Bacterial Control of Marine Humic‐Like Substance Production, Composition, Size, and Transfer to Sea Spray Aerosols During Phytoplankton Blooms.

Humic‐like substances (HULIS) are present in every environmental reservoir, including the ocean and the atmosphere. Ocean‐derived HULIS can be transferred to the atmosphere in the form of sea spray aerosols (SSA). Little information exists on the factors controlling this transfer process or how HULIS alter SSA physicochemical properties, cloud‐forming ability, and atmospheric reactions. Here, using excitation‐emission matrix spectroscopy and isolated ocean‐atmosphere systems, we investigated how ocean biology affects HULIS sea‐to‐air transfer during multiple phytoplankton bloom experiments. We posit that bacterial enzymatic activity on phytoplankton‐derived organic matter control HULIS size, production, and chemical composition. We found that changes in fluorescence indices and shifts in the HULIS fluorescence emission spectra reveal changes in HULIS chemical composition induced by bacteria. High bacterial enzymatic activity on the proteinaceous, lipid, and phosphorus‐rich organic pools enhanced HULIS production and its transfer to SSA. Seawater HULIS consistently accumulated across all experiments. The majority of HULIS was smaller than 0.2 μm or 50 kDa. Our results suggest that enzymatic‐processing bacteria transform the composition of HULIS in seawater, degrading dissolved organic matter into diverse chemical structures that are more efficiently transferred to the atmosphere in SSA. Plain Language Summary: Humic‐like substances (HULIS) are complex molecules found in the ocean. When waves break, HULIS can be launched into the atmosphere in tiny sea spray aerosol particles (SSA). While it is well documented that bacteria modify molecules in the ocean, far less is known about how they modify the characteristics and ocean‐to‐atmosphere transfer of HULIS. Here, we use a laboratory ocean‐atmosphere simulator to study how ocean biology affects HULIS properties in air and ocean samples. We found associations between ocean enzyme activities, HULIS chemistry, and the relative abundance of HULIS in SSA. Therefore, this study shows that marine bacteria enzymes can modify the chemical structures of HULIS, which in turn impacts their ability to transfer to the air via SSA. This study provides insight into the complex role marine biology plays in affecting SSA composition which controls marine clouds and climate. Key Points: Seawater humic‐like substances (HULIS) concentrations consistently increase during phytoplankton blooms, but remain mostly in the <0.2 μm or <50 kDa size fractionHigh bacterial enzymatic activity in seawater enhances HULIS productionHULIS chemical changes may play a role in HULIS sea‐air transfer [ABSTRACT FROM AUTHOR]