By Chris Michaud

NEW YORK (Reuters) - Bidders will be able to buy fine art, a vacation in the Maldives, or an internship with designer Donna Karan and help the planet at the same time in Christie's annual Green Auction, which is expected reap millions for environmental causes.

Proceeds from the third annual auction on April 11, which raised nearly a combined $5 million its first two years, will benefit four environmental charities -- Oceana, the Natural Resources Defense Council, the Central Park Conservancy and Conservation International.

"We must all take responsibility to protect the natural world for future generations," said philanthropist and environmentalist Susan Rockefeller a co-chair of the event, adding the auction conveys a message about the relevance and necessity to conserve the planet's finite resources.

David Rockefeller, Vanity Fair Editor Graydon Carter and actress Salma Hayek are other co-chairs of the auction.

Proceeds from the event will be directed toward each organization's water-conservation projects, according to Christie's, which was set to announce the auction this week.

"Our ocean waters, which cover 70 percent of this blue planet, are hovering on the brink of an irreversible collapse, with 90 percent of the ocean's big fish gone," said Oceana CEO Andrew Sharpless.

"Luckily, history and science show us that our oceans can rebound if we put in place and enforce sensible policies," he added in a statement.

People not able to attend the invitation-only auction can bid in a concurrent online auction, which will run from March 29 to April 19. It will feature hundreds of items including art, fashion, travel and once-in-a-lifetime experiences.

In previous Green Auctions bidders have paid as much as $100,000 to spend a day with former President Bill Clinton or a trip to Hollywood to attend the Oscar parties and $26,000 for a one-hour tennis lesson with John McEnroe.

Past sales have also featured a backstage meeting with Lady Gaga and artwork by prominent contemporary artists.

Other artists and celebrities donating works of art or experiences will be announced in the coming weeks. An interactive campaign at Facebook.com/ABidtoSavetheEarth, includes a video contest and a chance to win tickets to the auction.

As in past years Christie's is waiving all fees associated with the event.

(Editing by Chris Michaud; editing by Patricia Reaney)