I hope The conversion of seaweed to plastic only uses natural processes and has no harmful by products.
First it's not plastic but rather a cellulose based alternative. No natural process are used to create it. In fact, it does not exist in Nature in this form.
While you could eat it... I wouldn't as your internal biological processes my not be compatible... lots more research needed.
Based on plant cellulose, which are the building blocks of tree's, farm crops and seaweed. All these in a any combination will work together just fine as it's all cellulose.
Trees will work but you have forest fires.
Farm crops will also work but you have fertilizer issues as well as demand for Ethanol.
Seaweed is best because of it's fantastic grown rate and you don't have to worry abut forest fires or fertilizers.
Because any damage done will be underwater and in the ocean, it wont be seen. The ocean, after all, is so vast it cannot be be harmed by further exploitation.
We all know that's wrong... but now, for the first time, the ocean's bottom has value. Possibly a lot of it...
The question now is who owns it? Who is being hurt when passing ships dump their oil or diesel into the water? Who goes to court... and who goes to jail?
WE have the technology... now we have to work out the legal stuff...