**5. Perturbations and food webs**

Microbial communities have been evolved by modifications and adaptations in responses to natural stresses that finally allow them to get along with environmental change. The problem we are facing now resides in the velocity of environmental changes imprinted by human activities. The most important, but hardly the only one, resides in the use of fossil fuels because of the acceleration of climate change. The CO2 released as byproduct of combustion is just one of the causes of climate modification in the short term (in historical and geological times). Internal combustion engines also produce other greenhouse gases such as NxO or NO2, having a bigger capacity of keep heat, and this is a big problem generated only for the atmosphere. Hydrocarbons pose a permanent threat of contamination to aquatic and soil systems near the extraction zones, the transporting infrastructure to refineries, infrastructure for later transportation as fuel to expending places, and by illegal activities damaging oil ducts.

Soil microbiota react in different ways along the gradient of contamination when hydrocarbons reach soils. The plume of contamination normally eradicates the phototrophs and exert a strong selective pressure on bacteria and fungi, by killing or inhibiting the growth of sensitive species while enhancing the growth of resistant ones. These effects can be modified by the toxicity of the different compounds rupturing and/or changing the connections of the trophic networks [102, 103].

The effect of hydrocarbon contamination and others contaminants (pesticides, heavy metals) on communities will depend on the intensity, duration, and frequency of the perturbation. Then, lower species richness and abundance, shortening of the trophic webs, and the simplification of the trophic web are among the first observable damages contamination cause on microbial and protist communities [104]. Protists must at least tolerate the presence of the contaminant to achieve this function. Protists do not feed on hydrocarbons, but their grazing activity on the microorganisms that can keep the metabolization of the contaminant as high as another limiting factor allows them to.

Greater richness and abundance of ciliates species are associated with less perturbed areas; the greater the perturbation, the lesser species richness and abundance [105], regardless of the nature of the perturbing factor. For example, a significant reduction of ciliate diversity has been found in systems polluted by high hydrocarbon concentrations [106]. Medium concentrations only reduce the quantity of individuals from dominant species [106], while low concentrations produce an increase in the numbers of heterotrophic protists [107]. Saline accumulation forces the ciliates' diversity to decrease as salinity values increase [108, 109]. In the same way, acidic pollution produces lower species richness and abundance as the environment becomes more acidic [110, 111], and the same pattern is observed with heavy metals' contamination [104, 110].

Addition of organic matter in excess suddenly changes the base of production of the microbial food web, from phototrophs' productivity to heterotrophic bacteria and yeasts' productivity. The time of reaction is also different along the different microbial groups surviving the contamination event. Bacteria may start their biological activities several hours after the pollution event, whereas yeast and protists will delay from days to weeks, depending on the size of the organism.

Changes of primary producers from phototrophs to heterotrophs scale to functional groups, accommodating species richness and abundance of bacterivores protists, followed by omnivores. This is due to hydrocarbons stimulation of bacterial growth and the consequently increase of bacterivores species [112, 113]. Some species of genera *Colpoda* and *Vorticella* dominate aquifers receiving constant hydrocarbon discharges [114]. The bacterivorous ciliates, *Parauronema virginianum,* strongly dominate sites highly polluted with hydrocarbons and are replaced by *Pseudocohnilembus*

and *Euplotes* later [115]. Additionally, organic contamination and heavy metals increase the abundance of bacterivorous ciliates in water and sediments [116].

An increase in diversity and complexity of food webs are direct effects of these perturbations. Oil spill in deep waters increase the richness of the microbial community species and the complexity of their corresponding relationships, and the oil stimulated microbial activity supports greater variety of ciliates functioning along several trophic levels [117].

Other events of enriching oligotrophic systems with organic matter produce similar changes in the community structure of ciliates. Tirjaková and Vďačný [118] analyzed the changes in the communities of ciliates before and after a windstorm hit a stream, and they found a significant increase of ciliates' species' richness and abundance after the storm. Several weeks later, the community of ciliates presented the typical values of oligotrophic sites. The increase in resources availability is the factor indirectly responsible of these changes of ciliate community, but later, communities tend to return to states similar to the initial ones after resources exhaustion, which my take place around six months [118]. However, Shabarova et al. [119] report that the microbial community recovers from perturbation to a preflood state within two weeks after the event.

Regarding the connections' shrinkage of the trophic networks, a gradual narrowing of the planktonic size spectrum has been reported in hypersaline lakes, correlated to salinity increases during the summer, resulting in a simplification of the community represented by the ciliated *Fabrea salina*, diatoms, and *Dunaliella* spp. [120]. Simplifications of food webs have also been described as consequence of heavy metal contamination, herbicide use, and lake acidification [104, 121, 122]. Loss of connections have consequences on carbon transfer in food webs. The decrease of bacterivores species allows an excessive increase in bacterial biomass, which may produce up to 300-fold reduction in the transfer of carbon from the bacteria to higher levels of the trophic networks [104].

Communities' characteristic of hypersaline lakes are dominated by *Fabrea salina,* which has a broad tolerance to salinity and contributes to high proportion of the biomass of ciliates in hypersaline lakes [108, 109, 120, 123]. In addition, its abundance is strongly related to the microalgae, *Dunaliella* sp. [123], and can act as a competitor to shrimp, *Artemia salina*, in saline environments [108].

Regarding the perturbances in the soil ciliated communities, similar effects have been described as in aquatic ecosystems. Exposure of ciliate communities to heavy metals induces a reduction in the biomass of ciliates and this effect lasts for 20 weeks [124]. Insecticides also generate a decrease in ciliates species immediately after contamination, they also generate a change in the dominance of ciliates, the bacterivores (*Colpoda* spp. and *Paracolpoda steinii*) and macrophage (*Grossglockneria*) considerably increased their abundance after 90 days, while that other genera of ciliates decreased [125]. In soils contaminated with hydrocarbons, a decrease in diversity and a lower functional diversity have also been observed, the ciliated communities in soils with hydrocarbons are dominated by the Colpodea class [96, 126, 127]. It has also been observed that along with the decrease in the diversity of ciliates there is a decrease in the trophic groups after an intense pulse of contamination by hydrocarbons. However, the community recovers its diversity and trophic groups after a month of contamination [127].

### **6. Conclusions**

Protists in general, and ciliates in particular, play a key role in nutrient cycling and food web functioning in both aquatic and terrestrial ecosystems. In the world *Food Webs DOI: http://dx.doi.org/10.5772/intechopen.97252*

experiencing climate change and other kind of anthropogenic menaces, protists may be useful partners to tell us how aquatic and terrestrial systems are dealing with these issues while mesmerizing the observer with their great diversity of beautiful forms.
