Chlorophyta

Chlorophyta, commonly known as green algae, is a major division of eukaryotic organisms. These organisms are essential to Earth's ecosystems because they fix more than one billion tons of carbon every year. They are part of a larger group called Viridiplantae, which also includes land plants. While many people think of algae as simple, they are structurally diverse. Some are unicellular, meaning they consist of only one cell. Others, like those in the classes Chlorophyceae and Ulvophyceae, show an evolutionary trend toward complex colonies or multicellularity.

At the cellular level, chlorophytes are defined by their specialized chloroplasts. These are green organelles surrounded by a double-membrane envelope. Inside, they contain chlorophylls a and b, which capture light for energy. They also contain various carotenoids, such as lutein and zeaxanthin. These pigments help the cell manage environmental stress. For example, algae may accumulate extra carotenoids during nitrogen deficiency, high salinity, or intense sunlight. The chloroplasts also serve as storage sites for starch, which acts as a carbohydrate reserve. Unlike the Ochrophyta division, chlorophytes do not have a chloroplast endoplasmic reticulum.

Movement in many chlorophytes is achieved through flagella, which are whip-like tails. These cells often possess two or four flagella of equal length. A unique feature of chlorophytes is their flagellar apparatus. They use a symmetrical, cross-shaped root system known as an "X-2-X-2" arrangement. This means ciliary rootlets with many microtubules alternate with rootlets containing only two microtubules. You can also distinguish them from streptophytes by where the flagella attach. In chlorophytes, flagella are inserted directly at the cell apex. In contrast, streptophyte flagella are inserted sub-apically, or slightly to the side of the apex.

Reproduction in chlorophytes can be asexual or sexual. Asexual reproduction is very common and includes methods like cell division, fragmentation, or the formation of zoospores. Some colonial groups create autocolonies, where new colonies match the parent's arrangement exactly. Sexual reproduction can be isogamous, where gametes are identical, or oogamous, where there are distinct sperm and egg cells. Most chlorophytes follow a diplontic life cycle. In this cycle, gametes fuse to form a zygote, which then grows and eventually undergoes meiosis. Some exceptions use a haplodiplontic life cycle, which involves an alternation of generations similar to land plants.

Early-branching unicellular groups, called prasinophytes, have unique survival strategies. They often produce resistant stages known as cysts or phycoma. To create these, a cell discharges mucilage vesicles to build an inner wall. The cell also increases its internal lipids to improve buoyancy. Once the outer wall forms, the nucleus and cytoplasm divide into many new flagellate cells. These daughter cells are eventually released when the cyst wall ruptures. Some of these cells may even function as gametes for sexual reproduction.

The history of naming these organisms has changed as science advanced. In 1828, Heinrich Gottlieb Ludwig Reichenbach first used the name Chlorophyta. However, his definition was very broad and included mosses and ferns. In 1914, Adolf Pascher narrowed the definition to include only green algae with chlorophylls a and b. Throughout the 20th century, many scientists proposed different classification systems. In 2004, the modern usage of Chlorophyta was established. Scientists used molecular phylogenetics, the study of genetic relationships, to separate chlorophytes from the streptophyte lineage.

Chlorophytes inhabit a vast range of environments. While about 90% of known species live in freshwater, many are marine. They are vital components of phytoplankton in both habitats. Some species are highly specialized, living in extreme places like deserts, arctic regions, or deep-sea hydrothermal vents. Others live in symbiosis, forming partnerships with other organisms. For instance, some chlorophytes live with fungi to create lichens. Some are even used as food, such as the expensive Monostroma kuroshiense. However, some can be harmful, like the genus Prototheca, which causes protothecosis in humans.