Responses of photosynthesis and nitrogen assimilation in the green-tide macroalga Ulva prolifera to desiccation

Abstract

The Yellow Sea in China has experienced annual large-scale green tides since 2007. Ulva prolifera, the dominant causative species, originating from Pyropia yezoensis aquaculture rafts, experiences periodic tidal emersion in the intertidal nursery. It was proposed that physiological adaptations of U. prolifera may enable it to survive the harsh intertidal environment and contribute to subsequent blooms. In this paper, we measured the responses of photosynthesis and nitrogen assimilation of U. prolifera during desiccation of 0–6 h and subsequent rehydration for 0–24 h. The results suggested that both PSII photosynthesis and nitrogen assimilation were significantly reduced at the onset of desiccation. These reductions were reflected by decreases in the maximum quantum yield (Fv/Fm), effective photochemical quantum yield (YII), maximal electron transport rate (ETRmax), light utilization efficiency (α), NO3-N and NH4-N uptake rates, tissue nitrogen concentration (TN), and nitrate reductase activity (NRA). Desiccation temporarily lowered PSII photosynthesis, but rates recovered after algae were submerged for 24 h. Assimilation of nitrogen was also reduced during exposure to air. Although both NO3-N and NH4-N uptake rate recovered within 12 h upon rehydration, TN and NRA were not completed within 24 h. U. prolifera therefore appears well adapted to survive prolonged periods of desiccation, as it experiences growing in Pyropia farms along the Chinese Jiangsu coast. These traits, along with its ability to grow fast, may explain its success and one of the mechanisms behind the formation of the extreme blooms in the Yellow Sea.