Love, infidelity and stable finances are in the air at the Atlanta Opera, with Mozart’s subversive “Cosi Fan Tutte” opening Saturday, and with the realization that a recent $9 million bequest has opened a new chapter for the 31-year-old company.

Alarming news has been frequent from across the local performing arts scene. The mighty Atlanta Symphony Orchestra groans under the weight of a debt beyond $8 million. The smaller Cobb Symphony issued an urgent plea to its patrons to bridge a budget gap of almost $50,000. Tiny arts groups across the region struggle to make payroll.

But there have been a few breakthrough stories in recent years, such as the Atlanta Ballet’s big capital campaign, big endowment gifts and deluxe new dance studio.

The Atlanta Opera was in financial hot water, too, until an unexpected bequest from the late Barbara D. Stewart, a corporate economist, arrived in February. Soon after, the opera sold out its run of “Porgy and Bess,” the cap to what might be remembered as the best month the company has ever had.

Suddenly its future is looking as buoyant as its upcoming Mozart production at the Cobb Energy Performing Arts Centre with four performances through April 17. “Cosi Fan Tutte” is the third and final production of the Atlanta Opera’s 2010-11 season.

By the terms of Stewart’s will, half of the $9 million must go to the endowment, which brings the total to $5.4 million. (If the opera gets 5 percent interest off the endowment, that means $270,000 will be added to the budget annually.)

The rest, general director Dennis Hanthorn says, will go into the checking account. “We haven’t actually received the money, but we’ll need it to replenish our cash reserves,” he said. “Our balance sheet has been weak, and we were known as a pay-as-you-go organization. These funds will help us cover the bills when cash is low, to cover the months between when a grant has been announced and when it’s actually paid.”

He continued: “The funds won’t allow us to add another production — each costs about $600,000 — but it gives us stability. Her gift legitimizes the organization. Before, we’d be asked [by foundations and individual donors] ‘How long you gonna stay in business?’ Well, now we are not going out of business.”

In addition to the opera’s $9 million, Stewart also left $1.5 million each to the High Museum of Art and the Atlanta Symphony Orchestra. Although Stewart had been a steady patron of the arts over the decades, and had served on the Atlanta Opera’s board, Hanthorn was surprised by the size of the gift.

“Barbara was a very modest person,” he said. “The Woodruff Arts Center wasn’t aware she’d give that much away.

“I suspect few people knew she had that kind of money,” Hanthorn said, “and no one was suspecting that level of generosity for the arts in Atlanta. I’m humbled by it, and the best way we can honor her memory is to double or triple her bequest.”

The other way to honor Stewart is to produce great opera. “Cosi Fan Tutte” is the third and most subtle, and slightly louche, of the trio of masterpiece operas Mozart composed with librettist Lorenzo da Ponte. Like “Marriage of Figaro” and “Don Giovanni,” “Cosi” is brought to a boil by the subject of sex that is risqué or unwelcome, depending on your perspective.

The title is an imprecise translation from Italian meaning “all women are like that,” a bit of barbed chauvinism that sets the plot in motion. On a bet, two young soldiers test their fiancées’ fidelity. The girls are sisters, and to trick them each guy appears in disguise to woo the other’s betrothed. Dispiritingly, each soldier seems to have more chemistry with his pal’s gal than with his own. The seductions are successful.

“‘Cosi Fan Tutte’ is a statement about how we lie to each other but really lie to ourselves,” said Carter Joseph, who has taught an Evenings at Emory class on the opera. “Historically, it opens the door to the end of the Enlightenment, where we’ve tried rational thought and it doesn’t work: Everyone is disillusioned at the end of the opera and our passions won out.”

Many elements of the plot, however, would fit the pop culture of our own time. By classical convention, da Ponte’s unerringly brilliant libretto has all the action take place in a single day and in (mostly) one location, including spontaneous dual weddings at the end.

“That sounds far-fetched and decadent, but Britney Spears got married for just a day,” Joseph pointed out. “The whole plot could be the premise for a reality television show. But the music is by Mozart, and he doesn’t pass judgments. He finds their humanity.”

Pierre Ruhe is classical music critic of www.ArtsCriticATL.com.