elements in biology, but did not serve in their time as stepping stones towards a comprehensive theory of the role of elements in biological interactions. The field of ecology had to wait until the early nineties for such a theory, called “Ecological Stoichiometry”, to emerge. It is a recent, exciting theory, presented in the third section, which tackles the role of elements in ecological interactions with a novel and promising approach. It views organisms as a single molecule, made of a combination of the various essential elements (C, N, P, Fe...). Accordingly, it treats ecological interactions as chemical reactions, during which elements are exchanged between a consumer, its resource and the environment. It generally assumes that the organisms stoichiometry is constant, i.e., that their elemental composition is homeostatic. But this assumption is not essential to the theory. More essential is the mass conservation principle, which constrains the ecologists to track the fate of all the important elements exchanged in an ecological interaction. This theory has led to major advances in our understanding of ecological interactions across biological scales. Among them, there are: the realization that the growth of higher, complex organisms can be limited by the availability of one specific element in their food (Urabe & Watanabe 1992); the uncovering of indirect effects from plants on their supply of mineral nutrients through herbivores, because of mismatches between their elemental compositions (Sterner 1990); the exposure of a causal relationship between the elemental compositions of organisms and their growth rates (Elser et al. 2003a). Surprising insights from the theory also extend to other fields of biology, such as reproductive biology (Bertram et al. 2006), human cancer (Elser et al. 2007), evolution (Souza et al. 2008) and genomics (Acquisti et al. 2009). The earliest contributions of ecological stoichiometry to biology are covered in the 4 th section. The latest contributions are covered in the 5 th section. These advances led to the coinage of a new term, “Biological Stoichiometry”. This term is meant to emphasize the potential of the theory to link processes across all the scales of biology, from molecules to the biosphere. The last section of this chapter will briefly evaluate how far the theory has gone in this unifying endeavour and what are the challenges ahead of biological stoichiometry, before it can claim to realistically portray some of the important interrelations between molecular and ecosystemic processes. Justus von Liebig was probably the first influential scientist to apply chemistry to study plant and animal physiologies in a systematic way. It is probably his vision, that there was no distinction between chemical reactions within and outside organisms, that led him to investigate the elemental compositions of organisms and the effects of this composition on biotic processes such as plant growth and decomposition (Playfair & Liebig 1843). He came to realize that plant nutrition could be entirely satisfied by inorganic compounds, as long as they contained all the elements that made up plant biomass. Liebig’s law of the minimum (Figure 2) emerged from this work and has become a central law of ecology. This law was the first example of an application of the principle of conservation of matter to the biological realm, albeit restricted to plants. For society at large, Liebig’s work led to the invention and large-scale application of inorganic fertilizers, in other words, what was later known as the “Green Revolution”. Liebig’s work on plant mineral nutrition started a long tradition of research on plant growth limitation by elements. It also oriented researchers towards the pursuit of the original source of mineral nutrients in soils. Quickly, it was understood that microbes (bacteria and fungi) were the main providers of mineral nutrients to plants through their decomposition of organic matter in soils. This organic matter itself originates from dead parts of plants (shed leaves, fallen twigs and trunks, dead roots...) or from animal waste. To a lesser degree, Liebig’s work also attracted attention to the role of animals as resuppliers of elements to plants. Overall, it helped entrench a prevalent view of ecosystems where plants are at the centre, bringing inorganic nutrients into the world of organic matter, and consumers are dissipators of energy and resuppliers of inorganic nutrients to plants, with decomposers taking the largest stack. The abiotic components of ecosystems considered are mainly those that affect mineral uptake by plants and the decomposition of organic matter into minerals (e.g., atmospheric deposition of minerals, leaching, temperature, light conditions...). This model came to reinforce the energy-based food chain model formalized by Lindeman (1942). The effect of elements on the growth and reproduction of animals and microbes was seldom considered, although Liebig himself invoked the possibility that the availability of elements in an animal’s diet could limit its growth (Playfair & Liebig 1843). Elements were not yet seen as a factor able to affect the food webs structure and dynamics. Later developments, however, made it harder to ignore the importance of the elemental needs of some consumers, both for their growth and for their recycling of nutrients. For example, it was known since a long time that microbial decomposers could, in some circumstances, take up inorganic elements instead of mineralize them (Waksman 1917). This uptake of mineral nutrients by heterotrophic microbes was called immobilization. It was quickly understood that the main controlling factor for microbial decomposition or immobilization of nutrients was the mineral content of the microbial biomass in relation to the content of organic matter (Figure 3). The ecological consequences of the microbial decomposers making up shortages of essential elements in their resource by tapping into the stocks of inorganic nutrients were worked out later on (Bratbak & Thingstad 1985; Harte & Kinzig 1993; Daufresne & Loreau 2001a; Cherif & Loreau 2007). Nutrient-limited primary producers tend to generate carbon-rich organic matter, promoting microbial immobilization. This leads to a paradox, with nutrient-limited plants driving themselves towards stronger nutrient limitation by promoting immobilization. How, then, do limiting nutrients cycle back to plants and support continuous primary productivity if they are locked into the biomass of microbial decomposers? The solution to this paradox was found when food web studies in both aquatic and terrestrial ecosystems established that most bacterial production is generally consumed by heterotrophic predators also called microbivores (mainly protists, such as amoebae, ciliates and flagellates and, in soils, nematodes). The elements locked in the biomass of the ingested microbial decomposers are then mineralized as catabolic by- products, or because microbivores themselves fall prey to other predators higher up the food web, closing what was called the “microbial loop” (Caron 1994; Clarholm 1994; figure 4). Following the breakthrough of the microbial loop concept, a change in the paradigm of decomposition occurred. Now, the whole detrital food web, not only microbial decomposers, was seen as contributing to the mineralization of the elements essential to plant growth. In this new model, the elemental composition of microbial decomposers plays a central role. On one hand, it determines the extent of immobilization occurring. On the other hand, it also affects the excretion of elements by microbivores, since elements in excess of microbivores needs are excreted (Nakano 1994). Unfortunately, this increasing awareness of the roles of the elemental compositions of microbes, organic matter and microbivores within the communities of researchers interested in soils, did not spread to other fields of ecology, at least until the emergence of the ecological stoichiometry theory. The closest to an early stoichiometric thinking, anterior to the ecological stoichiometry theory, appeared in oceanography, a discipline ...