Iron bioaccumulation in coral reef organisms and biodilution in the reef food web: A mesocosm approach

Iron (Fe) is an essential metal for marine organisms, but at excessive environmental concentrations, it can cause ecophysiological adverse effects. In the present study, Fe bioaccumulation was investigated in several groups of reef organisms: turf algae, rhodoliths, fleshy macroalgae (Sargassum sp. and Lobophora variegata), corals (Millepora alcicornis, Mussismilia harttii and Siderastrea sp.), fish (Stegastes fuscus), and brittle star (Ophioderma appressum). Organisms were kept in seawater without Fe addition (control condition) or in incrementing concentrations of dissolved Fe (100, 300, and 900 μg L^-1) for 28 days, using a marine mesocosm system. In seawater without Fe addition, coral skeletons and brittle star arms showed the lowest Fe concentration (1.91-4.61 μg g^-1 dry weight) while macroalgae and fish liver exhibited the highest Fe concentration (184-210 μg g^-1 dry weight). After exposure (14 or 28 days) to the incrementing concentrations of Fe in seawater, turf algae, corals (skeleton and tissue), fish (muscle, gill, and liver) and brittle star (arm and disc) did not accumulate Fe. On the other hand, Fe bioaccumulation was observed in rhodoliths (4.35-fold) and macroalgae (Sargassum sp.: 5.81-fold; L. variegata: 4.51-fold). Bioconcentration factor values were markedly higher in macroalgae (646‒1319) than in the other biological matrices evaluated (8.14-229). This finding evidences Fe bioaccumulation in key primary producers and highlights the potential risk of Fe impacts in coral reefs because of excessive dissolved concentrations of this metal. Food web magnification factors (FWMF) in the control condition and after Fe exposure (900 μg L^-1) for 28 days corresponded to 0.481 and 0.404, respectively. Across all tested dissolved Fe concentrations and exposure times, a general biodilution of Fe (FWMF <1.00) was observed throughout the analyzed reef food web.