QCD and strongly coupled gauge theories : challenges and perspectives

We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe

Adaptive divergence in pigment composition promotes phytoplankton biodiversity

The dazzling diversity of the phytoplankton has puzzled biologists for decades(1-5). The puzzle has been enlarged rather than solved by the progressive discovery of new phototrophic microorganisms in the oceans, including picocyanobacteria(6,7), pico-eukaryotes(8), and bacteriochlorophyll-based(9-11) and rhodopsin-based phototrophic bacteria(12,13). Physiological and genomic studies suggest that natural selection promotes niche differentiation among these phototrophic microorganisms, particularly with respect to their photosynthetic characteristics(14-16). We have analysed competition for light between two closely related picocyanobacteria of the Synechococcus group that we isolated from the Baltic Sea(17). One of these two has a red colour because it contains the pigment phycoerythrin, whereas the other is blue-green because it contains high contents of the pigment phycocyanin. Here we report theory and competition experiments that reveal stable coexistence of the two picocyanobacteria, owing to partitioning of the light spectrum. Further competition experiments with a third marine cyanobacterium, capable of adapting its pigment composition, show that this species persists by investing in the pigment that absorbs the colour not used by its competitors. These results demonstrate the adaptive significance of divergence in pigment composition of phototrophic microorganisms, which allows an efficient utilization of light energy and favours species coexistence