Red algae are any of the species of algae belonging to the taxonomic Division Rhodophyta. They are characterized by the pigment phycoerythrin, which reflects red light and absorbs blue light. That makes rhodophyta looks red, and let them live in greater depth water with photosynthesis. Some rhodophyta have very little phycoerythrin, so they may appear green or blue from chlorophyll and other pigments possessed by them.
In Asia, Rhodophyta is one of important sources as food like nori. They contain high vitamin and protein content and this fact make them attractive sources. In Japan, cultivation of Rhodophyta began more than 300 years ago. 
Rhodophyta are autotrophic, which is an organism that obtains and stores Floridian starch from photosynthesis. There are a few exception, but most Rhodophyta are multicellular organisms. They are pigmented with allophycocyanin, phycocyanin, and phycoerythrin that are located in phycobilisomes. Rhodophyta don’t have any centrioles or flagella. They don’t have chloroplast endoplasmic reticulum either. But they have unstacked thylakoids in plastid. 
Pit connections and pit plugs
Some species have pit connections between cells. Features of red algae, pit connections and pit plugs are very unique and distinctive. In the case of red algae, cytokinesis is incomplete. Normally, a small pore is located in left-middle part of the newly formed partition. The pit connection is formed in which the daughter cells that remain in contact. Then, the pit connection is built cytoplasmic continuity is blocked by the generation of a pit plug. The generation of a pit plug is deposited in the wall gap that connects the cells. Connections between cells have a common parent cell, called primary pit connections. Cells mostly have two primary pit connections because apical growth is normal in red algae.
After forming of a pit connection, tube-shaped membranes appear. A granular protein, which is called the plug core, forms around these membranes. The tubular membranes disappear while some of red algae orders have a plug core, others have an associated membrane at each side of cap membranes. The pit plug exists between the cells until one of them dies. When one dies, the living cell makes a layer of wall that seals off the plug. 
Rhodophyta reproduce sexually. Usually, life cycles of rhodophyta are diplohaplontic, with alternation between diploid stages and haploid stages, with few exceptions. For instance, the Porphyra nereocystis has a heteromorphic alternation of generations. Sexual reproduction is oogamous, that involves unmovable spermatia and closed mitosis. In the case of tetraspores, they are produced in the tetrasporangia while meiosis is on process. If they reproduce asexually, it does so through aplanospores. 
Rhodophyta live in both freshwater and salt water, even though mostly are marine organisms. They range tropical, temperate, and cold water places. Their pigments absorb blue light, that penetrates water to deeper than other wavelengths, for they are designed to live at very deep water. Rhodophyta are primary producers, for they provide habitats to other aquatic organisms. In addition, Rhodophyta is in an important part in the maintenance and establishment of coral reefs. These species that are found in coral reefs are called coralline algae. 
The oldest fossil record of Rhodophyta is also the oldest fossil of eukaryote. Bangiomorpha pubescens, a multicellular fossil from arctic Canada is very close to modern red algae, Bangia. 
- Rhodophyta Wikipedia
- Rhodophyta MicrobeWiki
- Rhodophyta UC Berkeley
- Rhodophyta Integrated Taxonomic Information System