Chlorophyta

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Chlorophyta, or green algae, are a diverse group of photosynthetic organisms. They range from single cells to larger colonies and even simple multicellular forms. Most chlorophytes have a single green chloroplast in each cell and store starch as an energy reserve. They contain chlorophylls a and b and a set of carotenoids, which is why they look green and are related to land plants.

What they look like and how they live
- Structure: Many chlorophytes are unicellular, especially in the earliest-branching groups. In two major lineages, Chlorophyceae and Ulvophyceae, they also form colonies and simple multicellular bodies.
- Cells and flagella: Most chlorophyte cells can swim, usually with two or four flagella of the same length. Some groups have different flagella shapes, and a few have lost their flagella entirely. A distinctive flagellar arrangement called X-2-X-2 helps identify chlorophytes.
- Reproduction: Asexual reproduction is common and can happen through cell division, spores, or fragmentation. Some chlorophytes reproduce sexually, with different types of gametes (isogamous, anisogamous, or oogamous). Life cycles are mostly diplontic (gametes fuse to a zygote that grows and then makes more spores) but some have alternation of generations.

Where they live and why they matter
- Habitat and abundance: Chlorophyta are important in both freshwater and marine plankton. They contribute a lot of the world’s primary production by fixing carbon. They also appear as seaweeds along rocky shores.
- Diversity and adaptation: About 90% of known chlorophyte species live in freshwater. Some can live on land, in deserts, or in extreme environments like Arctic regions, hypersaline areas, deep oceans, or near hydrothermal vents.
- Relationships and nutrition: Many chlorophytes live in symbioses with other organisms (including fungi to form lichens, as well as sponges and corals). Some are heterotrophic or mixotrophic, feeding in addition to photosynthesis. A few Prototheca species can cause disease in humans and animals.

Evolution and classification
- History: Chlorophyta was named in the 19th century and has been reorganized many times as scientists learned more from cells and genomes. Today, Chlorophyta is kept separate from the Streptophyta, the group that includes land plants.
- Fossils and origins: One of the oldest possible multicellular chlorophytes is Proterocladus antiquus, about a billion years old. Genetic studies suggest chlorophytes originated roughly 2.0 to 1.2 billion years ago.
- Current view: There are many classes within Chlorophyta (at least eleven), including well-known groups like Chlorophyceae and Ulvophyceae. New discoveries continue to refine how these algae are related.

Uses and applications
- Biodiesel and bio-products: Green microalgae are explored for biofuels and valuable compounds such as carotenoids, vitamins, and fatty acids.
- Industry and research: Genera like Chlorella, Scenedesmus, Haematococcus, Dunaliella, and Tetraselmis are cultivated for biomass, nutrients, and cosmetics. The volvocine algae (including Chlamydomonas and Volvox) are studied to understand how multicellularity and cell specialization evolved.

In short, Chlorophyta are the green algae that range from tiny single cells to simple multi-cellular forms, live in a wide variety of habitats, and play a key role in ecosystems and biotechnology.