Publication Summary

As part of a project to define sediment criteria, The Washington State Department of Ecology is evaluating several bioassays for use on freshwater sediments. This report reviews the several variations in methods of Microtox sediment bioassay, the current uses of Microtox sediment bioassays by different agencies, and recommendations for its use in testing freshwater sediments.

As part of a project to define sediment criteria, The Washington State Department of Ecology is evaluating several bioassays for use on freshwater sediments. This report reviews the several variations in methods of Microtox sediment bioassay, the current uses of Microtox® sediment bioassays by different agencies, and recommendations for its use in testing freshwater sediments.

Microtox is a relatively simple and inexpensive bioassay that indicates toxicity through a reduction in light output of the luminescent bacterium, Photobacterium phosphoreum. There are currently three Microtox tests available for sediment testing. The Basic Test and the 100% Test detect contaminants that have been extracted into an aqueous phase. The Solid-Phase Test can detect both aqueous phase contaminants and those bound to sediment.

The Basic Test is the only test that is precise enough to use for the development of sediment criteria. The choice of which aqueous phase to use (pore water, saline elutriate, distilled water elutriate or organic extract), depends mainly on project goals and the contaminants of interest. The 100% Test can be used with aqueous samples of lower toxicity, while the Solid-Phase Test is useful for determining potential toxicity to burrowing organisms where the main route of contaminant exposure is through ingestion.

Microtox data can be analyzed by calculating EC50 (Effective Concentration 50%) values based on dose response curves. For sediment criteria development purposes, the determination of a "hit" based on the Sediment Management Standards ("one-hit no adverse effects" and "two-hit minor adverse effects" approaches) and a 20% light reduction from reference, is preferred.

Microtox can be used separately to screen many samples. However, it is most appropriate for inclusion in a "battery of tests" where sediment is evaluated based on the results of several different bioassays.