<P> Agar - agar is an allowed nonorganic / nonsynthetic additive used as a thickener, gelling agent, texturizer, moisturizer, emulsifier, flavor enhancer, and absorbent in certified organic foods . </P> <P> An agar plate or Petri dish is used to provide a growth medium using a mix of agar and other nutrients in which microorganisms, including bacteria and fungi, can be cultured and observed under the microscope . Agar is indigestible for many organisms so that microbial growth does not affect the gel used and it remains stable . Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium . Other ingredients are added to the agar to meet the nutritional needs of the microbes . Many specific formulations are available, because some microbes prefer certain environmental conditions over others . Agar is often dispensed using a sterile media dispenser . </P> <P> As a gel, an agar or agarose medium is porous and therefore can be used to measure microorganism motility and mobility . The gel's porosity is directly related to the concentration of agarose in the medium, so various levels of effective viscosity (from the cell's "point of view") can be selected, depending on the experimental objectives . </P> <P> A common identification assay involves culturing a sample of the organism deep within a block of nutrient agar . Cells will attempt to grow within the gel structure . Motile species will be able to migrate, albeit slowly, throughout the gel and infiltration rates can then be visualized, whereas non-motile species will show growth only along the now - empty path introduced by the invasive initial sample deposition . </P>

Name two algae from which agar is obtained