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In three microcosm experiments, we exposed microbial communities of a natural sediment to environmentally relevant concentrations of the fungicide captan, the herbicide isoproturon, and the insecticides deltamethrin and pirimicarb. Exposure concentrations were estimated negligible concentrations (NCs), maximum permissible concentrations (MPCs), and 100 times MPC (100MPC). Experimental endpoints were microbial community respiration and biomass, bacterial activity, and denitrification. All four pesticides inhibited bacterial activity by 20 to 24% at MPC, which corresponded to concentrations in the range of µg/kg dry-weight sediment. Treatments with deltamethrin and isoproturon showed inhibiting effects on bacterial activity at NC exposures. Surprisingly, for captan, deltamethrin, and isoproturon, this inhibiting effect was not observed at 100MPC treatments. Microbial biomass was negatively effected in MPC treatments with deltamethrin and in NC treatments with isoproturon. The tested pesticides did not affect community respiration and denitrification rates. These results show that exposure to the tested pesticides may induce toxic responses in sediment microbial communities at concentrations that are predicted to be environmentally safe. [ABSTRACT FROM AUTHOR]

Under estuarine conditions, hydrophobic pesticides undergo physical/chemical partitioning into dissolved and particulate fractions of sediments. Our experimental study using 14C-chlorpyrifos (14C-CHPY) showed that sorptive associations of this common insecticide with particulate (>0.2 µm) and dissolved fractions (<0.2 µm) of sediments strongly influenced uptake, tissue absorption, and elimination in the estuarine bivalve Mercenaria mercenaria. Bivalves were fed algae mixed with particulate and dissolved fractions of 14C-CHPY-contaminated estuarine sediment particles, components of estuarine sediments (silica, humic acids), and a naturally occurring food source (phytoplankton) to quantify relative uptake, tissue absorption efficiencies, and elimination of 14CCHPY. Measurements of 14C-CHPY tissue absorption efficiencies (AE%) indicated that particle source qualities influenced much of the insecticide bioavailability. Mean tissue AE% of 14C-CHPY associated with different particulate fractions ranged from 23 to 31%, while uptake from the dissolved/colloidal forms resulted in low (7-17%) AE%. Our data indicated that 14C-CHPY is more likely to bioaccumulate through ingestion of contaminated particulate material than through filtration/ingestion of dissolved/colloidal material. The high selectivity and digestibility of algae particles by M. mercenaria, a potentially ingested food source, may play an important role in the bioavailability of chlorpyrifos in this animal. [ABSTRACT FROM AUTHOR]

The development and activity of microbiota in artificial sediment may have strong implications for the fate of test compounds and the outcome of toxicity tests. In this study, we compare a number of microbial variables in the artificial sediment commonly used in toxicity testing with that of natural sediments. Bacterial abundance of artificial sediment ranged 5.7 to 7.1 x 108 cells/g wet weight, which is about two orders of magnitude lower than values commonly reported for natural sediments. Similarly, alternative estimates of microbial biomass (sum of phospholipid fatty acid, ergosterol, adenosine triphosphate) were several times lower for artificial sediment compared with natural sediment. Bacterial activity (³H-thymidine incorporation) ranged 4.0 to 7.4 pmol g-1 h-1 (or 0.062-0.113 µg C g-1 h-1) in artificial sediments, which is low compared with values commonly reported for freshwater sediments. Community respiration in artificial sediment was 34 to 93 µg CO2 g-1 d-1. Bacterial community composition assessed by terminal restriction fragment length polymorphism of polymerase chain reaction amplified 16S rRNA genes demonstrated that natural bacterial communities appear to be more diverse than their counterparts in artificial sediment. The average similarity of the microbial communities obtained by this method was less than 40%, and different operational taxonomic units appeared to dominate the artificial and natural sediment, respectively. These results and supporting data from previous studies in natural sediments suggest that the artificial sediment has a poorly developed microbial component that differs substantially from that in natural sediments. [ABSTRACT FROM AUTHOR]