Effects of salinity, light intensity and sediment on growth, pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

Abstract

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 mmol photons m-2 s-1) and sediment (0, 0.67 and 2.28 mg L-1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 mmol photons m-2 s-1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 3.0 %DW) was at 25 psu, 150-400 mmol photons m-2 s-1 and no sediment. The highest pigment contents were phycoerythrin (0.8 0.5 mg g-1FW) and phycocyanin (0.34 0.05 mg g-1 FW) produced in low light conditions, at 150 mmol photons m-2 s-1. The highest photosynthesis rate was 161.3 32.7 mg O2 g-1 DW h-1 in 25 psu, 400 mmol photons m-2 s-1 without sediment in the short period of cultivation, (3 days) and 60.3 6.7 mg O2 g-1 DW h-1 in 25 psu, 700 mmol photons m-2 s-1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid-January to the end of April (dry season), and from mid-July to the end of September (first rainy season) when provided sediment was restricted.important raw materials for producing agar and they are relatively easy to farm. Gracilaria species contribute 50% of the world’s agar, which has a world market value of US$ 82.2 million (McHugh, 2003) In Thailand, although wet and dry Gracilaria production has reached 80 tons and 20 tons per year, respectively, the commercial requirement is for more than 2400 tons (Tachanaravong and Daroonchoo, 1988). Songkhla Lagoon, which is mesotrophic and the largest natural lagoon in Thailand, is an important source of Gracilaria, especially the harvests from both wild and planted beds at Koh Yor. However, in recent times the yields of Gracilaria have declined because of developments in and near the lagoon, which have dramatically changed the salinity and turbidity (Angsupanich & Rakkheaw 1997; Angsupanich & Kuwabara 1999; Panapitukkul et al. 2005). Salinity is an important factor for photosynthesis, respiration, and growth of Gracilaria spp (Israel et al. 1999; Li-hong et al. 2002). Lower salinities often inhibit growth of seaweeds, affect branching patterns and promote changes in their chemical composition (Ekman et al. 1991; Choi et al. 2006). Light intensity is also important for photosynthesis, and ultimately for all their biological processes. The quality and quantity of light depends on the depth, the density of particles in the water (turbidity) and day length (Lobban et al. 1985). The ability of seaweeds to absorb light energy varies, depending on their quantity of pigment and density of their photosynthetic units (Darley 1982). Adaptation is always important for the algae populations at all depths. Red algae adapt well to variations in light. The ratio of phycoerythrin increases with increased depth or low light intensities, and is a good example of chromatic adaptation (Beer & Levy 1983; Carnicas et al. 1999). An increase of sediment load has been recognized as a major threat to marine biodiversity on a global scale.