Adsorption in Ecology

Elements cycle in the ecosphere following different pathways through the different sphere: biosphere, hydrosphere, atmosphere, and lithosphere. These cycles play a fundamental role in ecology because they permit, for instance, the supply of nutrients to life, contribute to the homeostasis of the system, and allow the continuous flux of matter that sustains an ecosystem.

Adsorption is one important process that contributes to the accumulation of the elements in the lithosphere: thanks to this process dissolved compounds can be trapped temporarily or permanently on solids where they can stay inert or react with the solid matrix.

The cycle of phosphorus in the ecosphere is a good example of how much adsorption is important in ecology. Phosphorus is one of the five major constituents of all living organisms, and one of the two (with nitrogen) that usually can limit growth of primary producers (C, H, and O usually being abundant). Hence, its availability is fundamental for the maintenance of the ecosystem. Inorganic phosphorus is usually present in the environment as orthophosphate (PO3-) and its reduced forms HPO2- and H2PO4 that are more common at environmental pH. Dissolved phosphates can be physisorbed and chemisorbed on soil. The latter can occur through formation of a complex with metal cations through hydroxyl groups, in particular with iron and aluminum hydrous oxides (Fe(OH)3, Al(OH)3), or other metal cations (e.g., Ca2+ in CaCO3). Another good matrix is clay, and especially its aluminosilicates: in this case sorption occurs into the clay lattice replacing surface water molecules or, at higher concentration of phosphates, silicates.

As a main result of this tendency to adsorption, phosphorus is mostly trapped into particulate and is not much available in its dissolved inorganic forms. This is the reason, for instance, why the main pathway to travel from land to sea is connected more to solid transport (e.g., erosion) than the liquid one (e.g., leaching). Particulate P is not only less mobile than the dissolved form, but also not, or little, bioavailable; this is one of the reasons why it is more limiting than nitrogen even if the amount necessary to living organisms is lower. In case of deficiency of P, plants are able to boost desorption from soil-changing environmental conditions.

Desorption can also occur without the boosting effect of vegetation: in aquatic ecosystems, sediments can become anoxic, for instance, as consequence of aerobic degradation of huge amount of organic matter due to eutrophication of the water body. In these conditions, P desorbs (e.g., due to reduction of iron) and is released from sediments. The amount released can be very significant, and even the largest source of phosphorus in the ecosystem (e.g., in the Baltic Proper) overshadowing in the short term the effect of reduction policies.

Adsorption, together with ionic exchange, plays also a key role in another important ecological process: acidification. This is a natural process that consists of the deposition of acid substances in the environment with a series of relevant consequences on the ecosystems (see Acidification). One of the elements largely responsible for acidification is sulfates, mainly coming from fossil fuel combustion. After the deposition of sulfuric acid, sulfates leach into the soil where they can be trapped and stored, adsorbed mainly on the surface of Fe3+ and Al3+ hydrous oxides. Furthermore, part of the acidic compounds can be neutralized via ionic exchange using metallic cations present on the soil like Mg2+, Ca2+, and K+. Thus, the acidic anions are fixed in the soil and acidification of surface waters is prevented or at least reduced. At the same time soil ecosystem suffers several problems. pH in the soil decreases, leading to an increase of mobility of Al in porewater, that can be toxic for plants if in excess. Furthermore, neutralization of sulfates 'confiscates' important oligoelements for primary production: if they become scarce, plants grow less healthy, and they can also stop growing if the deficiency of these elements is critical. Forests are usually the more affected because of their higher needs of nutrients and longer life cycle.

Adsorption is not an irreversible process; hence, if concentration of sulfates in pore water decreases (due, for instance, to a decrease of deposition related to improvement in abatement technology of emissions), they can be desorbed and leach to surface waters. In this case, the effects of a reduction of emissions cannot be observed in a short term: this is the case of some European catchments (mainly in the Central and Northern Europe) with high sulfate storage in which no reduction of release was observed despite the reduction of emissions and deposition achieved in Europe.

Adsorption occurs not only on inorganic solid surface -it concerns any kind of surface, even the cell membrane. This is indeed the mechanism used by viruses to stick on a cell and then inoculate their genetic information inside in order to replicate themselves. Adsorption of viruses on cell membrane is a two-step process: the first concerns the contact and bond of the virus on some specific sites, called receptors. The receptors are proteins that are able to bind with other proteins that are on the virus surface (the VAP -virus attachment protein). Adsorption bond between receptors and VAP is mostly due to steric reasons, more than electrochemical affinity. Despite the selectivity of this bond, this is not unique: a receptor can attach different viruses, provided with the same or morphologically similar VAP, and larger viruses can be equipped with more than one VAP in order to increase the probability of infection. This step is still reversible: viruses can desorb (especially at higher temperature) and move freely in the environment looking for another cell to adsorb to. The second step is irreversible, and involves directly the structure of the virus.

Viruses adsorb not only on living cells, but also on inorganic soil particles, such as sand or, especially, clay. Several mechanisms have been proposed, all based on electrochemical forces like van der Waals forces: the bond can occur directly between anionic groups on viruses and cations on clay, or between two anionic groups mediated by water molecules. Adsorption on soil plays an ecologically important role, not only because it is a way to disinfect waters, but also because it is a good way for transport of viruses, and then diseases (for animals and also for bacteria or algae). Indeed, viruses adsorbed on soil are more resistant to the different disinfectant agents, and then their survival time increases. If the soil particles are mobile, viruses can be transported (e.g., via runoff) in other ecosystems where they can desorb from particles and infect their targets.

See also: Acidification; Enzymatic Processes.

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