Trichodesmium is a genus of filamentous cyanobacteria, a type of algae. They are found in nutrient poor tropical ocean waters and grow at such a scale that they can be seen from satellites; features that can cover 200,000 square kilometres. The crucial question for the Air-to-Sea team is what impact such features have on the exchange of gas and energy through the air water interface, and how this impacts ocean surface heating and upper ocean mixing. The answers may well be found within the sea surface microlayer. Learn about the first week of the #AirToSea expedition and how the scientists are collecting samples on the top one millimeter of the ocean, and why they are so interested in a Trichodesmium slick.