Modeling the global cycle of marine dissolved organic matter and its influence on marine productivity

Macroscale vertical power-law distribution of bacteria in dark oceans can emerge from microscale bacteria-particle interactions

Microbes in the dark oceans are a key determinant of remineralization of sinking carbon particles. However, most marine ecosystem models overlook how microbes aggregate on particles and the microscale interactions between particle-associated microbes, making it difficult to obtain mechanistic insights on their vertical power-law decay pattern. Here, we present a spatial population model where the attachment and detachment processes of bacterial cells depend on local density of particle-associated bacteria. We show that the power-law relationship can emerge when the non-random aggregated distribution of bacteria is considered without any depth-specific environmental parameters. Furthermore, the comparison between model behavior and empirical patterns in the Pacific and Southern Ocean indicated that temperature-dependent hydrolysis rate and nutrient-dependent sinking rate of particles are key parameters to explain the regional variations of the power-law exponent. The mechanistic approach developed here provides a pathway to link micro-scale interactions between individuals to macro-scale food chain structures and carbon cycle.

Marine viruses and climate change: Virioplankton, the carbon cycle, and our future ocean

Interactions between marine viruses and microbes are a critical part of the oceanic carbon cycle. The impacts of virus-host interactions range from short-term disruptions in the mobility of microbial biomass carbon to higher trophic levels through cell lysis (i.e., the viral shunt) to long-term reallocation of microbial biomass carbon to the deep sea through accelerating the biological pump (i.e., the viral shuttle). The biogeochemical backdrop of the ocean-the physical, chemical, and biological landscape-influences the likelihood of both virus-host interactions and particle formation, and the fate and flow of carbon. As climate change reshapes the oceanic landscape through large-scale shifts in temperature, circulation, stratification, and acidification, virus-mediated carbon flux is likely to shift in response. Dynamics in the directionality and magnitude of changes in how, where, and when viruses mediate the recycling or storage of microbial biomass carbon is largely unknown. Integrating viral infection dynamics data obtained from experimental models and field systems, with particle motion microphysics and global observations of oceanic biogeochemistry, into improved ecosystem models will enable viral oceanographers to better predict the role of viruses in marine carbon cycling in the future ocean.

Eco-evolutionary responses of the microbial loop to surface ocean warming and consequences for primary production

Predicting the response of ocean primary production to climate warming is a major challenge. One key control of primary production is the microbial loop driven by heterotrophic bacteria, yet how warming alters the microbial loop and its function is poorly understood. Here we develop an eco-evolutionary model to predict the physiological response and adaptation through selection of bacterial populations in the microbial loop and how this will impact ecosystem function such as primary production. We find that the ecophysiological response of primary production to warming is driven by a decrease in regenerated production which depends on nutrient availability. In nutrient-poor environments, the loss of regenerated production to warming is due to decreasing microbial loop activity. However, this ecophysiological response can be opposed or even reversed by bacterial adaptation through selection, especially in cold environments: heterotrophic bacteria with lower bacterial growth efficiency are selected, which strengthens the "link" behavior of the microbial loop, increasing both new and regenerated production. In cold and rich environments such as the Arctic Ocean, the effect of bacterial adaptation on primary production exceeds the ecophysiological response. Accounting for bacterial adaptation through selection is thus critically needed to improve models and projections of the ocean primary production in a warming world.

Molecular characterization of dissolved organic nitrogen during anoxic/oxic and anammox processes using ESI FT-ICR MS

Dissolved organic nitrogen (DON) is a component of wastewater with a negative influence on the environment. The removal of DON is conducted through the anoxic/oxic (A/O) and anammox processes. However, the mechanisms and chemical preferences in the removal of DON compounds have not been understood and compared so far. This study, for the first time, comparatively investigated the molecular-level characteristics of DON during both processes by using FT-ICR MS (Fourier transform ion cyclotron resonance mass spectrometry). The results indicated that the number of DON formulas increased from 1844 to 1935 during A/O process, and from 2784 to 3242 during anammox process, highlighting the increase in complexity of DON after undergoing both processes. DON with high saturation and aliphatic structures was removed by A/O process, whereas highly unsaturated and aromatic structures were removed by anammox process. For DON without S atom, Lignin-like and tannin-like ones were resistant to both processes and protein-like and condensed aromatic structures were resistant to anammox process. The complementarity of these two processes provided a sequential combination with sufficient theoretical support to improve DON removal efficiency. PRACTITIONER POINTS: Molecular components of dissolved organic nitrogen characterized by ESI FT-ICR MS. DON removal preferences of A/O and anammox processes evaluated. A/O and anammox processes are effective to remove aliphatic and aromatic DON, respectively. Complementarity in removal preferences of A/O and anammox processes can remove recalcitrant DON of each other. Sequential A/O and anammox processes can improve DON removal.

Long-term stability of marine dissolved organic carbon emerges from a neutral network of compounds and microbes

Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean. The reason behind this recalcitrance is unknown. We test whether the long-term stability of DOC requires the existence of structurally refractory molecules, using a mechanistic model comprising a diverse network of microbe-substrate interactions. Model experiments reproduce three salient observations, even when all DOC compounds are equally degradable: (i) >15% of an initial DOC pulse resists degradation, but is consumed by microbes if concentrated, (ii) the modelled deep-sea DOC reaches stable concentrations of 30–40 mmolC/m³, and (iii) the mean age of deep-sea DOC is several times the age of deep water with a wide range from <100 to >10,000 years. We conclude that while structurally-recalcitrant molecules exist, they are not required in the model to explain either the amount or longevity of DOC.