Print PageThe Laguna Madre, South Texas/ North-Eastern Mexico is one of just five hypersaline estuaries in the World and the only one of its kind on the North American continent. The estuary harbors extensive seagrass beds which serve as habitat for a high diversity of animal species. Several anthropogenic chemicals and in particular, Arsenic has been introduced to the Laguna Madre via agricultural runoff, illegal disposal and other inputs. Our goals were to 1) Survey the spatial distribution and concentration of total arsenic in water, sediment, drift algae, seagrass, and consumers in Lower Laguna Madre, 2) Examine interactions between microorganisms and arsenic, especially Arsenic-oxidizing bacteria, and 3) Investigate possible bioaccumulation of Arsenic among trophic levels in the food web and links in the transfer of Arsenic between aquatic and terrestrial ecosystems. Samples of sediment, water, seagrass, and penaeid shrimp were obtained from eight locations in the Lower Laguna Madre. In addition, oysters, crabs and two species of fish were collected on a separate occasion. Samples were assayed via Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). In addition, sediment samples were cultured on synthetic media with 100 μM arsenite. Densities of arsenite-tolerant bacteria at each site were determined and bacterial isolates were gram-stained, tested for the enzyme catalase and profiled using API 20E® test strips. Bacterial isolates were also tested for arsenite tolerance at different concentrations.
Results/Conclusions
The concentration of total Arsenic showed a pronounced spatial variation in the Lower Laguna Madre. However, in general, the Arsenic concentration in water, sediment, drift algae, and seagrass was relatively low (2-7 ppm), higher in some of the consumers (penaeid shrimp and gastropods 4-12 ppm), and highest in oysters (17-22 ppm). Surprisingly, primary consumers harbored much higher concentrations than secondary and tertiary consumers (< 1 ppm) indicating that the latter effectively excretes rather than store Arsenic. A high density of arsenite tolerant bacteria (8,250 to 2,455,000 cfu g-1 of wet sediment) and Streptomyces were identified and cultured on up to 1000 μM As(III) suggesting that those organisms may be essential in the immobilization of arsenic in Laguna Madre. All isolates displayed API® phenotypes that were >95% similar, and most (43/49) had identical phenotypes suggesting that they are the same organism. Adult tiger beetles occupying the shoreline contained Arsenic in various body parts and may represent a link in the transfer of Arsenic from aquatic to terrestrial ecosystems.
