## 6 Bioremediation: Feasibility and Limits  
Nutrient limitation in Antarctic soils is one of the biggest constraints in bioremediation. Most Antarctic soils are oligotrophic and particularly low in nitrogen and phosphororous content, except highly localized coastal soils, sites of penguin rookeries and elephant seal colonies [39]. A stoichiometric imbalance occurs when a significant amount of hydrocarbon is spilled on these soils. Ex situ bioremediation techniques which mimic the environment condition of the indigenous or transient microbes can be enhanced by incorporating applications such as aeration, irrigation and nutrients as well as leachate collection systems to achieve maximum degradation rate [40]. As the C:N:P ratio in Antarctic soils moves away from the optimal index that promotes microbial growth, the application of a nutrient-amended bio-pile to contaminated soil is widely used at polar regions such as Casey Station, Antarctica [41]. In a study on crude oil bioremediation in intertidal sediments, pristine soils were artificially contaminated with Arabian light crude oil (topped at 150 \({}^{\circ}\)C) in several 1 m\({}^{2}\) enclosures and treated with different fertilisers for one year [42]. The result only showed enhanced biodegradation efficiency on \(n\)-alkanes in the first three months on fertilised plots and reached the same level of biodegradation with the unfertilised control in the following months. Similar biostimulation methods using Inipol EAP 22 evidenced a positive effect on two sub-Antarctic soil samples (organic and mineral soils) in mesocosm experiments for two months [43]. Inipol EAP 22 is a slow-release commercial oleophilic fertiliser containing a solution of urea, oleic acid and lauryl phosphate. The authors attributed the enhanced hydrocarbon biodegradation to the stimulatory effect of fertilisers in hydrocarbon-degrading microbe abundance. The result was especially pronounced in mineral soil with more than three orders of magnitude higher in terms of biostimulation efficiency. Nonetheless, excessive nitrogen addition can cause inhibitory effect on microbial growth and lower degradation rate due to osmotic soil water potential depression. In line with this, [PERSON] et al. [44] studied the optimal nitrogen addition at 604 mg N kg\({}^{-1}\) H\({}_{2}\)O and inhibition threshold of 1200 mg N kg\({}^{-1}\) H\({}_{2}\)O for petroleum-contaminated soil collected from Macrquarie Island. Similar experimental designs can produce varying results depending on the previous history of pollution and soil properties. This was evidenced by two different findings using a similar fertiliser protocol of complex organic matrix (fish meal and commercial fertiliser, OSE II) at Carlini Station, King George Island. With the enhancement of fish meal in biopile, a significant loss of 71% of total hydrocarbon concentration was reported in 2010 [45]. In addition, a shift in bacterial structure was observed over 50 days of incubation. The treated sample was a chronically diesel-contaminated soil exposed to fresh hydrocarbon contamination. The predominant phylum, Proteobacteria, was overtaken by Actinobacteria, which adapted better to lower levels of available hydrocarbons and depleted nutrient concentrations. Both bacteria groups are among the frequently reported hydrocarbon-degraders in polar soils [46; 47]. In contrast with the finding, similar field trial in year 2007 suggested unspecific fish meal-induced soil microbiota growth that did not promote differential growth of hydrocarbon-degrading microbes [48]. Hydrocarbon reduction on aged diesel-contaminated soil was reported to be insignificant in the fish meal amended system compared to the control system at the end of the assay. Conversely, the application of OSE II produced a significant hydrocarbon reduction after 45 days. The efficiency of the salt-mediated system in bioremediation was also not discerned despite proof from previous study [49]. Different soil contamination histories accounted for the divergence in the two findings.