COME under Kingdom Monera.Blue-green algae are also known as cyanobacteria.Blue-green algae are photosynthetic autotrophs.Explanation-Some of the Blue-green algae can perform Nitrogen fixation.Blue-green algae fix nitrogen through specialized cells called heterocysts.They convert atmosphere nitrogen into NITROGENOUS compounds LIKE amino acids, nitrates. These nitrates increase the FERTILITY of the soil.Hence blue-green algae improve the fertility of the soil by nitrogen fixation.Examples of blue-green algae that fix nitrogen are Nostoc.Cyanobacteria were the first organisms that produced oxygen on our earth.Brown algae, green algae, and RED algae are the algae that come under Kingdom Plantae.Member of Chlorophyceae is commonly known as 'green algae'.Member of Phaeophyceae is commonly known as 'brown algae'.Member of Rhodophyceae is commonly known as 'red algae'.