Algae, singular alga, members of a group of predominantly aquatic photosynthetic organisms of the kingdom Protista. Algae have many types of life cycles, and they range in size from microscopic Micromonasspecies to giant kelps that reach 60 metres (200 feet) in length. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. In addition to their ecological roles as oxygen producers and as the food base for almost all aquatic life, algae are economically important as a source of crude oil and as sources of food and a number of pharmaceutical and industrial products for humans. The taxonomy of algae is contentious and subject to rapid change as new molecular information is discovered. The study of algae is called phycology, and a person who studies algae is a phycologist.
In this article the algae are defined as eukaryotic (nucleus-bearing) organisms that photosynthesize but lack the specialized multicellular reproductive structures of plants, which always contain fertile gamete-producing cells surrounded by sterile cells. Algae also lack true roots, stems, and leaves—features they share with the avascular lower plants (e.g., mosses, liverworts, and hornworts). Additionally, the algae as treated in this article exclude the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria).
Beginning in the 1830s, algae were classified into major groups based on colour—e.g., red, brown, and green. The colours are a reflection of different chloroplast pigments, such as chlorophylls, carotenoids, and phycobiliproteins. Many more than three groups of pigments are recognized, and each class of algae shares a common set of pigment types distinct from those of all other groups.
The algae are not closely related in an evolutionary sense, and the phylogeny of the group remains to be delineated. Specific groups of algae share features with protozoa and fungi that, without the presence of chloroplasts and photosynthesis as delimiting features, make them difficult to distinguish from those organisms. Indeed, some algae appear to have a closer evolutionary relationship with the protozoa or fungi than they do with other algae.
This article discusses the algae in terms of their morphology, ecology, and evolutionary features. For a discussion of the related protists, see the articles protozoan and protist. For a more complete discussion of photosynthesis, see the articles photosynthesis and plant.
For other uses, see Spirogyra (disambiguation).
Spirogyra (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous charophytegreen algae of the orderZygnematales, named for the helical or spiral arrangement of the chloroplasts that is diagnostic of the genus. It is commonly found in freshwater areas, and there are more than 400 species of Spirogyra in the world.Spirogyra measures approximately 10 to 100 μm in width and may grow to several centimeters in length.
Spirogyra is very common in relatively clean eutrophic water, developing slimy filamentous green masses. In spring Spirogyra grows under water, but when there is enough sunlight and warmth they produce large amounts of oxygen, adhering as bubbles between the tangled filaments. The filamentous masses come to the surface and become visible as slimy green mats.
Spirogyra can reproduce both sexually and asexually. In vegetative reproduction, fragmentation takes place, and Spirogyra simply undergoes the intercalary mitosis to form new filaments.
Sexual Reproduction is of two types:
- Scalariform conjugation requires association of two different filaments lined side by side either partially or throughout their length. One cell each from opposite lined filaments emits tubular protuberances known as conjugation tubes, which elongate and fuse, to make a passage called the conjugation canal. The cytoplasm of the cell acting as the male travels through this tube and fuses with the female cytoplasm, and the gametes fuse to form a zygospore.
- In lateral conjugation, gametes are formed in a single filament. Two adjoining cells near the common transverse wall give out protuberances known as conjugation tubes, which further form the conjugation canal upon contact. The male cytoplasm migrates through the conjugation canal, fusing with the female. The rest of the process proceeds as in scalariform conjugation.
The essential difference is that scalariform conjugation occurs between two filaments and lateral conjugation occurs between two adjacent cells on the same filament.
The following species are currently accepted: