Adaptations of a green tide forming Ulva linza (Ulvophyceae, Chlorophyta) to selected salinity and nutrients conditions mimicking representative environments in the Yellow Sea

Abstract

:The massive green tide species Ulva linza undergoes an apparent rafting in a counterclockwise direction in the Yellow Sea from Jiangsu Province, China, to the west coast of Korea. During this dispersal, it undergoes development from early life history stages to senescence and experiences extensive salinity and nutrient gradients. We investigated the physiological responses of different developmental stages of U. linza to three representative environmental conditions that mimicked (1) a brackish environment at salinity 10 (S = 10) and 200 μM NO3−, (2) an inshore environment at S = 30 and 200 μM NO3−, and (3) an offshore environment at S = 30 and 4 μM NO3−. Chlorophyll a fluorescence of the early development stages (spores and germlings) of U. linza was measured using pulse amplitude modulation fluorometry to determine their photosynthetic efficiency. In addition, the photosynthesis and respiration rates of adult thalli were measured using chamber incubation. Within two hours of release, spores showed no difference in photosynthetic performance in the three conditions. Subsequently, the photosynthesis of germlings was enhanced at high-nutrient conditions regardless of salinity, suggesting that brackish and inshore waters are suitable nursery areas for early development. The adult thalli in the brackish environment showed two and three times higher growth rates than in the inshore and offshore environments, respectively. The adult thalli experienced synergistic effects of high nutrients and low salinity on photosynthesis that facilitated rapid growth in brackish environments. Overall, we suggest that brackish areas facilitate the initiation of blooms of U. linza, and the subsequent movement of floating fronds from estuaries to the inshore and then the offshore provides a continuous nutrient inoculum that maintains the population in the offshore environment.