The Ocean’s Elevator: Evolution of the Air–Seawater Interface during a Small-Scale Algal Bloom

The composition and lifetime of sea spray aerosols are driven by the molecular and biological complexity of the air–seawater interface. We explore in situ the surface properties of marine algal bloom diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum-frequency generation (SFG) spectroscopy, and infrared reflection–absorption spectroscopy (IRRAS). Over the course of the bloom, the marine algae produce surface-active biogenic molecules that temporally partition to the topmost interfacial layers and are selectively probed through surface imaging and spectroscopic measurements. BAM images show morphological structural changes and heterogeneity in the interfacial films with increasing density of surface-active biogenic molecules. Film thickness calculations quantified the average surface thickness of a productive bloom over time. The image results reveal an ∼5 nm thick surface region in the late stages of the bloom, which correlates with typical sea surface nanolayer thicknesses. Our surface-specific SFG spectroscopy results show significant diminishing in the intensity of the dangling OH bond of surface water molecules consistent with organic molecules partitioning and replacing water at the air–seawater interface as the algal bloom progresses. Interestingly, we observe a new broad band appear between 3500 and 3600 cm–1 in the late stages of the bloom that is attributed to weak hydrogen bonding interactions of water to the surface-active biogenic matter. IRRAS confirms the presence of organic molecules at the surface as we observe an increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band over time. Our work shows the often overlooked but vast potential of tracking changes in the interfacial regime of small-scale laboratory marine algal blooms. By coupling surface imaging and vibrational spectroscopies to complex, time-evolving, marine-relevant systems, we provide additional insight into unraveling the temporal complexity of sea spray aerosol compositions.