Seawater nitrogen concentration and light independently alter performance, growth, and resource allocation in the bloom-forming seaweeds Ulva lactuca and Ulvaria obscura (Chlorophyta)

Ulva lactuca and Ulvaria obscura are seaweeds that form green tides on Salish Sea shores. They have similar macroscopic morphologies but differ in their biochemistries and physiological responses. To understand how they are affected by changes in environmental conditions, a factorial experiment was conducted in which algae were grown in artificial seawater with either low (10 μM) or high (160 μM) nitrate (NO3−) concentrations at high (29 mol photons·m−2.day−1) and low (4 mol photons·m−2.day−1) light levels. Light and NO3− affected algal responses, but always independently. After two weeks, U. lactuca grown in high light were larger, had lower maximum quantum yields (MQYs), and lower nitrogen (N), carbon (C), pigment, and dimethylsulfoniopropionate (DMSP) concentrations, respectively, relative to algae in low light. In contrast, U. obscura growth was unaffected by light. Like U. lactuca, U. obscura grown in high light had lower MQYs, and N, pigment, and DMSP concentrations. In high light, U. obscura also had 89% higher dopamine concentrations and a tendency to fragment. Both U. lactuca and U. obscura grown in 160 μM NO3− were larger, had higher MQYs, and higher N, pigment, and DMSP concentrations, respectively, than algae in 10 μM NO3−. Also, when U. obscura was grown in the 160 μM NO3− medium, it significantly increased its surface area/mass ratio. Although both species grew faster in high NO3−, high light only promoted the growth of Ulva, which may explain the dominance of Ulva in summer months. High light was physiologically stressful to both species and caused increases in photoprotective mechanisms, such as the production of dopamine, a melanin precursor, in Ulvaria, and DMSP lysis in Ulva to generate antioxidants. Growing in 10 μM NO3- produced responses that were consistent with nitrogen limitation and had greater impacts on Ulvaria than Ulva, suggesting that Ulvaria responds more strongly to eutrophication.