Which type of living organisms are classified as microscopic and consist of a single cell?

Bacteria are classified as microscopic organisms that consist of a single cell, making them a vital part of various ecosystems and essential for processes like decomposition and nutrient cycling. Unlike viruses, which are not considered living organisms because they cannot reproduce independently and require a host cell, bacteria are self-sufficient and capable of carrying out all necessary life processes within their single-cell structure.

In contrast to algae and fungi, which can be multicellular, bacteria exist in a wide range of sizes and shapes but are fundamentally unicellular. Algae, while some species can be unicellular, are often associated with larger, multicellular forms as well. Fungi, similarly, typically exist as multicellular organisms, although there are unicellular fungi like yeast, they do not belong in the same classification as bacteria. Hence, bacteria are the definitive answer as the group defined by being microscopic and living as a single cell.