Monitoring and Mitigation of Heavy Metals by Porphyra
Introduction and Significance
Heavy metals occur in all soil and water systems. Several approaches have been used to monitor or remove metal complexes from contaminated waters, either at the site of contamination or in treatment areas downstream. Most monitoring of metals is done by intermittent chemical analysis of water samples, while some attempts at continuous monitoring in small bodies of water are being made, particularly where the metal content of water fluctuates frequently.
The number of such monitoring devices needed to cover large areas is often cost prohibitive. Mitigation treatments involve chemical precipitation, filtration or catalytic binding. Such methods can be quite expensive and often involve chemicals that themselves can contribute to a different kind of pollution. A logical approach would be to use naturally occurring organisms to monitor and remove toxic metals from aquatic systems. Such organisms could be harvested at regular intervals, dried and disposed of as contaminated solid waste or used to recover valuable heavy metals. It is the feasibility and future optimization of this approach with marine macroalgae that forms the basis of the proposed studies.
Bioavailability of heavy metals is highly dependent upon several environmental factors. Biomonitors utilizing plants growing under "natural" conditions where biotic and abiotic factors are intercalated reduces the need for making assumptions regarding bioavailability of metals. Plants themselves can alter the microenvironment around them, thus altering the amount of metals that are biologically available. Bulk water analysis may not measure the conditions at the membrane level where changes occur. Benthic plants can provide valuable information regarding past environmental conditions over weeks and months. This is particularly important in plants growing within the intertidal zones where the metal content of water may fluctuate continuously.
The three long-term objectives of the proposed research are:
1) To establish profiles for the ability of several native Porphyra species growing at diverse sites for heavy metal accumulation
2) To determine the nature and content of heavy metal sequestering compounds (phytochelatins and metallothioneins) in Porphyra
3) To clone and characterize genes for metallothioneins (MT) and phytochelatins (PC) synthases from Porphyra.
Objective 1: Gametophytic thalli of different Porphyra species collected from several sites along the New England coast will be monitored for their heavy metal (Cd, Cr, Cu and Hg) content. The distribution of different Porphyra species along the northeastern coast has been well documented by Mathieson and Yarish. The selection of collection sites will be based upon sound environmental impacts, ranging from pristine waters to the most impacted areas within the Great Bay estuary system and the Long Island Sound. The heavy metal content of the water and plant samples will be enumerated in order to establish a statistically reliable relationship between the tissue metal content and that of the surrounding water. The data obtained during the first year will establish a benchmark for the heavy metal status of the different Porphyras growing under variable levels of contaminants. The metal contents of plants growing under minimally impacted areas will also establish a benchmark for the ability of different Porphyra species to accumulate heavy metals.
Objective 2: Samples of the six to eight Porphyra species collected from extremes of environmental conditions (objective 1) will be analyzed for their PC contents and nature. The content of different MT will be analyzed by using acrylamide gel electrophoresis and specific plant MT antibodies. The results of this objective will provide detailed profiles of the content and nature of different PCs, which generally differ in peptide chain lengths. This information will lead to the cloning of genes involved in the biosynthesis of both MT and PC.
Objective 3: MT are direct gene products, thus it should be possible to directly clone their genes using heterologous probes from similar genes in other plants (Zhou and Goldsbrough, 1995). On the other hand, PC are produced by enzymes called PC synthases. Their gene sequences have also been cloned and are available for use as heterologous probes to clone similar genes from Porphyra. The techniques of gene cloning are well established for land plants as well as Porphyra. The availability of these genes will open the way for future enhancement of the metal sequestration ability of Porphyra and other seaweeds in heavy metal mitigation of marine environments.