Rules, Structures and Effects in Ecosystems

The respective set of metabolic processes exerted on a given sub-set of substrates in turn defines one “ecological niche” within some ecosystem by its trophic demands and network structure/position. So there is multi-level autocatalysis by reproduc-tion of organisms, and these are likely to use fairly abundant elements for the respective transformations rather than those which would appear best-suited to the catalytic chemist (for corresponding lists and comparisons, see Ochiai 1968;

Fra¨nzle 2010).

Normally, in an ecosystem total biomass of the next-higher trophic level (TL + 1) is about 10–15 % (one-tenth to one-seventh) of that of the given trophic level TL. Only some part of the biomass thus produced is “shuttled” into offspring;

another part of the “missing” 85–90 % becomes simply oxidized to obtain meta-bolic energy just to keep the consumer alive, converting most of available plant or bacterial or phytoplankton biomass back into CO₂and soluble odd-nitrogen species or N₂, N₂O. By definition there can be only one level of producers whereas consumers might arrange in three or even four more trophic levels. Essential trace metals contained therein are released from food biomass and can be used by the predator/grazer for its own purposes of running metabolism and permitting reproduction. Hence it holds for carbon (C content is about constant among trophic levels):

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7 C_{org TLð Þ}þ n oxidants ! 6 CO2þ C_{biom TLþ1ð Þ};

while the fate of n depends on the respective C/N ratios (cp. textbooks of ecological stoichiometry).

SNA exactly states what might happen then, due to the very fact there is autocatalysis in this system, and going to take over dynamics. This means SNA can be used to analyse and predict ecosystems dynamics particularly for rather contrived systems with strange redox biochemistry and/or extant chemosynthesis, like in famous Movile cave (near Mangalia, Dobrogea, SE-Romania, Fig.4.1). The Warwick University research group concerned with Movile cave describes the situation as follows (Kumaresan et al. 2014):

Movile Cave is a totally unique environment, situated in the south of Romania. . . In spite of being totally sealed and being devoid of light, the Cave is a thriving ecosystem filled with all manner of life, from tiny crustacea to isopods, molluscs and arachnids. On the rest of the planet, ecosystems are supported by primary producers, such as plants or algae, which convert carbon dioxide from the air into living matter that can be eaten by higher organisms—this process is driven by light and is known as photosynthesis. In the dark reaches of Movile Cave, the primary producers are bacteria that convert carbon dioxide into living matter in the form of vast floating “mats” on the surface of the Cave waters. Primary production in the dark is driven by chemical energy obtain by the bacteria from the oxidation of sulfur compounds and ammonia in the Cave waters—a process called chemosynthesis. . .to better understand the roles of chemosynthetic and methanotrophic bacteria in the floating mats of Movile Cave and their interactions with other species at the base of the food web. Limited productivity of some ecosystem commonly causes an increase in biodiversity².

Fig. 4.1 Microbial mats in Movile cave, Romania. A mat is a complex microbial habitat, comprised of fungal hyphae and an assortment of bacteria. The mat is very thin (like wet tissue paper) and is kept afloat due to air bubbles. There are also submerged mats sticking to the ground of this pond which cannot be seen in this picture. The white deposits along the shoreline are mainly elemental sulphur (S₈). Image courtesy of Kumaresan et al. (2014)

2Yet compare Movile cave biodiversity to that seen in some usual soil mesofauna.

As here in- and output and the lines of production (chemosynthesis) and trophic connections are fairly easy to spot, it should be looked upon in some more detail.

There is but limited inflow of reactants from different sources: CH₄, NH₄⁺, and H₂S from warm springs in the cave, CO₂/HCO₃^ from (probably bioassisted) weathering of its walls while some O₂must come from outside. Oxygen is required for running S oxidation by “chemical and biological processes” (Fig.4.2) beyond the pyrrhotite + H₂S/pyrite system, and the diverse arthropods inside the cave for sure are not fully anaerobic.

Normally, matter exchange is less contrived but much harder to understand also with respect to its ramifications to “mode of operation” of single species within an ecosystem. This is why we selected this truly peculiar example. Due to the interaction of autocatalytic (i.e., metabolic for this purpose) and other modes to Fig. 4.2 Reaction network within Movile cave. Note the reactions to the left involving N₂, CH₄ are really hard to accomplish unless for photo- or radiochemistry so every organism getting capable of doing them will take over dynamics while then secondary steps, and H₂S and thiosulfate oxidations and methylotrophy based on both CH₃OH and methyl amine are much simpler (there, chemical processes are indicated to take place and compete with biology). Accordingly, as the unique arthropod biota of Movile cave necessarily is mainly aerobic, as are many of the bacteria degrading ammonium ions and methanol, and they feed on the microbial mats they must compete with inorganic processes for sustaining themselves while the primary producers are in a better position. Image courtesy of the Murrell Lab, University of East Anglia

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fulfil some reaction chemistry will dramatically change locally once an access is made for living beings to thrive somewhere; this is about i.a. mining sites where sulphide oxidation first produces sulphuric acid (+ substantial biomass) and the former then etches away and mobilizes large amounts of Al and heavy metals, compromising living conditions especially for fishes and aquatic arthropods, with the latter being even less tolerant towards acidic conditions than many (limnetic) fishes, requiring to maintain pH 5.