High tolerance of vegetative branch and germlings of the brown alga Sargassum hemiphyllum to low salinity effects

Increasing intensity and frequency of storms has been predicted as one of the consequences of global climate change. In which case, changes in salinity are likely to become an important factor affecting the survival of coastal marine organisms. Macroalgae are important primary producers along the coast. In Hong Kong, the brown alga Sargassum hemiphyllum is an important canopy species. It occupies a critical niche in the low intertidal and shallow subtidal. It is therefore susceptible to low salinity exposure during low tide. We examined two growth stages of S. hemiphyllum, the vegetative and germling stages, for their vulnerability to low salinities. Secondary shoots with length of around 5 cm were collected from separate Sargassum individuals and kept in 250 ml conical flasks, each with 150 ml seawater culture medium. They were then subjected to salinity treatments at 0 psu, 10 psu, 20 psu, 30 psu and 33 psu (as ambient control). Likewise, germlings of Sargassum were induced to settle on Petri dishes and exposed to 0 psu, 5 psu, 10 psu, 15 psu, 20 psu, 25 psu and 33 psu (as ambient control). The ambient control was the salinity level of the seawater at the time when the samples were collected from the field. Both experiments lasted for 40 days and growth and mortality, as indicated by complete loss of pigments or decay of the branch or germlings, were used as the end points. All vegetative branches survived their exposure to different salinities, except at 0 psu. Vegetative branches attained the best growth under the ambient condition of 33 psu. On the other hand, all germlings cultured under different salinities demonstrated a type three survivorship curve, in which high mortality occurred early in the developing stage. All germlings died at day 6 under 0 psu, whereas most germlings could survive under 15 psu or higher salinities. The highest survivorship (80%) was surprising recorded at 15 psu as germlings exposed to higher salinities needed also to compete with the growth of epiphytic algae. Germlings under 25 psu obtained the highest increase in growth of 7 time of their original size at Day 0. Our results indicated that S. hemiphyllum is very tolerant to low salinities. They are likely to survive in the future even with lowering of salinity due to global climate change.