Seaweeds Along KwaZulu-Natal Coast of South Africa—4: Elemental Uptake by Edible Seaweed Caulerpa racemosa (Sea grapes) and the Arsenic Speciation

Abstract

The elemental uptake by edible seaweed Caulerpa racemosa (Sea grapes), a marine macroalgae (chlorophyta, green alga) grown richly along KwaZulu-Natal coastline. The total concentrations of seven elements, namely Mn, Fe, As, B, Ti, Zn and Hg in Caulerpa racemosa were monitored for a one-year cycle (June 2002 to May 2003) at four selected sampling sites spread over 150 km wide from North to South. The C. racemosa possess high arsenic, boron and titanium accumulating ability, but low iron uptake. A typical C. racemosa sample at Treasure Beach in the vicinity of Durban Metropolis in autumn contained Mn (5.2 ± ppm), Fe (0.21 ± 0.01 ppm), As (8.5 ± 0. ppm), B (1090 ± ppb), Ti (159 ± ppb), Zn (3.8 ± 0.1 ppb), and Hg (189 ± ppb). The general trend found at all sites was high elemental concentrations in winter and a decrease in concentrations from winter to spring and summer. C. racemosa recorded highest mercury levels (>205 ppb) during the summer season at the Zinkwasi site. The arsenic speciation in four abundant seaweeds from the beaches of Indian Ocean in the KwaZulu-Natal coast is investigated. The speciation of arsenic in two rhodophyta seaweeds, Plocamium corallorhiza and Gelidium abbottiorum, and two chlorophyta seaweeds, Ulva lactuca and Caulerpa racemosa at four sampling sites during the summer of 2003, is elucidated. Caulerpa racemosa had highest total arsenic (in ppb) reaching (8850 ± 200) at Zinkwasi, and the concentrations of the other arsenic species analysed are As (III) (194 ± 10), As(V) (568 ± 27), methylarsonic acid (494 ± 22) and dimethylarsinic acid (373 ± 12). In an indirect estimation, C. racemosa had 81 ± 2% aresenosugars.