On the backdrop behind the 97X Next Big Thing stage on Saturday, you couldn’t miss the phrase “Tampa Bay’s New Alternative.”What does “new alternative” mean at a time when alternative rock has less mainstream cachet than ever? When the night’s headlining act, Bastille, had a hit this year only by pairing with a hit pop DJ named Marshmello?What it meant on Saturday was a handful of acts who didn’t share a whole lot in style, but who all felt like alternatives to something.For the second year, 97X split its Next Big Thing lineup into two days -- Panic! at the Disco headlines Sunday -- with the opening night drawing a healthy but smaller crowd than Sunday will.“Most of our music’s really depressing, so I’m sorry about that,” singer Dan Smith told the crowd.Actually, the British synth-rock act has brightened up quite a bit in five years, to the point where people dance and cheered heartily for that Marshmello collaboration literally titled Happier. Smith moved almost constantly, hopping on the balls of his feet as he sang with just the right amount of gospel in his quasi-Benedictine moan.As headliners, they could bend the 97X stage to their will, with smoke and mood lighting that shot across the stage -- sirenlike reds on Bad Blood, joyous blues on Of the Night, pops and flashes everywhere on the dramatic Fake It.Musically, the music had more muscle than mopiness, despite a hint of the blues on Things We Lost in the Fire and waves of prickly misery on The Draw. Before closing with huge hit Pompeii, Smith even took a lap around the amphitheater floor on the jaunty piano-pop track Flaws. So see? Even Bastille can have fun.And they weren’t alone, especially when it came to audience interaction. Increasingly popular synth-pop openers LANY ventured into the audience during their set. Piano-pounding singer-songwriter Andrew McMahon, a perennial Next Big Thing favorite, ventured into the crowd not once but twice, once on an inflatable rubber duck and again beneath a giant parachute.Elle King didn’t go into the crowd, but she spiced the attitude way up, twisting speakeasy rockabilly and wild-child rawk into her set, from spitfire opener Baby Outlaw to heel-kicking closer Little Bit of Lovin’.And then there was the band that delivered a strong contender for set of the weekend: Nathaniel Rateliff and the Night Sweats, whose shaggy, rug-cutting rock ‘n’ soul revue channeled Sly Stone, Ike and Tina and the Blues Brothers. With a three-piece horn section pumping and blasting away on Shoe Boot, Intro and revival-like closer S.O.B.Twice, Rateliff flung his guitar back to a stagehand; once, he spiked his tambourine so hard it bounced back up to head level, then rolled off the stage like a boulder. Never once did he stop shuffling and singing his soul out.It didn’t look like traditional alternative music. But new alternative? Yeah, that definitely applied.97X Next Big Thing returns Sunday with Panic! at the Disco, Foster the People, Young the Giant, AJR, Chvrches and more. Click here for details.Contact Jay Cridlin at [email protected] or (727) 893-8336. Follow @JayCridlin.