Rust Seaweed

The rust seaweed split from rust weed. It lives off the coast if the iron rich region of Ittiz. Like its ancestors metabolizes iron oxide from the rusty red soil it lives in. This gives it its rusty color. However it still photosynthesizes with it's purple pigment, which is hidden by the red rust. Going from a salty river to a salty sea was not much of a leap.