Hydrodynamic Treadmill Reveals Reduced Rising Speeds of Oil Droplets Deformed by Marine Bacteria.

In marine environments, microscopic droplets of oil can be transported over large distances in the water column. Bacterial growth on the droplets' surface can deform the oil-water interface to generate complex shapes and significantly enlarge droplets. Understanding the fate of spilled oil droplets requires bridging these length scales and determining how microscale processes affect the large-scale transport of oil. Here, we describe an experimental setup, the hydrodynamic treadmill, developed to keep rising oil droplets stationary in the lab frame for continuous and direct observation. Oil droplets with radii 10 < R < 100 µm were colonized and deformed by bacteria over several days before their effective rising speeds were measured. The rising speeds of deformed droplets were significantly slower than those of droplets without bacteria. This decrease in rising speed is understood by an increase in drag force and a decrease in buoyancy as a result of bio-aggregate formation at the droplet surface. Additionally, we found sinking bio-aggregate particles of oil and bacterial biofilms and quantified their composition using fluorescence microscopy. Our experiments can be adapted to further study the interactions between oil droplets and marine organisms and could significantly improve our understanding of the transport of hydrocarbons and complex aggregates.

Tubulation and dispersion of oil by bacterial growth on droplets.

Bacteria on surfaces exhibit collective behaviors, such as active turbulence and active stresses, which result from their motility, growth, and interactions with their local surroundings. However, interfacial deformations on soft surfaces and liquid interfaces caused by active growth, particularly over long time scales, are not well understood. Here, we describe experimental observations on the emergence of tubular structures arising from the growth of rod-shaped bacteria at the interface of oil droplets in water. Using microfluidics and timelapse microscopy, the dimensions and extension rates of individual tubular structures as well as bulk bio-aggregate formation are quantified for hundreds of droplets over 72 hours. Tubular structures are comparable in length to the initial droplet radius and are composed of an outer shell of bacteria that stabilize an inner filament of oil. The oil filament breaks up into smaller microdroplets dispersed within the bacterial shell. This work provides insight into active stresses at deformable interfaces and improves our understanding of microbial oil biodegradation and its potential influence on the transport of droplets in the ocean water column.

Hyperactivation is sufficient to release porcine sperm from immobilized oviduct glycans.

Fertilizing sperm are retained by adhesion to specific glycans on the epithelium of the oviduct forming a reservoir before sperm are released from the reservoir so fertilization can ensue. Capacitated sperm lose affinity for the oviduct epithelium but the components of capacitation that are important for sperm release are uncertain. One important correlate of capacitation is the development of hyperactivated motility. Hyperactivation is characterized by asymmetrical flagellar beating with high beat amplitude. We tested whether the development of full-type asymmetrical motility was sufficient to release sperm from immobilized oviduct glycans. Sperm hyperactivation was induced by four different compounds, a cell-permeable cAMP analog (cBiMPS), CatSper activators (4-aminopyridine and procaine), and an endogenous steroid (progesterone). Using standard analysis (CASA) and direct visualization with high-speed video microscopy, we first confirmed that all four compounds induced hyperactivation. Subsequently, sperm were allowed to bind to immobilized oviduct glycans, and compounds or vehicle controls were added. All compounds caused sperm release from immobilized glycans, demonstrating that hyperactivation was sufficient to release sperm from oviduct cells and immobilized glycans. Pharmacological inhibition of the non-genomic progesterone receptor and CatSper diminished sperm release from oviduct glycans. Inhibition of the proteolytic activities of the ubiquitin-proteasome system (UPS), implicated in the regulation of sperm capacitation, diminished sperm release in response to all hyperactivation inducers. In summary, induction of sperm hyperactivation was sufficient to induce sperm release from immobilized oviduct glycans and release was dependent on CatSper and the UPS.

Effect of dispersants on bacterial colonization of oil droplets: A microfluidic approach.

Bacteria biodegradation of immiscible oil requires cell-droplet encounters, surface attachment, and hydrocarbon metabolism. Chemical dispersants are applied to oil spills to reduce the mean dispersed droplet size, thereby increasing the available surface area for attachment, in attempts to facilitate bacterial biodegradation. However, their effectiveness remains contentious as studies have shown that dispersants can inhibit, enhance, or have no effect on biodegradation. Therefore, questions remain on whether dispersants affect surface attachment or cell viability. Here, using microfluidics and time-lapse microscopy, we directly observe the attachment and growth of the marine bacterium, Alcanivorax borkumensis, on stationary crude oil droplets (5 µm < R < 150 µm) in the presence of Corexit 9500. We show that the average colonization time, or the time comprised of encounters, attachment, and growth, is dependent on droplet size and primarily driven by diffusive encounters. Our results suggest that dispersants do not inhibit or enhance these biophysical processes.