The genus of Gracilaria is cosmopolitan in distribution, and has been reported from the arctic, temperate and tropical regions. Greville set up the genus Gracilaria in 1830, which comprised then only four species. Agardh reexamined the genus in 1852, and increased it to 23 species. In 1876 and 1901 he re-identified them again and reported 61 species altogether. Since then, the number of Gracilaria species reported from many places over the world have reached about 150, with 24 species reported in China.
Erect thallus arise from a small discoid holdfast. The thalli are generally cylindrical, depressed or blade-shaped, with lateral, alternate or subdichotomous branches. Sometimes several different branches may be found in one plant. The external appearance of thalli may be used to identify species. The style of the apex and the base of branches are different with species.
The thalli of most species are cylindrical. Several commercially important species will be discussed. Some species (such as G. eucheumaides) are compressed, their thalli growing horizontally along the substratum and form secondary holdfasts from the margin of branches. A few species (such as G. textorii) are blade-shaped.
Anatomy of main axis
The vegetative thalli of Gracilaria consist of cortex and medulla. Cortical cells are smaller. The outermost 1–2 layers are pigment cells. Medulla comprise large parenchymatous cells (Fig.1). The layers of cortex, the size and number of medullary cells and the change of cells from cortex to medulla are used for identification of species.
Fig. 1. Transection and longitudinal Gection of main axis of Gracilaria
The tetrasporangia are densely scattered on the cortex surface. Each tetrasporangia is composed of four tetraspores arranged in a cruciate manner (Fig. 2).
Fig. 2. Surface view and transection of tetrasporangia of Gracilaria
The spermatangia are globular or oval, scattered over the surface of thalli. The location and type of spermatangial conceptacle are three important considerations for identification of species (Fig. 3).
Fig. 3. Transections of spermatangis of Gracilaria
Cystocarp is prominent, protruding, globose or hemispherical, scattered over the surface of the fronds. It may be divided into 4 portions (Fig. 4).
Fig.4 Transections of cystocarps of Gracilaria
Consisting of several layers of cells, outmost layer is composed of pigmented cells.
In the center of cystocarps, consists of parenchymatous cells.
Formed at the top of the gonimoblast, round or ovoid in shape.
D. Absorbing filaments
Extended from the gonimoblast tissue to the pericarpic-layer. Some species possess absorbing filaments.
Introduction of several important species
Thallus erect, solitary or caespitose, cylindrical throughout, arising from a small discoid holdfast; purplish brown to dark brown, sometimes greenish or yellowish; branches irregularly alternated.
G. asiatica is widespread along the China coast, and was first reported in 1866 as Gracilaria verrucosa Papenfuss. In 1985, Chinese phycologists Zhang and Xia compared G. verrucosa specimens collected from England with Chinese and Japanese specimens both identified as “G. verrucosa”. They found that the Chinese and Japanese specimens were of the same species (Japanese phycologists were of the same opinion), but since the Chinese species differed from the English G. verrucosa species, they named the Chinese plant Gracilaria asiatica.
The asiatica species may be readily distinguished from English G. verrucosa by the deeper spermatangial conceptacles, larger tetraspores, smaller carpospores and especially by the structure of the pericarp (Fig. 5).
Fig. 5. Comporicon in pericarps of asiatica from China and Japan with verrucosa from England
G. lemaneiformis is one of the commercially important species because of its fast growth and high quality gel. It is distributed along the coasts of north China, and is similar in appearance to G. asiatica. Thallus is purplish red, arises from a larger red holdfast, and usually grows in gravel in the lower intertidal zone, with the base portions of thallus often covered with sand.
There are two varieties in south China coasts: v. tenuistipitata and v. liui. Variety tenuistipitata grows on gravels and shells in the sublittorial region of lower salinity. It is characterized by its extreme slenderness near the base. The thallus arises from a small disklike holdfast, and is simply, moderately, or alternately branched near the base. The branches tend to become like the main axis.
Variety liui differs from the variety tenuistipitata by the slender thalli bearing numerous, delicate, short to long lateral branchlets, branching mostly from the percurrent axes. Its cystocarpic structure and shallow spermatangia are similar to those of v. tenuistipitata (Fig. 6).
This variety frequently occurs naturally in muddy substrate in fish ponds and shallow intertidal areas. When cultivated, as in Hepu, Guangxi Province and in Taiwan, the thalli are usually detached and without holdfasts, and may tumble about in fairly large masses. The Taiwanese “G. verrucosa” (Shang 1976 and Chiang 1981) has been examined and is being place among the variety liui of G. tenuistipitata.
Fig. 6. Comparison between G. tenuistipitata var. liui and G. tenuistipitata var. tenuistipitata 1. Main axis. 2. Tetrasporangia. 3. Cystocarp. 4. Spermatangia. 5. Pericarp.
Sporophytes and gametophytes of Gracilaria occur alternately in its life cycle (Fig. 7).
Fig. 7. Life history of Gracilaria
Mature tetrasporophytes (2n) produce (meioses) tetrospores, some of which develop to male gametophytes (n), while others grow to female gametophytes. The male mature gametophyte forms spermatangia, the female forms carpogonia. After fertilization and cystocarps are formed on the female plant, carpospores (2n) are released and develop to tetrasporophytes again.
The vegetative tetrasporophyte, male gametophyte and female gametophyte have no obvious difference. Wild Gracilaria collected consist of tetrasporophytes, spermaphytes, and carposporophytes in different stages, respectively.
Gracilaria tenuistipitata v. tenuistipitata is the dominant species of some natural Gracilaria beds. The yield and quality of phycocolloid from this species is high. Spores adhering experiments have been carried out on how to efficiently protect and enhance those resources.
A 300 m2 protected experimental ground was set off in the natural beds. Triplicate of 1 m2 nets were settled in the ground every half month. The experiment lasted one year. Observations and studies were made on the growth condition of the wild plants in the bed and of those adhering to the nets, the development of sporelings, the rapid growth period, and the duration of spores discharge. The results are presented below:
Except July and August, mature tetrasporophytes and mature male and female gametophytes were found the year round, and spores were adhering to the nets. No spores were found adhering to nets placed in July and August until when the temperature was falling.
The development of tetrasporophytes was similar to that of the gametophytes. Cell division occurred 15 minutes to one day after the adhesion of spores. After 2–7 days, hemispheres were formed and then, discs arose from hemispheres. The center of the discs began to arch by the end of 10 days, and one or more erect fronds occurred gradually. The whole process generally took more than 30 days.
After adhering for 40–70 days, the sporelings grew to 2 mm in length, and could be seen with the naked eye. The duration of the process differed with seasons, from January to April it took 60–70 days, in other months about 40 days.
November to April was the rapid growth season, when there were more and larger (30–60 cm long) thalli in the bed and nets. In May, when the water temperature rose to 29°C, the thalli stopped growing. In June, thalli decayed from their apex to the basic portions of plants and disappeared finally, the larger plants first and the smaller ones later. By the end of June, only small matured thalli could be found (the smallest matured fronds were only 1 cm long).
When seawater temperature reached 31°C, no old thalli could be found in the bed or the nets. In the meantime, some new sporelings yellow in color and less than 2 mm could be detected under microscope. They were almost not growing in July and August, but recovered normal color and started growing well again when temperature dropped.
Studies on Gracilaria are of special interest because of its increasing market value as a source of agar, its importance in human diet and as food in the cultivation of abalone. As the supply of Gracilaria from the natural population has not substantially increased, Gracilaria is cultivated to supplement harvests from natural populations. The commercial beds of Gracilaria in China are so limited that the supply is unstable and can not meet the ever-increasing demands.
There are two varieties of Gracilaria tenuistipitata along the coast of south China: G. var. tenuistipitata and the faster-growing G. var. liui adaptable to brackish seawater. G. tenuis-tipitata var liui is, thus, the most common species under pond cultivation in Guangxi and Hainan Provinces. Unfortunately the growth rate sometimes fluctuates greatly. The effect of temperature, salinity, nitrogen, culture density and depth on the growth were investigated between April 1985 and March 1986 in outdoor ponds. The purpose of the study was to determine the relationship between the main environmental factors and the growth rate, so that knowledge gained could be applied to stabilize and increase pond production.
Triplicate samples of 500 g 5–10 cm long Gracilaria fragments were cultured at 0.5–1 m water depth in a one square meter rectangular net cage and weighed every 15 days. Changes in the wet weight were used to calculate specific growth rates from the formula:μ = where N0 is the initial biomass and Nt is the final biomass at time (t). Algal densities were maintained at desired levels by harvesting at the proper growth period.
Fig. 8. shows the year long growth of G. tenuistipitata v. liui in the experimental field. The mean daily growth rate was 2.4%. This means the weight was doubled each month. March, April, and May and September, October, and November were months of maximum growth when mean daily growth rate was 3.3%. In winter and summer it was only 1.5%. In the course of the experiment, temperature ranged from 15–32°C. The recorded growth rates were lowest at 15°C (lowest temperature) and 32°C (highest temperature). Growth rate was over 2% from 20–30°C. Thus 20–30°C may be considered as the temperature range most favorable for growth.
Fig. 8. Growth rate and sea water temperature
Gracilaria were cultured in two ponds with different salinities by using similar net cages as described above. The salinity of pond A was 30–34 %, that of pond B was 24 %. Total nitrogen content of the two ponds was controlled to the same level by adding nitrogen fertilizer to pond A. Outdoor tank culture experiments and measurement of the effect of salinity on photosynthetic rate were concurrently conducted.
After one month of experimentation the best algal growth was obtained in pond B, where yield in weight of G. tenuistipitata was 1.3 times higher than that in pond A. This clearly shows that salinity is an especially important factor influencing the growth of this alga. Results obtained from tank culture (Table 1) indicate that growth peaked at 21%, with a broad plateau between 7– 27%. Much lower or higher salinity was unfavourable for growth. At 3% salinity decolorization of apical segments occurred within two days and necrosis occurred after four days while at 34%, segments grew slender and branches were softer, while at 47%, segments decolored after 2 weeks.
Fig.9. Photosynthesis as a function of salinity (light intensity 240 υE.m-2.sec-1)
Effect of salinity on the photosynthetic rate is shown in Fig. 9. Maximum photosynthetic rate of G. tenuistipitata v. liui was obtained at 21% and was not markedly affected when the salinity was reduced to 14% or increased to 27%.
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