Water depth-dependant photosynthetic and growth rates of Gracilaria lemaneiformis, with special reference to effects of solar UV radiation

To optimize the farming strategy of economically important seaweeds, we grew Gracilaria lemaneiformis at different seawater depths under natural solar radiation with or without UV irradiances. Our results showed that solar UV radiation (UVR, 295-395 nm) significantly inhibited its growth and photosynthesis in the surface seawater; however, such a negative effects became inverse at 1.7 m with levels of UVR being about 11.7 W m(-2). The maximal growth rates were found at 1.7, 0.8 m when the thalli grown at PAR + UVR or PAR only conditions, respectively. UV-absorbing compounds in the algal thalli were up to 85% higher under PAR + UVR compared to PAR alone treatment and they increased with decreased depths even in the absence of UVR. The enhanced growth rate in the presence of UVR could be attributed to enhanced photosynthesis under moderate levels of solar radiation by UV-A. Our study clearly showed that UV-induced growth inhibition could be minimized or avoided by optimizing the alga's farming depth in different waters of different transparency, so that Gracilaria's biomass production could be increased.