Many have been puzzled about large intra-specific genetic differentiation over small spatial scales in aquatic protists without any apparent dispersal barriers. We did a transplant experiment with two populations of common diatom from a fjord and the open sea and we found that both populations displayed significant higher fitness in their native water and they were also much more competitive in their native water compared to the foreign water. Read more on Environ Microbiol doi:10.1111/1462-2920.13372