**Preface to "Lipids in the Ocean 2021"**

From a chemical viewpoint, lipids are biological molecules that are insoluble in water but soluble in organic solvents. Within this broad definition, lipids are divided in a large number of types, including the most common categories of "oil", "fatty acid", "cholesterol", and "lecithin". Lipids are vital to life and are thus present in all living organisms from animals to plants as well as fungi and bacteria, where they serve as energy reserve and functional molecules.

Scientific interest for lipids in environmental sciences has rapidly increased in the last two decades. Recent research has provided evidence that some lipids (and not only the well-known omega-3 EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid)) play essential roles in maintaining vital biological functions that are related to the fitness components of marine organisms. In particular, lipids are key players in an organism's ability to respond to and cope with environmental changes (temperature, light, salinity, nutrients, and pH). Yet, sources, the biosynthetic pathways and functions of many lipids in marine organisms remain poorly described and understood.

Lipids also allow shedding light over the recurrent question of "who is eating what" and revealing local to regional adaptations of marine organisms to shifting biotic and abiotic conditions, a feature of extreme relevance if one considers the changes ongoing in the oceans of today and tomorrow.

From a conceptual and methodological viewpoint, investigations on marine lipids are based on resolutely interdisciplinary approaches that include specialists in analytical chemistry, biochemistry, cell biology, molecular biology, physiology, ecology, and biogeochemistry. Marine lipids allow researchers to work at multiple organizational levels, from unicellular marine organisms to the functioning of entire ecosystems.

From a more applied perspective, framed by blue growth opportunities, a better understanding of lipids from the ocean is paramount to fostering new biotechnological solutions as well as more sustainable fisheries and aquaculture practices. Lipid fingerprints can be used to pinpoint the geographic origin of seafood, either harvested from the wild or farmed under different aquaculture practices, as well as to fight illegal, unreported, and unregulated fishing. The growing interest on the smart valorization of "green lipids" produced by microalgae, seaweeds, and other salt-tolerant macrophytes must also be highlighted, as only now, with the advent of lipidomics, can one truly start to understand the remarkable chemical diversity of marine lipids present in these and other marine taxa.

Multidisciplinary projects on marine lipids are now essential to develop groundbreaking and integrative research programs and to acquire relevant concepts on biological mechanisms associated with lipids. Such knowledge is necessary to address broader scientific questions related to climate change, conservation, and human health.

> **Maria do Ros´ario Domingues and Philippe Soudant** *Editors*
