Algae, singular alga, are members of the kingdom Protista that are primarily aquatic photosynthetic organisms. Seaweed have a variety of life cycles and range in size from microscopic Micromonas species to giant kelps that can grow to be 60 metres (200 feet) long. Their photosynthetic pigments are more diverse than those of plants, and their cells have characteristics not shared by plants or animals.
Aside from their ecological roles as oxygen producers and the food source for almost all aquatic life, seaweed are economically significant as a source of crude oil, food, and a variety of pharmaceutical and industrial products for humans. Seaweed taxonomy is contentious and subject to rapid change as new molecular information becomes available. Phycology is the study of seaweed, and a phycologist is someone who conducts such research. According to definitions, Algae are eukaryotic (nucleus-bearing) creatures that can photosynthesize but lack the specialised multicellular reproductive structures of plants, which always comprise fertile gamete-producing cells surrounded by sterile cells. Like the avascular lower plants, seaweed likewise lack true roots, stalks, and leaves (e.g., mosses, liverworts, and hornworts). Additionally, the prokaryotic (nucleus-free) blue-green seaweed are not included in the list of seaweed in this article (cyanobacteria). Seaweed were divided into major groups based on their colour, such as red, brown, and green. Different chloroplast pigments, such as chlorophylls, carotenoids, and phycobiliproteins, are reflected in the colours. There are many more pigment groups recognised, and each class of seaweed shares a common set of pigment. The phylogeny of the group has not yet been determined, and the Algae are not closely related in an evolutionary sense. Without the existence of chloroplasts and photosynthesis as distinguishing characteristics, several types of seaweed are difficult to identify from protozoa and fungus. In fact, compared to other seaweed, some seaweed seem to share a closer evolutionary relationship with protozoa or fungi. Seaweed physical and ecological characteristics Size range and structure diversity Discoidea Halimeda. The seaweed have a size range of seven orders of magnitude. Many seaweed have only one cell, while the largest have millions. Groups of cells in large, macroscopic seaweed are specialised for specific functions such as anchorage, transport, photosynthesis, and reproduction; such specialisation indicates a level of complexity and evolutionary progress. Based on the morphology of their vegetative, or growing, state, Algae can be classified into several types. Filamentous forms have cells arranged in chains similar to bead strings. Some filaments are unbranched (e.g., Spirogyra), whereas others (e.g., Stigeoclonium) are branched and bushlike. The gross morphological form of many red seaweed (e.g., Palmaria) is created by numerous adjacent filaments joined laterally. Parenchymatous (tissuelike) forms, such as the giant kelp (Macrocystis), can grow to be several metres long. Coenocytic seaweed, such as the green seaweed Codium, can grow to be quite large without forming distinct cells. Coenocytic seaweed are essentially unicellular, multinucleated seaweed with no cell walls that divide the protoplasm (cytoplasmic and nuclear content of a cell). Some Algae have flagella and swim in water. These flagellates range from single cells like Ochromonas to colonial organisms like Volvox with thousands of cells. Coccoid organisms, such as Scenedesmus, typically have an exact number of cells per colony, which is obtained through a series of rapid cell divisions when the organism is first formed; once the exact cell number is obtained, the organism grows in size but not in cell number. Chrysocapsa and other capsoid organisms have a variable number of cells.
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