Green Algae — Phylum Chlorophyta
Most green algae, the Chlorophyta, inhabit freshwater environments. Some species of green algae are unicellular and microscopic (e.g., Chlamydomonas sp.), but most are larger (e.g., Ulva sp.). Green algae often attach themselves to submerged rocks or plants, or form free-floating mats, balls, or long strands. Other green algae form plate-like colonies, thread-like filaments, or net-like tubes. Many green algae have one or more flagella – whip-like appendages that propel the alga through the water.
Some green algae thrive in snow banks, adding colour to the snow (e.g., Hormidium subtile and Raphidonema nivale), while grey patches of snow can be caused by the green alga Mesotaenium berggrenii. Red snow is generally due to the presence of spores of green algal species such as Chlamydomonas nivalis. Green algae also grow attached to ice, adding colour to frozen lakes. Life on snow and ice means that algae must have adaptations to prevent their cells from freezing. Since the free water content in their cells has the highest risk of freezing, algae survive by binding this water to structures within the cell.