Can the xanthophyll cycle help extract the essence of the microalgal functional response to a variable light environment?

How do pelagic microalgae cope with light changes in marine ecosystems? Is photosynthesis enhanced or depressed by frequent light fluctuations and how? These long-lasting questions are still under debate. A great diversity of responses has been documented relative to the spatial and temporal scale of study, the groups and species, etc. The answers to these questions probably lie in the interplay between fast photoregulation and longer term photoacclimation whose framework (amplitude, kinetics) is defined by environmental genetic adaptation. One of the main photoregulatory processes is the light-dependent operation of the so-called xanthophyll cycle (XC). XC is activated when the incident light becomes excessive with respect to its optimum necessary for maximization of photosynthesis. The present paper aims at (i) describing a short state-of-the-art on the XC functioning in microalgae with an ecophysiological point of view, (ii) discussing the relevance of the in situ XC operation and its response to the underwater light climate, (iii) addressing a series of open questions for filling the gap between XC functional features and field studies with respect to its role in the photophysiology of microalgae and its potential use for characterizing the hydrodynamics of water bodies.