Marine microbial community response to inorganic and organic sediment amendments in laboratory mesocosms

Sediment amendments provide promising strategies of enhancing sequestration of heavy metals and degradation of organic contaminants. The impacts of sediment amendments for metal and organic remediation including apatite, organoclay (and apatite and organoclay in geotextile mats), acetate, and chitin on environmental microbial communities in overlying water and sediment profiles are reported here. These experiments were performed concurrent with an ecotoxicity evaluation (data submitted in companion paper) and X-ray absorption spectroscopy of zinc speciation post apatite amendments. X-ray absorption spectra showed that a modest modification of zinc speciation occurred in amended treatments. Significant changes in both bacterial cell densities and populations were observed in response to amendments of apatite+organoclay, chitin, and acetate. The enriched bacteria and breakdown of these amendments were likely attributed to water quality degradation (e.g. ammonia and dissolved oxygen). Molecular fingerprints of bacterial communities by denaturant gradient gel electrophoresis (DGGE) showed that distinct bacterial populations occurred in overlying waters from different amendments: apatite+organoclay led to the dominance of Gammaproteobacteria, acetate enriched Alphaproteobacteria, and chitin treatment led to a dominance of Bacteroidetes and Alphaproteobacteria. In amended sediments, Firmicutes, Bacteroidetes, and Deltaproteobacteria (Desulfovibrio) were commonly found with chitin and apatite+chitin treatments. Finally, sulfate-reducing bacteria (e.g. Desulfovibrio) and metal-reducing bacteria were also recovered with most probable number (MPN) analyses in treatments with acetate, chitin, and apatite+chitin. These geochemically important bacteria were stimulated by amendments and may play critical functional roles in the metal and organic contaminant remediation process for future investigations of contaminated sediments.

Research highlights
► Organic amendments impacted both microbial density and microbial population structure.
► Sediment amendments enriched distinct bacterial populations in overlying waters and sediments.
► Modest modifications of metal speciation occurred during amendment treatments.
► Sulfate and metal-reducing bacteria were recovered and may play critical roles under sediment amendment conditions.
► Chitin provides a potential for practical application in sediment amendments to enhance bioremediation processes.