Lithogenic material such as Saharan dust can be incorporated into organic aggregates and act as ballast, potentially enhancing the marine carbon export via increased sinking velocities of aggregates. We studied the ballasting effects of Saharan dust on the aggregate dynamics in the upwelling region off Cape Blanc (Mauritania). Aggregate formation from a natural plankton community exposed to Saharan dust deposition resulted in higher abundance of aggregates with higher sinking velocities compared to aggregate formation with low dust. This higher aggregate abundance and sinking velocities potentially increased the carbon export 10‐fold when the aggregates were ballasted by Saharan dust. After aggregate formation in the surface waters, subsequent sinking through suspended Saharan dust minerals had no influence on aggregate sizes, abundance, and sinking velocities. We found that aggregates formed in the surface ocean off Mauritania were already heavily ballasted with lithogenic material and could therefore not scavenge any additional minerals during their descent. This suggests that carbon export to the deep ocean in regions with high dust deposition is strongly controlled by dust input to the surface ocean while suspended dust particles in deeper water layers do not significantly interact with sinking aggregates.

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In the presence of bromide, ozonation as applied in water treatment results in the formation of bromate, an ion with carcinogenic properties. The reduction of bromate by mixed bacterial populations as well as pure cultures was studied under laboratory conditions. Bromate was reduced to bromide by a mixed bacterial population with and without a preceding nitrate reduction step in an anaerobically incubated medium with ethanol as the energy and carbon source at 20 and 25؇C. The predominating bacteria isolated from the batches showing bromate reduction were identified as Pseudomonas spp. Strains of Pseudomonas fluorescens reduced BrO 3 ؊ to Br ؊ but at a much lower rate than the mixed bacterial population did. Nitrate is a preferred electron acceptor for the bromate-reducing bacteria. Bromate reduction did not occur in the presence of NO 3 ؊ , and the rate of bromate reduction was at least 100 times lower than the rate of nitrate reduction. Bromate was completely converted to Br ؊ , indicating that intermediates, e.g., BrO 2 ؊ , did not accumulate during bromate reduction. on July 6, 2020 by guest http://aem.asm.org/ Downloaded from 240 HIJNEN ET AL. APPL. ENVIRON. MICROBIOL. on July 6, 2020 by guest http://aem.asm.org/ Downloaded from a This group included isolates Br1, Br4, Br5, Br14, Br16, Br22, and Br23. b This group included isolates Br15, Br17, Br18, Br19, Br21, Br24, and Br25. c This group included isolates Br2, Br3, Br6 to Br13, and Br20.