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SUMMARY The effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of two Vietnamese brown algae, Sargassum mcclurei and S. oligocystum (Fucales), were determined by field and laboratory measurements. Dissolved oxygen sensors and pulse‐amplitude modulated (PAM) fluorometry were used for the measurements of photosynthetic efficiency. A Diving‐PAM revealed that underwater measurements of the effective quantum yield (Φ PSII ) of both species declined with increasing incident PAR, with minimum Φ PSII occurring during noon to early afternoon. Φ PSII recovered in the evening, indicating photo‐adaptation to excessive PAR. In laboratory experiments, Φ PSII also decreased under continuous exposure to 1000 μmol photons m−2 s−1; and full recovery occurred after 12 h of dark acclimatization. The net photosynthesis – PAR experiments of S. mcclurei and S. oligocystum conducted at 28°C revealed that the net photosynthetic rate quickly increased at PAR below the saturation irradiance of 361 and 301 μmol photons m−2 s−1 and nearly saturated to maximum net photosynthetic rates of 385 and 292 μg O2 gww − 1 min−1 without photoinhibition, respectively. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (12–40°C), revealed that the maximum gross photosynthetic rates of 201 and 147 μg O2 gww − 1 min−1 occurred at 32.9 and 30.7°C for S. mcclurei and S. oligocystum, respectively. The dark respiration rates increased exponentially over the temperature ranges examined. The estimated maximum value of the maximum quantum yield occurred at 19.3 and 20.0°C and was 0.76 and 0.74, respectively. Similar to the natural habitat of the study site, these two species tolerated the relatively high temperatures and broad range of PAR. The ability of these species to recover from exposure to high PAR is one of the mechanisms that allow them to flourish in the shallow water environment.

The responses of photochemical efficiency to desiccation and salinity gradients in an intertidal edible brown macroalga, Sargassum fusiforme (Harvey) Setchell (Sargassaceae, Fucales), were determined using a pulse amplitude modulation (PAM)-chlorophyll fluorometer. The effective quantum yields (ΔF/F m '; = Φ PSII ) of photosystem II (PSII) dropped to zero after 360-min aerial exposure under low irradiance (20 μmol photons m À2 s À1 ) and 120-min exposure under high irradiance (700 μmol photons m À2 s À1 ) for this species at 20 C and 50% relative humidity. Under these conditions, ΔF/ F m ' failed to recover to initial levels even after 1-day rehydration in seawater. In general, ΔF/F m ' decreased as desiccation reduced the absolute water content (AWC, %). Nevertheless, when AWC was above ca. 20%, ΔF/F m ' was mostly restored to initial levels after 1-day rehydration in seawater, suggesting strong tolerance to dehydration. Furthermore, S. fusiforme appeared to tolerate a broad range of salinity (i.e. 15-50 psu) during six days of culture; however, ΔF/F m ' declined when salinity was <10 and 60 psu. Strong tolerance to dehydration and salinity stress likely provides S. fusiforme an advantage that allows it to flourish in the intertidal habitat.

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