The seaweed/algea story is pushed strongly by meat industry greenwashing efforts. It isn't large scale deployed anywhere and the potential benefits are overblows.
https://www.wired.com/story/carbon-neutral-cows-algae/
"What’s more, feeding cattle algae is really only practical where it’s least needed: on feedlots. This is where most cattle are crowded in the final months of their 1.5- to 2-year lives to rapidly put on weight before slaughter. There, algae feed additives can be churned into the cows’ grain and soy feed. But on feedlots, cattle already belch less methane—only 11 percent of their lifetime output. ... This means that even if algae diets on feedlots worked perfectly, it wouldn’t help with the 89 percent of cows’ belches that occur earlier in their lives."
https://www.wired.com/story/carbon-neutral-cows-algae/ "What’s more, feeding cattle algae is really only practical where it’s least needed: on feedlots. This is where most cattle are crowded in the final months of their 1.5- to 2-year lives to rapidly put on weight before slaughter. There, algae feed additives can be churned into the cows’ grain and soy feed. But on feedlots, cattle already belch less methane—only 11 percent of their lifetime output. ... This means that even if algae diets on feedlots worked perfectly, it wouldn’t help with the 89 percent of cows’ belches that occur earlier in their lives."