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

Abstract

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.

New Insights into Cation- and Temperature-Driven Protein Adsorption to the Air–Water Interface through Infrared Reflection Studies of Bovine Serum Albumin

“…The IRRAS spectra presented herein are surface-sensitive; therefore, such shifts indicate that stronger interactions are occurring at the surface when BSA and a stearic acid monolayer is present. This is indicative of greater surface organization, which is established in the literature through water surface measurements using Brewster angle microscopy (BAM) 59,72 and surface pressure−area Langmuir isotherms. 66,73−75 The observed wavenumber shifts are not necessarily related to tertiary protein structure conformation change but to a change in the water surface structure at the air−water interface.…”

“…Other factors convolute ocean surface adsorption; for example, algal blooms and their subsequent senescence result in the injection of organic molecules into the water column and enrichment of the surface layer. 59 While not presently investigated, their effect is nonetheless present and impacting SSA chemistry.…”

“…However, our experimental design is ultimately not comprehensive or exhaustive in its inclusion of all oceanic and atmospheric biological, chemical, or physical components. For example, our experiments exclude the influence of microbial communities, 58−62 the diverse and complex array of molecules adsorbed from biogenic 9,59,63 or anthropogenic sources, 1 and wind dynamics 64 that are destabilized by decreased surface tension from adsorbed organic monolayers. Instead, the inclusion of salinity and temperature are used to understand the fundamental process of protein surface adsorption.…”