Sequential bioavailability of sedimentary organic matter to heterotrophic bacteria

Abstract

Aquatic sediments harbour diverse microbial communities that mediate organic matter degradation and influence biogeochemical cycles. The pool of bioavailable carbon continuously changes as a result of abiotic processes and microbial activity. It remains unclear how microbial communities respond to heterogeneous organic matrices and how this ultimately affects heterotrophic respiration. To explore the relationships between the degradation of mixed carbon substrates and microbial activity, we incubated batches of organic-rich sediments in a novel bioreactor (IsoCaRB) that permitted continuous observations of CO production rates, as well as sequential sampling of isotopic signatures (δ C, Δ C), microbial community structure and diversity, and extracellular enzyme activity. Our results indicated that lower molecular weight (MW), labile, phytoplankton-derived compounds were degraded first, followed by petroleum-derived exogenous pollutants, and finally by higher MW polymeric plant material. This shift in utilization coincided with a community succession and increased extracellular enzyme activities. Thus, sequential utilization of different carbon pools induced changes at both the community and cellular level, shifting community composition, enzyme activity, respiration rates, and residual organic matter reactivity. Our results provide novel insight into the accessibility of sedimentary organic matter and demonstrate how bioavailability of natural organic substrates may affect the function and composition of heterotrophic bacterial populations.

Illuminating microbial species‐specific effects on organic matter remineralization in marine sediments

“…The upper 0–10 cm of sediment cores were homogenized and freed of carbonate species by titration to pH 2–2.5 with 10% hydrochloric acid (HCl) in an ice bath (Mahmoudi et al ., ). Sediments were subsequently freeze dried and sterilized by gamma‐irradiation via a 137 Cs (radioactive caesium) source to receive a total dose ~40 kGy.…”

“…For example, Δ 14 C measurements of bacterial lipids extracted from Beggiatoa mats in Guaymas Basin (Gulf of California) showed that the bacterial community was net heterotrophically consuming hydrothermal petroleum‐derived carbon (Pearson et al ., ) rather than autotrophically fixing carbon from dissolved inorganic carbon (DIC) as had been suspected. More recently, δ 13 C and Δ 14 C measurements of CO 2 collected during laboratory incubations of coastal sediment revealed that organic matter was degraded in a sequential, step‐wise manner by heterotrophic bacteria: small, phytoplankton‐derived compounds were degraded first, followed by petroleum‐derived exogenous pollutants, and finally by polymeric plant material (Mahmoudi et al ., ).…”

Microbial Processing of Sediment‐Derived Dissolved Organic Matter: Implications for Its Subsequent Biogeochemical Cycling in Overlying Seawater

“…Approximately 80% of marine sedimentary organic matter is buried in the shelf and slope areas (Hedges & Keil, 1995;Mahmoudi et al, 2017). Recycling of organic material in marine sediments is an important component of biogeochemical cycles because these sediments are recognized to be critical for long-term carbon sequestration (Arndt et al, 2013;Orsi et al, 2018).…”

Illuminating microbial species-specific effects on organic matter remineralization in marine sediments

“…Gas flow is then changed to 100 ml min −1 of 20% 185 Gaseous CO 2 concentration measurements were corrected for baseline drifts and then 194 rescaled to agree with the higher-precision manometric yields obtained from the trapped CO 2 195 (Beaupré et al, 2016). These normalized CO 2 concentrations were corrected for the 196 confounding effects of mixing in the culture vessel headspace and decreasing slurry volume (as 197 described in Mahmoudi et al, 2017) to calculate the rate of CO 2 generation per unit volume of 198 growth medium (µg C l −1 min −1 ), which serves as a proxy for the microbial CO 2 production rate. 199 A total of six incubations were performed: one on-axis and two off-axis samples for each 200 of the two bacterial isolates.…”

“…Beggiatoa mats in Guaymas Basin (Gulf of California), showed that the bacterial community 74 was net heterotrophically consuming hydrothermal petroleum-derived carbon (Pearson et al,75 2005) rather than autotrophically fixing carbon from dissolved inorganic carbon (DIC) as had 76 been suspected. More recently, δ 13 C and Δ 14 C measurements of CO 2 collected during laboratory 77 incubations of coastal sediment revealed that organic matter was degraded in a sequential, step-78 wise manner by heterotrophic bacteria: small, phytoplankton-derived compounds were degraded 79 first, followed by petroleum-derived exogenous pollutants, and finally by polymeric plant 80 material (Mahmoudi et al, 2017). 81…”