The term 'algae' is used for some lower plants and many, often unrelated groups of microorganisms that are able to perform photosynthesis. Photosynthesis (converting light energy into chemical energy) is performed in parts of the cell called chloroplasts. They can be found in different shapes and colours and in many different organisms. Not all these organisms are green. Diatoms, Chrysophytes and dinoflagellates have yellow to brown chloroplasts.
Freshwater algae are the Chlorophytes or Green algae. Based on the compounds of the photosynthetic pigments and several other characteristics they seem closest related to plants .A common green algae is Hydrodictyon, the water net. It is a related to Pediastrum (top image) But it forms a bag-shaped colony. Like Pediastrum each individual cell can develop into a new colony. You can imagine that since the colony contains thousands of cells Hydrodictyon can reproduce very rapidly.
And unlike Pediastrum, Hydrodictyon can grow large, almost 30 cm. in length. Blooms of Hydrodictyon can be a real problem for water treatment plants .The image shows a part of a small colony (left) and three individual cells of a big colony. Inside each of these cells a new colony can be formed. Many green algae form long filaments. The cells stay attached after they divide. Some genera, like Spirogyra, Mougeotia and Zygnema can become so numerous they form dense mats of growth in surfaces of ponds, so-called pond scum. This pond scum is interesting for a study under the microscope. If you squeeze it in a jar you will collect many interesting organisms. And the filamentous algae are at least as interesting. The chloroplasts have distinct shapes. In Spirogyra the chloroplast runs through the cell like a helix. The image also shows the nucleus hanging on fine threads.