In situ assessment of Ulva australis as a monitoring and management tool for metal pollution

We investigated physiological responses of Ulva australis to metals and their implications for biomonitoring and management tool. To determine the capacity of Ulva to accumulate metals over the short-term, we undertook an in situ experiment where we transplanted thalli to sites with different levels of metal pollution. After 12 days, arsenic, copper, lead, and zinc accumulation was observed. Zinc was significantly greater (p = 0.001) at the most polluted site and was highly correlated (r = 0.87) with seawater total Zn concentration. We also assessed whether metal exposure can compromise U. australis performance by evaluating physiological responses and changes in thalli ultrastructure. We observed an increase in electron-dense bodies in the cell walls and vacuoles, which clearly indicates metal accumulation. However, there was no change in physiological performance (i.e. growth rate, F_v/F_m, rETR_max, or in photosynthetic pigments content) between the control and transplanted thalli (p > 0.05). Bioaccumulation capacity of U. australis was assessed by deploying thalli at a highly polluted site for 45 days, where zinc in Ulva markedly increased over time and was highly correlated with the environmental concentrations (total Zn in seawater, r = 0.85). The metal uptake rate increased steadily over time, confirming that Ulva is clearly capable of bioaccumulation. However, visual examination of the thalli suggested degradation over time, which might limit deployment time (20 days). Clearly, U. australis has potential as a biomonitor/management tool, particularly for zinc, but the results suggest it may be a useful tool for removing metals from the environment.