Uptake and Degradation of Discharged Produced Water Components in Marine Microorganisms
Authors: Brakstad O.G. , Olsen A.J. , Nordtug T. , Frost T.K. , Aunaas T. , Johnsen S. 
Published: December 2007

Uptake and Degradation of Discharged Produced Water Components in Marine Microorganisms.

Produced waters from offshore oil production are a significant source of aromatic compounds discharged to the seawater. Exposure studies have revealed toxic effects of alkylated phenols and PAH compounds to various marine organisms. In this study the fate of aromatic compounds in seawater was investigated, using a dynamic exposure system which simulated dilution effects of discharged chemicals and "natural" conditions in the seawater recipient. 14C-labelled alkylated phenols (para-cresol) and polyaromatic hydrocarbons (PAH; naphtalene or phenanthrene) were applied to exposure tanks at sub-ppb concentrations by the aid of a computer-controlled injector device. Natural seawater, with normal seawater bacteria, cultures of the phytoplankton Isochrysis galbana, or the ciliate Euplotes bisulcatus), passed the exposure system at a residence time of approximately 5 hours, creating a short and defined exposure time between compounds and microorganisms. Compounds bound to or taken up by the organisms were collected on filters downstream the exposure system. The results showed that marine microorganisms may take up portions of aromatic compounds within a short period of time. Uptake mechanisms were expected to be passive events. Comparison of bioconcentration factors to the wateroctanol coefficients of the components indicated alternative uptake mechanisms to a passive incorporation in the lipid membranes of the organisms. Binding to surface protein and carbohydrate moieties may play a central role during uptake. Studies in static systems with exposure of components to normal seawater bacteria showed a significant uptake and mineralization only for p-cresol. Standard seawater BOD testing indicated that all compounds tested were potentially biodegradable in normal non-acclimated seawater. The results demonstrate that uptake and degradation of produced water components are important to consider during studies of the fate of these components. Microbial adaption may be an important factor, since this will enhance uptake and the subsequent degradation.

Introduction

The discharge of produced water from the Norwegian sector of the North Sea is expected to increase from 16 million m3 in 1990 to 90 million m3 by the year 2000. The composition of produced water includes a variety of hydrocarbons, fatty acids, heavy metals and production chemicals. According to the Paris Commission discharges of produced water to the North Sea should not have hydrocarbon contents above 40 mg/l. Several investigations have pointed out the potential effects of defined groups of chemicals of the discharged water on organisms in the water column, with specific focus on alkylated phenols and polyaromatic hydrocarbons. Effect studies have mostly been conducted in the laboratory with pure cultures of test organisms and high concentrations of chemicals. The relevance of test organisms and test conditions may also be questioned in several of the investigations.

A major objective for the oil companies and the Pollution Authorities is to develop predictive models for the fate and behavior of seawater discharges from offshore activities. These models include transport and binding mechanisms, uptake and biodegradation, and accumulation in the food chain. Attention has been made on the considerable amounts of potential toxic aromatic compounds like alkylated phenols and polyaromatic hydrocarbons (PAH).

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