Agar from different algae have different structures, and, therefore, have differing properties. While some marine bacteria can use agar as food and form pits when spread on a plate of agar, most bacteria can't damage agar. Some agars gel around 30C and are used when 40 or 45C would be harmful. For example, one often wants to mix bacteria or antiserum with melted agar and then pour this into individual wells of well plates, test tubes, or petri plants for special assays. Since agar contains many different molecules and impurities, some companies fractionate agar and sell special products for use in foods and laboratories.
Alginic acid is used in ice cream as a smoothener.
In the presence of calcium ions alginic acid forms gels. Therefore, if you mix bacteria with alginic acid and drip the suspension into calcium ion solution, you obtain soft spheres containing trapped bacteria. You can then pass food thru the bed of spheres and food would diffuse into the spheres and the products will difusse out. Agarose is another product obtained by fractionation of crude agar. Ordinary agar and chemical gels have networks of polymer threads too dense to permit passage of the huge DNA molecules.