Algae were traditionally thought to be plants, but are now known to be members of several quite distantly-related groups. They include all the photosynthetic protists.
They can be differentiated by both morphology and the color of their photosynthetic pigments. Many algae are multicellular, forming sheets, filaments, and other structures of varying size and complexity.
Some organisms were formerly classified as both algae and protozoans, because they are photosynthetic and also motile. Furthermore, many groups of algae have members that are not photosynthetic, such as the chrysophytes and the dinoflagellates. The entries for such organisms are cross-referenced between the different categories they may belong to..
A checklist of algae from Singapore has recently been published by the Raffles Museum of Biodiversity Research (MN Pham, HTW Tan, S Mitrovic, HHT Yeo (2011) A checklist of the algae of Singapore). Nomenclatural information can also be found on Algaebase.
The Public Utilities Board has recently published A Guide to Freshwater Phytoplankton in Singapore Reservoirs, which is no. 45 in the Science Centre Guidebooks series. The fully-illustrated volume is compact and handy for reference. An older, but still useful guidebook is How to Know the Fresh-Water Algae, by GW Prescott, published in 1954. It includes an identification key illustrated with line drawings. The entire book has been digitized by the Biodiversity Heritage Library and can be browsed or downloaded in pdf form from the Internet Archive.