Activity and control of methanol metabolism in the seawaters across the estuary–coast–shelf continuum

Methanol metabolism is important in marine carbon cycling, while the control of metabolic mode and activity remains poorly constrained, particularly in coastal waters. Here, we investigated the biogeochemical cycling of methanol across the estuary–coast–shelf continuum of the East China Sea. Methanol concentrations varied from < 12.3 to 617.3 nmol L−1, decreasing from coast to shelf. The total uptake rates of methanol in the estuary (143.4 ± 29.6 nmol L−1 d−1) and coast (92.3 ± 86.9 nmol L−1 d−1) were much higher than those in the shelf (20.4 ± 33.8 nmol L−1 d−1). While methanol was primarily used as an energy source in the coast and shelf (oxidation/total uptake: > 72.2%), much more methanol was channeled to the biomass (assimilation/total uptake: 31.9–49.7%) in the estuarine waters. As supported by incubation experiments with the addition of nutrients and other C1 compounds, enhanced methanol assimilation suggests the use of methanol as a supplementary source of carbon, while reduced methanol oxidation indicated the competitive utilization of other C1 compounds as an energy source. The estuary and coastal waters acted as net sources of methanol to the atmosphere, while ventilation accounted for < 5.7% of total loss, suggesting that microbial consumption was the dominant pathway of methanol removal. In the shelf, atmospheric deposition replenished surface methanol pools and contributed up to 17.5% ± 16.9% of microbial consumption. These results provided insights into the control of methanol metabolism and assessed the relative importance of different methanol sources and sinks along the estuary–coast–shelf continuum.