The role of nitrogen-fixing bacteria in nitrogen-limiting marine waters (Chapter 4)

Abstract

Crude oil spills are a major threat to the marine flora and fauna. Oil spills like the Exxon Valdez in Alaska and the Deepwater Horizon in the Gulf of Mexico have resulted in devastating environmental and socio-economic losses. Oil spills persist in the ocean for a long time due to their recalcitrant nature and low bioavailability. Therefore, there is a pressing need to develop non-invasive, cost-effective, and eco-friendly methods for the removal of oil spills. Bioremediation is favoured, as chemical and physical methods are either costly or contribute to pollution and hence, inefficient. Marine biologists consider nitrogen limitation to be a crucial factor for regulating natural oil degradation by indigenous microorganisms. Biostimulation is a common strategy for the bioremediation of oil spills, wherein external nutrients like nitrogen are added to kick-start the growth of indigenous hydrocarbon-degrading microorganisms. But this method is costly and the nutrients are continuously lost or diluted by the strong ocean currents at the site of application and thus, need to be regularly replenished. Bioaugmentation is another approach for oil spill management involving the use of effective exogenous pre-selected oil-degrading microorganisms. However, the long term in situ survival of these exogenous microbes becomes difficult due to their competition with indigenous microbes or their inability to acclimatise to the prevailing environmental conditions. They may also be washed away by the strong water currents, or they may not degrade oil effectively due to nitrogen limitation. The isolation of indigenous marine oildegrading nitrogen-fixing bacteria and their application to oil-polluted sites may compensate for the strong limitation in the bioavailable nitrogen in marine ecosystems, rendering a more effective oil remediation strategy. Also, nitrogen-fixing marine bacteria possessing enzymes like carragenase, agarase, alginate lyase, cellulase, protease, and amylase can be productively used in the treatment of seaweed waste dumped in the marine environment. This chapter focuses on the role of nitrogen-fixing bacteria in the bioremediation of oil and seaweed waste in nitrogenlimiting marine waters.