They dont.
The invention of the microscope paved the way for the discovery of the microscopic world.
The world's largest single-celled organism is the green algae Caulerpa.
They are living, but they are not organisms. Cells are the "building blocks" of tissues, which group together into organs, which group together into organ systems. Those organ systems comprise an organism. The individuals cells are not organisms themselves.
The first organism on Earth was a bacterium, from what current science tells us. The Earth was composed of single-celled organisms in the beginning of the world.
Protists, fungi, and viruses can also be grouped with bacteria and amoeba as microscopic organisms. These organisms are typically single-celled and have diverse characteristics, roles, and habitats in the natural world.
There is a limit to how big a cell can get, and that nearly always means we need a microscope to see them, Size is limited by the need for substances to diffuse in and out of the cell. If it gets too big there simply isn't enough surface to allow enough exchange of substances for the cell to maintain its life processes.
have photosynthetic organisms in their tissues
The two largest groups of organisms are bacteria and fungi. Bacteria are single-celled microorganisms found in diverse environments, while fungi are a diverse group of multicellular organisms that can be found in various habitats, including soil, water, and living organisms.
Cells are present in all living organisms, from the simplest single-celled microorganisms to complex multicellular organisms like plants and animals. Cells make up the basic units of structure and function in living organisms.
most algae in the world is single celled but there are a few algae that are multicellular
The largest known single celled organism is called a "Valonia Ventricosa".These organisms typically grow in tropical or subtropical waters around the world.
Approximately 99% of all eukaryotic organisms are classified as protists. These single-celled eukaryotes are incredibly diverse, ranging from microscopic algae to parasitic organisms. Despite their small size, protists play important roles in various ecosystems and are crucial to the functioning of the natural world.