Mercury and flooding cycles in the Tapajós River basin, Brazilian Amazon: the role of periphyton of a floating macrophyte (Paspalum repens).

Methylmercury (MeHg) increases mercury (Hg) toxicity and is biomagnified in the trophic chain contaminating riverine Amazon populations. Freshwater macrophyte roots are a main site of Hg methylation in different Brazilian environments. Paspalum repens periphyton was sampled in four floodplain lakes during the dry, rainy and wet seasons for measurement of total Hg (THg), MeHg, Hg methylation potentials, %C, %N, δ(13)C, δ(15)N and bacterial heterotrophic production as (3)H-leucine incorporation rate. THg concentration varied from 67 to 198 ng/g and the potential of Me(203)Hg formation was expressive (1-23%) showing that periphyton is an important matrix both in the accumulation of Hg and in MeHg production. The concentration of MeHg varied from 1 to 6 ng/g DW and was positively correlated with Me(203)Hg formation. Though methylmercury formation is mainly a bacterial process, no significant correlation was observed between the methylation potentials and bacterial production. The multiple regressions analyses suggested a negative correlation between THg and %C and %N and between methylation potential and δ(13)C. The discriminant analysis showed a significant difference in periphyton δ(15)N, δ(13)C and THg between seasons, where the rainy season presented higher δ(15)N and the wet period lighter δ(13)C, lower THg values and higher Me(203)Hg formation. This exploratory study indicates that the flooding cycle could influence the periphyton composition, mercury accumulation and methylmercury production.

Mercury methylation and bacterial activity associated to tropical phytoplankton.

The methylated form of mercury (Hg), methylmercury (MeHg), is one of the most toxic pollutants. Biotic and/or abiotic methylation, often associated to sulfate-reducing bacteria metabolism, occurs in aquatic environments and in many tropical areas, mostly in the periphyton associated to floating macrophyte roots. Data about mercury methylation by phytoplankton are scarce and the aim of this study was to verify the biotic influence in the methylation process in Microcystis aeruginosa and Sineccocystis sp. laboratory strains and in natural populations of phytoplankton from two different aquatic systems, the mesotrophic Ribeirão das Lajes reservoir and hypereutrophic oligohaline Jacarepaguá lagoon, Rio de Janeiro state, Brazil. Adapted radiochemical techniques were used to measure sulfate-reduction, mercury methylation and bacterial activity in phytoplankton samples. Methyl-(203)Hg formation from added inorganic (203)Hg and (3)H-Leucine uptake were measured by liquid scintillation as well as sulfate-reduction, estimated as H(2)(35)S produced from added Na(2)(35)SO(4). There was no significant difference in low methylation potentials (0.37%) among the two cyanobacterium species studied in laboratory conditions. At Ribeirão das Lajes reservoir, there was no significant difference in methylation, bacterial activity and sulfate-reduction of surface sediment between the sampling points. Methylation in sediments (3-4%) was higher than in phytoplankton (1.5%), the opposite being true for bacterial activity (sediment mean 6.6 against 150.3 nmol gdw(-1) h(-1) for phytoplankton samples). At Jacarepaguá lagoon, an expressive bacterial activity (477.1 x 10(3) nmol gdw(-1) h(-1) at a concentration of 1000 nM leucine) and sulfate-reduction ( approximately 21% H(2)(35)S trapped) associated to phytoplankton (mostly cyanobacteria M. aeruginosa) was observed, but mercury methylation was not detected.