Nostoc, genus of blue-green algae with cells arranged in beadlike chains that are grouped together in a gelatinous mass. Ranging from microscopic to walnut-sized, masses of Nostoc may be found on soil and floating in quiet water. Reproduction is by fragmentation. A special thick-walled cell (akinete) has the ability to withstand desiccation for long periods of time. After 70 years of dry storage, the akinete of one species germinates into a filament when moistened. Like most blue-green algae, Nostoc contains two pigments, blue phycocyanin and red phycoerythrin, as well as chlorophyll, and has the ability to fix nitrogen in specialized cells called heterocysts. A terrestrial species has been used as a supplementary food source in Asia.

The full form of PPLO is Pleuro Pneumonia Like Organisms.

Explanation:
PPLO is a group of bacteria that lack a cell wall and are typically found in the respiratory tracts of animals. These bacteria are known for their ability to cause various diseases, including pneumonia, arthritis, and mastitis. PPLO are also referred to as "mycoplasmas" because they belong to the class Mollicutes, which includes bacteria that lack a cell wall.

PPLO were first discovered in the 1890s when they were isolated from cattle with pleuropneumonia, a disease characterized by inflammation of the lungs and pleural membranes. Since then, PPLO have been identified in a wide range of animals, including humans, and have been associated with a variety of diseases.

PPLO are unique among bacteria because they are the smallest self-replicating organisms known, with a typical size of only 0.2 to 0.3 micrometres. They also have a very small genome, containing only about 600 to 1200 genes, which is much smaller than most other bacteria.

In summary, PPLO stands for Pleuro Pneumonia Like Organisms, which are a group of bacteria that lack a cell wall and are associated with various diseases in animals and humans.