They’re common structures in some red and brown algae.

它们是一些红藻和褐藻中的常见结构。

That is sodium alginate, that's made from brown algae, that holds the water inside.

那是海藻酸钠， 由褐藻制成，可将水保存在里面。

In temperate and polar climates, up to 40  meters below the surface, dense populations of giant kelp, the largest brown algae, grip  the rocky seafloor with branching roots called holdfasts.

在温带和极地气候下，在海面以下 40 米深的地方，巨型海带（最大的褐藻）密集种群通过称为固着物的分枝根抓住岩石海底。