Lotto Soudal rider Marczynski, who had never claimed a stage win in a Grand Tour event, pipped fellow Pole Pawel Poljanski (Bora-Hansgrohe) and Spaniard Enric Mas (Quick-Step Floors) to the line following a sprint finish on the streets of Sagunto.
The trio finished the 204.4-kilometre stage from Vila-Real in a time of four hours, 47 minutes and two seconds, with the pairing of Leon Luis Sanchez (Astana) and Jan Polanc (UAE Team Emirates) completing the top five as they crossed the line eight seconds later.
Overall leader Froome, meanwhile, was part of an 18-strong body of riders that finished next, 26 seconds back. That group included many of Froome's biggest rivals at the top of the classification - but not Van Garderen.
The American started the day 10 seconds behind Froome in second place overall, but the BMC Racing rider crashed twice in the closing stages on Thursday and he now lies 30 seconds adrift of the leader in fourth place.
That means Froome's lead at the top of the standings increases marginally to 11 seconds - the gap to Colombian Esteban Chaves of Orica-Scott - as he stays on course to become just the third man to win the Vuelta and Tour de France in the same year
Ireland's Nicolas Roche (BMC) moves up a place to third ahead of Van Garderen, remaining 13 seconds behind the red jersey.
Italy's Vincenzo Nibali, one of only six riders in history to have won the Vuelta, Tour de France and Giro d'Italia during their careers, is 36 seconds back in fifth spot after clocking the same time as Froome on Thursday.
Froome, the four-time Tour de France winner who is looking for his maiden Vuelta crown, felt Thursday's stage in sweltering conditions was the toughest up until now.
He said on www.teamsky.com: "That's the hardest day of the Vuelta so far.
"It didn't stop all day, the attacks and for us having Sanchez in front we had to pull really hard and keep the break really close.
"The team was fantastic and I want to thank my team-mates that I'm still in red.
"I think it was a tough stage and a lot of people spent a lot of energy."
