CAMBRIDGE, Mass. (AP) – Target offers shoppers an unusual message about its gift cards at some stores, advising that they are biodegradable. “Just make sure you spend them first,” the displays conclude.

This isn’t just a marketing gimmick. Plastics made from corn and other plants are carving a tiny niche from the market for conventional petroleum-based plastics and being touted as green alternatives for everything from bulk food containers to lipstick tubes and clothing fiber – as well as gift cards.

So-called “bioplastics” offer the world a way to wean itself off oil, and most biodegrade to varying degrees. But their makers’ green argument is complex, and environmentalists are cautious in their support.

Manufacturing bioplastics produces carbon dioxide, which contributes to global warming. The materials are made from crops – corn, switchgrass, sugar cane, even sweet potatoes – that require land and water to grow. Some sound alarms because genetically modified organisms are used to spur the fermentation that creates them. And recycling them presents still other pitfalls.

They also can cost three times more than conventional plastics, which gives businesses pause about adopting them. Yet, until bioplastics expand beyond their current tiny fraction of the overall plastics market, the road to popularity is expected to be rough.

“It’s almost a chicken-and-egg scenario,” said David Cornell of the Association of Postconsumer Plastic Recyclers. “It might someday reach that critical mass, but it has to happen very quickly, because in the meantime it can be a nuisance for us.”

Bioplastics’ main benefit would be to reduce from 10 percent the share of U.S. petroleum consumption that goes into plastic. The types that are biodegradable also could help compensate for the country’s slow progress in recycling – only about 6 percent of plastic made in the U.S. was recycled in 2005, according to the U.S. Environmental Protection Agency.

Bioplastics also lack toxins like polyvinyl chloride that have raised health concerns and led California Gov. Arnold Schwarzenegger this month to sign legislation banning chemicals called phthalates from toys and baby products.

“This is a promising new technology that faces some challenges,” said Mike Schade of the Center for Health, Environment & Justice, a Falls Church, Va.-based nonprofit. “But we don’t view them as insurmountable, if the industry is willing to face them head-on.”

The market’s newest entrant is Mirel, from Cambridge-based Metabolix Inc. It more easily biodegrades than rival materials and, unlike others, can break down in a backyard compost bin. Its first consumer application came in July when Target began using it in gift cards at 129 stores. Metabolix is talking with potential clients about dozens more applications for Mirel, from razor blade handles to a coating for disposable coffee cups.

Agricultural processor Archer Daniels Midland Co. provides corn feedstock for making Mirel, which requires genetically engineered bacteria to aid in fermentation.

The most widely used bioplastic, NatureWorks – also corn-based and biodegradable – is a product of a Minnesota-based subsidiary Cargill Inc. It is made without genetically modified bacteria, though some of the corn that goes into it had been modified. It already is used in dozens of products, including water bottles – an application unsuited to Mirel, which isn’t transparent.

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