Black carbon structuring marine microbial activities and interactions: a micro- to macro-scale interrogation

Black carbon (BC) consists of partially combusted organic matter deriving from biomass and fuels burning. According to the IPCC's reports, BC emissions are the second-largest contributor to global warming after CO₂. BC enters the marine system via dry deposition or river run-off. Once in the sea, BC has the potential to affect nutrient biogeochemical cycles. In a series of four incubation experiments (Adriatic Sea and Ligurian Sea) and a pilot study, we have challenged the microbes with heavy loads of BC (24 mg L^-1) in order to study the short-term BC effect on microbial dynamics and activities. Upon BC amendment, heterotrophic prokaryotes increased in abundance while viruses decreased. At the microscale, microbes became attached to BC particles, very heterogeneous in shape and size and enriched in proteins over time; these findings were confirmed by Fourier transform-IR spectroscopy and atomic force microscopy. Enzymatic degradative activities, proteases, and alkaline phosphatases were suppressed in the BC treatments despite an enhancement in prokaryotic carbon production. The 16S rRNA gene amplicon sequencing analysis did not show a significant shift in the microbial communities. Despite this, indicator species analysis revealed that Arcobacter and Pseudoalteromonas genera were statistically associated with the BC treatment at 48 h, thus suggesting their adaptive strategies to utilize BC. Our findings reveal that BC has the potential to stimulate intense carbon flow through microbial activity in the sea. Future studies should take account of the contribution of anthropogenic carbon, BC, into the marine biogeochemical C cycle.