SEAWEEDS USED AS A SOURCE OF AGAR

 Genera and species used

Most agar is extracted from species of Gelidium (Figure 1) and Gracilaria (Figure 2). Closely related to Gelidium are species of Pterocladia, and small quantities of these are collected, mainly in the Azores (Portugal) and New Zealand. Gelidiella acerosa is the main source of agar in India. Ahnfeltia species have been used in both Russia and Japan, one source being the island of Sakhalin (Russia).

  Natural habitats

First, a discussion is necessary of the terms used to describe where seaweeds grow. The vertical dimension of the shore is divided into zones. Common terms used to describe these are the intertidal zone and the subtidal zone. People are familiar with tides and readily understand these terms. The intertidal zone is the part of the shore that lies between the high and low tide levels, the subtidal zone is everything below the low tide level. Sometimes the intertidal zone is called the littoral zone. However, it has been found that, from a biologist's point of view, it is better to use terms that define the zones by what usually grows in them. So a line is drawn, and above this line is a zone where organisms receive irregular wetting and this is called the eulittoral zone. Below this line, organisms are immersed in water except on rare occasions (e.g. extremely low tides); this is called the sublittoral zone. These zones do not necessarily depend on tide levels; the upper limit of the eulittoral zone is set by the upper limit of barnacles, where they occur, and the lower limit is the highest point where the large brown algae can be found growing. Above the edge of the eulittoral zone, beyond the barnacles, is a zone that is only reached by spray water and this is called the littoral fringe.

FIGURE 1
Gelidium, rehydrated from dried material purchased by an agar producer. The coin diameter is 20 mm.

FIGURE 2
Gracilaria, rehydrated from dried material purchased by an agar producer. The coin diameter is 20 mm.

Gelidium grows best where there is rapid water movement, which is in the eulittoral and sublittoral zones. Depending on the species, it can be found in water from 2 to 20 m in depth. Gelidium prefers rocky areas with steep slopes, and is rarely found on muddy or sandy bottoms (compare Gracilaria below). It prefers partial shade and may be bleached by full intensity light in tropical latitudes. It usually grows best at 15-20°C, but can tolerate higher temperatures. It can survive in low nutrient conditions and some species adapt to low or high salinity.
For further detail see Santelices (1991).
Large beds of Gracilaria usually grow in the eulittoral zone, or just below it in the beginning of the sublittoral, on sandy or muddy sediments that are protected from waves. Sometimes it can be found free-floating in tidal lakes of salt or brackish water. It can adapt to large variations in growing conditions, such as freshwater dilution, increase in fertilizer concentration from runoff, and raised temperatures. Seawater temperatures of 20°C or higher are needed for at least three months of the year. It grows in a wide range of latitudes. It can survive being covered in sediment - growing again when water motion uncovers it.