What happens to biomass burning-emitted particles in the Ocean? A laboratory experimental approach based on their tracers

Biomass burning is a major contributor to the emission of particle matter in the atmosphere (up to 90 % of primary organic aerosol) and thus has an impact on climate, human health, and ecosystems. Once emitted, biomass burning-derived organic aerosol can be transported to the oceans. However, the impact and fate of this particulate matter and its major components in the marine biological carbon pump and the trophic chain are largely unknown. Understanding these processes is of paramount importance to better asses the carbon cycle. This work presents the first attempt to investigate the bioavailability of two biomass-burning tracers (i.e., levoglucosan and galactosan) in seawater inoculated with marine microorganisms. To do so, a novel method was developed to extract the anhydrosugar from their salty matrix and monitor their evolution for 44 days under controlled conditions. Along with the anhydrosugar concentration, multiple parameters (dissolved organic carbon, inorganic nutrient concentrations, prokaryotic production, heterotrophic prokaryotes abundance, and prokaryotic diversity) were monitored to achieve a complete picture of their fate in seawater. Furthermore, two control experiments (glucose- and non-amended) were run in parallel for comparison purposes. The results show that both levoglucosan and galactosan can be biodegraded at slow rates as their concentrations dropped from 2.5 ± 0.6 and 2.4 ± 0.3 μM to 0.1 ± 0.1 and 1.5 ± 0.7 μM, respectively, over a period of 44 days. The decrease in the levoglucosan and galactosan concentrations was accompanied by an increase in both prokaryotic production (up to 40 and 5 times greater, respectively, when compared to the non-amended experiment) and heterotrophic prokaryotes abundance (for levoglucosan, up to one order of magnitude greater than the non-amended experiment). While glucose was assimilated by heterotrophic prokaryotes within 1.7 days, levoglucosan and galactosan biodegradation did not start until at least 8.7 days after the experiments were set. These delays suggest that the chemical structure of these anhydrosugars can only be tackled by specific enzymatic abilities regulated by slow-growing heterotrophic prokaryotes. Prokaryotic diversity analyses revealed the predominance of a few bacterial genera from the Roseobacter clade that were specifically selected by the addition of the anhydrosugars. These results raise questions about the enzymatic capabilities of this widespread marine bacterial clade and the processing of semi-labile compounds accumulating in surface ocean waters. This work shows that biomass-burning organic compounds have the potential to be biodegraded by prokaryotic bacteria and thus contribute to the trophic chain and the production of CO₂.