Effects of emersion, temperature, dopamine, and hypoxia on the accumulation of extracellular oxidants surrounding the bloom-forming seaweeds Ulva lactuca and Ulvaria obscura

Macroalgal blooms are occurring in greater size and frequency and many of these blooms are composed of ulvoid green algae. Ulvoid algae can release a variety of allelopathic compounds that include reactive oxygen species (ROS), oxidants that have detrimental effects on other organisms. When algae are subjected to environmental stressors, oxidant production increases and can exceed the scavenging capacity of antioxidants, which can result in the passive diffusion of excess ROS into the extracellular environment. Because macroalgal blooms consist of high biomasses of algae and occur in intertidal or shallow subtidal habitats where they experience large and rapid changes in environmental conditions, there is the potential for significant releases of ROS into waters that contain them. To examine the effects of environmental stressors on the accumulation of oxidants around bloom-forming ulvoid algae, we manipulated emersion times, extracellular dopamine concentrations following emersion, seawater temperatures following emersion, and seawater oxygen concentrations and then measured the accumulation of extracellular oxidants around Ulva lactuca and Ulvaria obscura. In most cases, the accumulations of oxidants were similar among our experimental treatments except that the concentrations of extracellular oxidants resulting from Ulva were significantly lower when algae were exposed to either dopamine or high water temperatures than they were from controls or lower temperatures, respectively. The accumulations of oxidants in water surrounding Ulvaria were higher in unemersed than emersed algae, regardless of emersion time. These differences in concentrations of extracellular oxidants were most likely due to differences in algal photosynthetic rates among treatments, although oxidant decay and the upregulation or activation of antioxidants may also have affected our measurements. Based on the relatively high concentrations of oxidants that we measured and the interspecific and environmentally-driven differences in their accumulations, we hypothesize that accumulations of oxidants in waters near intertidal or shallow subtidal macroalgal beds will be larger in magnitude, more driven by biotic sources, and more seasonally and spatially variable than those measured farther offshore.