We explore in situ the surface properties of
marine algal blooms of diatom monocultures by utilizing surface techniques of Brewster angle
microscopy (BAM) imaging, vibrational sum frequency generation spectroscopy (SFG), 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 over time. The image results reveal an ~5 nm thick surface region in
the late stages of the bloom which correlates to 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 peak appear between 3500 cm<sup>-1</sup> to 3600 cm<sup>-1</sup> 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 increasing intensity of vibrational alkyl modes and the appearance of a
proteinaceous amide band. 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.
The Ocean’s Elevator: Evolution of the Air-Seawater Interface During a Small-Scale Algal Bloom
