Local adaptation through countergradient selection in northern populations of Skeletonema marinoi

Marine microorganisms have the potential to disperse widely with few
obvious barriers to gene flow. However, among microalgae, several studies
have demonstrated that species can be highly genetically structured with
limited gene flow among populations, despite hydrographic connectivity.
Ecological differentiation and local adaptation have been suggested as
drivers of such population structure. Here we tested whether multiple
strains from two genetically distinct Baltic Sea populations of the
diatom Skeletonema marinoi showed evidence of local
adaptation to their local environments; the estuarine Bothnian Sea and the
marine Kattegat Sea. We performed reciprocal transplants of multiple
strains between culture media based on water from the respective
environments, and we also allowed competition between strains of estuarine
and marine origin in both salinities. When grown alone, both marine and
estuarine strains performed best in the high salinity environment, and
estuarine strains always grew faster than marine strains. This result
suggests local adaptation through countergradient selection, i.e. genetic
effects counteract environmental effects. However, higher growth rate of
the estuarine strains appears to have a cost in the marine environment and
when strains were allowed to compete, marine strains performed better than
estuarine strains in the marine environment. Thus, other traits are likely
to also affect fitness. We provide evidence that tolerance to pH could be
involved, and that estuarine strains which are adapted to a more
fluctuating pH continue growing at higher pH than marine
strains.