RESUMO
Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand through respiration, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil's Hole, Bermuda is a model ecosystem to study OMZ microbial biogeochemistry because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop, with organic matter and nutrients accumulating at depth. In this study, the bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of suboxic waters was assessed using a simulated overturn experiment. The surface inoculated prokaryotic community responded to the deep (formerly suboxic) 0.2 µm filtrate with cell densities increasing 2.5-fold over 6 days while removing 5 µmol L-1 of DOC. After 12 days, the surface community began to shift, and DOC quality became less diagenetically altered along with an increase in SAR202, a Chloroflexi that can degrade recalcitrant dissolved organic matter (DOM). Labile DOC production after 12 days coincided with an increase of Nitrosopumilales, a chemoautotrophic ammonia oxidizing archaea (AOA) that converts ammonia to nitrite based on the ammonia monooxygenase (amoA) gene copy number and nutrient data. In comparison, the inoculation of the deep anaerobic prokaryotic community into surface 0.2 µm filtrate demonstrated a die-off of 25.5% of the initial inoculum community followed by a 1.5-fold increase in cell densities over 6 days. Within 2 days, the prokaryotic community shifted from a Chlorobiales dominated assemblage to a surface-like heterotrophic community devoid of Chlorobiales. The DOM quality changed to less diagenetically altered material and coincided with an increase in the ribulose-1,5-bisphosphate carboxylase/oxygenase form I (cbbL) gene number followed by an influx of labile DOM. Upon reoxygenation, the deep DOM that accumulated under suboxic conditions is bioavailable to surface prokaryotes that utilize the accumulated DOC initially before switching to a community that can both produce labile DOM via chemoautotrophy and degrade the more recalcitrant DOM.
RESUMO
Despite extensive studies of the development and dynamics of hypoxia in coastal oceans, factors controlling the decomposition rates and pathways of labile organic matter (OM) in hypoxic waters are not well understood. Here we investigate peptide decomposition in a stratified water column in the hypoxic region of the northern Gulf of Mexico by conducting on-deck incubation experiments amended with tetrapeptide ala-val-phe-ala (AVFA), a fragment of RuBisCO. Our results show that decomposition efficiency of AVFA was limited by the availability of soluble reactive phosphorus (Pi) in the surface water (<0.3 µM), as it was greatly enhanced after Pi addition to the incubation water. In contrast, peptide decomposition rate in the subsurface water, enriched with Pi (0.4-1.2 µM), was twice as high as that in the surface water, concomitant with the development of fast-growing bacteria during the incubation. Consistent with the Growth Rate Hypothesis, these results indicate that a high level of Pi is crucial in stimulating the growth of bacterial strains with high RNA contents and thus faster OM decomposition in marine environments. This high decomposition potential of OM in subsurface hypoxic waters presents a positive feedback on hypoxia formation in Pi-enriched coastal subsurface waters, as a higher OM decomposition rate leads to rapid consumption of dissolved oxygen (DO).