On the effect of low oxygen concentrations on bacterial degradation of sinking particles

In marine oxygen (O2) minimum zones (OMZs), the transfer of particulate organic carbon (POC) to depth via the biological carbon pump might be enhanced as a result of slower remineralisation under lower dissolved O2 concentrations (DO). In parallel, nitrogen (N) loss to the atmosphere through microbial processes, such as denitrification and anammox, is directly linked to particulate nitrogen (PN) export. However it is unclear (1) whether DO is the only factor that potentially enhances POC transfer in OMZs, and (2) if particle fluxes are sufficient to support observed N loss rates. We performed a degradation experiment on sinking particles collected from the Baltic Sea, where anoxic zones are observed. Sinking material was harvested using surface-tethered sediment traps and subsequently incubated in darkness at different DO levels, including severe suboxia (<0.5 mg l−1 DO). Our results show that DO plays a role in regulating POC and PN degradation rates. POC(PN) degradation was reduced by approximately 100% from the high to low DO to the lowest DO. The amount of NH4+ produced from the pool of remineralising organic N matched estimations of NH4+ anammox requirements during our experiment. This anammox was likely fueled by DON degradation rather than PON degradation

Composition and Vertical Flux of Particulate Organic Matter to the Oxygen Minimum Zone of the Central Baltic Sea: Impact of a sporadic North Sea inflow

Particle sinking is a major form of transport for photosynthetically fixed carbon to below the euphotic zone via the biological carbon pump (BCP). Oxygen (O2) depletion may improve the efficiency of the BCP. However, the mechanisms by which O2 deficiency can enhance particulate organic matter (POM) vertical fluxes are not well understood. Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two basins showed different O2 regimes resulting from the intrusion of oxygen-rich water from the North Sea that ventilated the water column below 140 m in GB, but not in LD, during the time of sampling. In June 2015, we deployed surface-tethered drifting sediment traps in oxic surface waters (GB: 40 and 60 m; LD: 40 and 55 m), within the oxygen minimum zone (OMZ; GB: 110 m and LD: 110 and 180 m) and at recently oxygenated waters by the North Sea inflow in GB (180 m). The primary objective of this study was to test the hypothesis that the different O2 conditions in the water column of GB and LD affected the composition and vertical flux of sinking particles and caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and LD. In GB, particulate organic carbon (POC) flux was 18 % lower in the shallowest trap (40 m) than in the deepest sediment trap (at 180 m). Particulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes decreased with depth, while particulate organic phosphorus (POP), biogenic silicate (BSi), chlorophyll a (Chl a) and transparent exopolymeric particle (TEP) fluxes peaked within the core of the OMZ (110 m); this coincided with the presence of manganese oxide-like (MnOx-like) particles aggregated with organic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28 %, 42 % and 56 %, respectively, from the surface to deep waters. POP, BSi and TEP fluxes did not decrease continuously with depth, but they were higher at 110 m. Although we observe a higher vertical flux of POP, BSi and TEPs coinciding with abundant MnOx-like particles at 110 m in both basins, the peak in the vertical flux of POM and MnOx-like particles was much higher in GB than in LD. Sinking particles were remarkably enriched in BSi, indicating that diatoms were preferentially included in sinking aggregates and/or there was an inclusion of lithogenic Si (scavenged into sinking particles) in our analysis. During this study, the POC transfer efficiency (POC flux at 180 m over 40 m) was higher in GB (115 %) than in LD (69 %), suggesting that under anoxic conditions a smaller portion of the POC exported below the euphotic zone was transferred to 180 m than under reoxygenated conditions present in GB. In addition, the vertical fluxes of MnOx-like particles were 2 orders of magnitude higher in GB than LD. Our results suggest that POM aggregates with MnOx-like particles formed after the inflow of oxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may alter the composition and vertical flux of POM, potentially contributing to a higher transfer efficiency of POC in GB. This idea is consistent with observations of fresher and less degraded organic matter in deep waters of GB than LD

Composition and vertical flux of particulate organic matter to the oxygen minimum zone of the central Baltic Sea: impact of a sporadic North Sea inflow

Particle sinking is a major form of transport for photosynthetically fixed carbon to below the euphotic zone via the biological carbon pump (BCP). Oxygen (O2) depletion may improve the efficiency of the BCP. However, the mechanisms by which O2 deficiency can enhance particulate organic matter (POM) vertical fluxes are not well understood. Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two basins showed different O2 regimes resulting from the intrusion of oxygen-rich water from the North Sea that ventilated the water column below 140&thinsp;m in GB, but not in LD, during the time of sampling. In June 2015, we deployed surface-tethered drifting sediment traps in oxic surface waters (GB: 40 and 60&thinsp;m; LD: 40 and 55&thinsp;m), within the oxygen minimum zone (OMZ; GB: 110&thinsp;m and LD: 110 and 180&thinsp;m) and at recently oxygenated waters by the North Sea inflow in GB (180&thinsp;m). The primary objective of this study was to test the hypothesis that the different O2 conditions in the water column of GB and LD affected the composition and vertical flux of sinking particles and caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and LD. In GB, particulate organic carbon (POC) flux was 18&thinsp;% lower in the shallowest trap (40&thinsp;m) than in the deepest sediment trap (at 180&thinsp;m). Particulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes decreased with depth, while particulate organic phosphorus (POP), biogenic silicate (BSi), chlorophyll a (Chl a) and transparent exopolymeric particle (TEP) fluxes peaked within the core of the OMZ (110&thinsp;m); this coincided with the presence of manganese oxide-like (MnOx-like) particles aggregated with organic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28&thinsp;%, 42&thinsp;% and 56&thinsp;%, respectively, from the surface to deep waters. POP, BSi and TEP fluxes did not decrease continuously with depth, but they were higher at 110&thinsp;m. Although we observe a higher vertical flux of POP, BSi and TEPs coinciding with abundant MnOx-like particles at 110&thinsp;m in both basins, the peak in the vertical flux of POM and MnOx-like particles was much higher in GB than in LD. Sinking particles were remarkably enriched in BSi, indicating that diatoms were preferentially included in sinking aggregates and/or there was an inclusion of lithogenic Si (scavenged into sinking particles) in our analysis. During this study, the POC transfer efficiency (POC flux at 180&thinsp;m over 40&thinsp;m) was higher in GB (115&thinsp;%) than in LD (69&thinsp;%), suggesting that under anoxic conditions a smaller portion of the POC exported below the euphotic zone was transferred to 180&thinsp;m than under reoxygenated conditions present in GB. In addition, the vertical fluxes of MnOx-like particles were 2 orders of magnitude higher in GB than LD. Our results suggest that POM aggregates with MnOx-like particles formed after the inflow of oxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may alter the composition and vertical flux of POM, potentially contributing to a higher transfer efficiency of POC in GB. This idea is consistent with observations of fresher and less degraded organic matter in deep waters of GB than LD.</p

Potential role of oxygen and inorganic nutrients on microbial carbon turnover in the Baltic Sea

Oxygen (O2) deficiency and nutrient concentrations in marine systems are impacting organisms from microbes to higher trophic levels. In coastal and enclosed seas, O2 deficiency is often related to eutrophication and high degradation rates of organic matter. To investigate the impact of O2 concentration on bacterial growth and the turnover of organic matter, we conducted multifactorial batch experiments with natural microbial communities of the central Baltic Sea. Water was collected from suboxic (<5 µmol L -1) depths in the Gotland Basin during June 2015. Samples were kept for four days under fully oxygenated and low O2 conditions (mean: 34 µmol L-1 O2), with or without nutrient (ammonium, phosphate, nitrate) and labile carbon (glucose) amendments. We measured bacterial abundance, bacterial heterotrophic production, extracellular enzyme rates (leucine-aminopeptidase) and changes in dissolved and particulate organic carbon concentrations. Our results show that the bacterial turnover of organic matter was limited by nutrients under both oxic and low O2 conditions. In nutrient and glucose replete treatments, low O2 concentrations significantly reduced the net uptake of dissolved organic carbon and lead to higher accumulation of more labile dissolved organic matter. Our results therewith suggest that the combined effects of eutrophication and deoxygenation on heterotrophic bacterial activity may potentially favor the accumulation of dissolved organic carbon in the Baltic Sea

Organic matter fluxes and biogeochemical processes in the OMZ off Peru, Cruise No. M138, 01 June - 03 July 2017, Callao (Peru) - Bahia Las Minas (Panama)

The oxygen minimum zone (OMZ) in the eastern tropical South Pacific Ocean is tightly connected to the coastal upwelling system off Peru. The high biological productivity off Peru is therefore, driven by the complex interplay between the amount of nutrients recycled by remineralisation processes in the OMZ and the upwelling which brings these nutrients to the surface layer. However, surprisingly little is known about organic matter cycling and its effects on biogeochemical processes in the OMZ off Peru. To this end we conducted a first comprehensive study on the role of organic matter for the biogeochemical processes and the maintenance of the OMZ off Peru. M138 combined measurements of marine biogeochemistry, microbiology, physical oceanography and air chemistry with foci on (i) the efficiency of the biological pump, (ii) the nitrogen cycle processes in the OMZ, (iii) the ventilation of the OMZ as well as (iv) the air/sea gas exchange across the ocean/atmosphere interface and (v) aerosol deposition. The METEOR cruise M138 was performed as part of the third phase of the SFB754 'Climate-Biogeochemistry Interactions in the Tropical Ocean' (www.sfb754.de)

Transparent exopolymer particles (TEP) and coomassie stainable particles (CSP): comparing their origin and behavior using flowcam visualization and imaging

The discovery of transparent gel-like particles, such as polysaccharide-containing TEP and protein-containing CSP has changed our conception of particle-organism interaction and raised questions about the origin, composition, and role of gel particles in aquatic systems. Using standard staining methods and a new FlowCAM 3D visualization and imaging technique, we studied whether TEP and CSP are sub-units of the same particle, are distinct particles with different characteristics and behaviors, or both. In a mesocosm, phytoplankton produced both TEP and CSP; however, maximum abundances occured at different phases in the bloom. In the Sargasso Sea, depth profiles of TEP and CSP were different; both were most abundant in the upper 100 m, but CSP followed fluorescence and total particle concentration while TEP decreased with depth. Visual examination using FlowCAM of stained and unstained TEP and CSP from seawater, freshwater, and algal cultures, showed that TEP and CSP have different shapes, sizes and particle-association behaviors. We conclude that TEP and CSP are distinct particles, produced by different species at different growth phases, and their significance to aggregation changes depending on the community composition

Composition and vertical flux of particulate organic matter to the oxygen minimum zone of the central Baltic Sea

Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep)

No detectable effect of CO2 on elemental stoichiometry of Emiliania huxleyi in nutrient-limited, acclimated continuous cultures

Effects of CO2 concentration on elemental composition of the coccolithophore Emiliania huxleyi were studied in phosphorus-limited, continuous cultures that were acclimated to experimental conditions for 30 d prior to the first sampling. We determined phytoplankton and bacterial cell numbers, nutrients, particulate components like organic carbon (POC), inorganic carbon (PIC), nitrogen (PN), organic phosphorus (POP), transparent exopolymer particles (TEP), as well as dissolved organic carbon (DOC) and nitrogen (DON), in addition to carbonate system parameters at CO2 levels of 180, 380 and 750 µatm. No significant difference between treatments was observed for any of the measured variables during repeated sampling over a 14 d period. We considered several factors that might lead to these results, i.e. light, nutrients, carbon overconsumption and transient versus steady-state growth. We suggest that the absence of a clear CO2 effect during this study does not necessarily imply the absence of an effect in nature. Instead, the sensitivity of the cell towards environmental stressors such as CO2 may vary depending on whether growth conditions are transient or sufficiently stable to allow for optimal allocation of energy and resources. We tested this idea on previously published data sets where PIC and POC divided by the corresponding cell abundance of E. huxleyi at various pCO2 levels and growth rates were available

Hypoxia-tolerant zooplankton may reduce biological carbon pump efficiency in the Humboldt current system off Peru

Abstract In the ocean, downward flux of particles produced in sunlit surface waters is the major component of the biological carbon pump, which sequesters atmospheric carbon dioxide and fuels deep-sea ecosystems. The efficiency of downward carbon transfer is expected to be particularly high in tropical upwelling systems where hypoxia occurring beneath the productive surface waters is thought to hamper particle consumption. However, observations of both particle feeders and carbon export in low-oxygen waters are scarce. Here, we provide evidence that hypoxia-tolerant zooplankton feed on sinking particles in the extensive Oxygen Minimum Zone (OMZ) off Peru. Using several arrays of drifting sediment traps and in situ imaging, we show geochemical and morphological transformations of sinking particles and substantial control of carbon export by zooplankton. Our findings challenge the assumption of a consistently efficient biological carbon pump in OMZs and further demonstrate the need to consider mesopelagic organisms when studying oceanic carbon sequestration

Hypoxia-tolerant zooplankton may reduce biological carbon pump efficiency in the Humboldt current system off Peru

In the ocean, downward flux of particles produced in sunlit surface waters is the major component of the biological carbon pump, which sequesters atmospheric carbon dioxide and fuels deep-sea ecosystems. The efficiency of downward carbon transfer is expected to be particularly high in tropical upwelling systems where hypoxia occurring beneath the productive surface waters is thought to hamper particle consumption. However, observations of both particle feeders and carbon export in low-oxygen waters are scarce. Here, we provide evidence that hypoxia-tolerant zooplankton feed on sinking particles in the extensive Oxygen Minimum Zone (OMZ) off Peru. Using several arrays of drifting sediment traps and in situ imaging, we show geochemical and morphological transformations of sinking particles and substantial control of carbon export by zooplankton. Our findings challenge the assumption of a consistently efficient biological carbon pump in OMZs and further demonstrate the need to consider mesopelagic organisms when studying oceanic carbon sequestration