NO₃-N 5mg/L : PO₄-P 0.5mg/L, not too much or little...potentially just right

Nitrogen and Phosphorus are needed for growth and can be considered potential limiting factors. Approximately 150mg (nitrogen) and 30mg (phosphorus) are consumed when converting 1g of hydrocarbon to cell material. A common strategy in bioremediation of oil polluted shorelines is adding nutrients to encourage the biodegrading efficiency of microorganisms.The study by Wen-xiang et al (2007) focuses on the concentration and frequency of nutrient addition in the bioremediation on oil–polluted shorelines, by using an N/P ratio of 10:1 in a simulated shoreline models.

Polyethylene boxes (50cmx15cmx 30cm) were used to simulate shoreline environments (fig 1.) within the boxes was a sand (taken from Jin beach) zone and a water zone which consisted of oil polluted water. The polluted water was created using seawater collected from Huiquan Bay, the seawater was sterilised then Diesel oil (from Shengli oilfields) was added with a surfactant and pumped into circulation for 2 hours. The experiments involved three nutrient trials and a non nutrient (control) trial.

For the nutrient trials inorganic nutrients were added to the microcosm at a 10:1 ratio:

• NO₃-N concentration (1mg/L, 5mg/L and 10mg/L)
•  PO₄-P concentration (0.1mg/L, 0.5mg/L,1.0mgmg/L)

Chemical and microbiological analyses for total petroleum hydrocarbons (TPH), numbers of heterotrophic bacteria (HB) and petroleum degrading bacteria (PDB) were performed to measure the oil biodegradation efficiency and microorganism counts.

Oil degradation efficiency was highest in the 5mg/L group and had the biggest counts of petroleum degrading microorganisms. The control group counts of HB were much lower than other groups indicating a lack of nutrients. The 10mg/L group was considered an excess amount of nutrients as too many HB were stimulated, PDB did not increase and degradation overall had a lower efficiency than the 5 mg/L group. The 1mg/L group was considered inadequate to maintain high level of bioremediation.

I was interested in this paper because of the application of mesocosms (in this case microcosm) to bioremediation. Compared to my previous post on oil biodegradation this experiment focuses on the function to polluted beaches and shorelines. The nutrients added to this experiment illustrate the impact of fertilizers on oil degradation, and shows results like cases in Norway and Canada after the Exxon Valdez spill where the rate of biodegradation increased by approximately 10 times (Munn, 2011). This study isolated an optimal concentration of NO₃-N 5mg/L, with varying results for the 10mg/L and 1mg/L concentrations, these results can be considered useful in terms of selecting the nutrient concentration for bioremediation of oil-polluted beaches. However it is impossible to maintain certain ratios because of the washout impacts of wave action and tides on nutrient levels. The authors also mention that manipulating N/P ratio may enrich different microbial populations and the optimal N/P ratio can be different for degrading different compounds.

Wen-xiang, X., Jin-cheng, L., Zhi-wen, S., Ying-jie, S., (2007), Effects of nitrate concentration in interstitial water on the bioremediation of simulated oil-polluted shorelines. Journal of Environmental Sciences, Volume 19, Pages 1491–1495

Referenced in blog post:

Munn, C.B. (2011) Marine Microbiology – Ecology and Applications,2nd edition, Garland Science, Oxford, Page 302