Standing genetic variation fuels rapid adaptation to ocean acidification.

Global climate change has intensified the need to assess the capacity for natural populations to adapt to abrupt shifts in the environment. Reductions in seawater pH constitute a conspicuous global change stressor that is affecting marine ecosystems globally. Here, we quantify the phenotypic and genetic modifications associated with rapid adaptation to reduced seawater pH in the Mediterranean mussel, Mytilus galloprovincialis. We reared a genetically diverse larval population in two pH treatments (pHT 8.1 and 7.4) and tracked changes in the shell-size distribution and genetic variation through settlement. Additionally, we identified differences in the signatures of selection on shell growth in each pH environment. Both phenotypic and genetic data show that standing variation can facilitate adaptation to declines in seawater pH. This work provides insight into the processes underpinning rapid evolution, and demonstrates the importance of maintaining variation within natural populations to bolster species' adaptive capacity as global change progresses.

Ocean pH fluctuations affect mussel larvae at key developmental transitions.

Coastal marine ecosystems experience dynamic fluctuations in seawater carbonate chemistry. The importance of this variation in the context of ocean acidification requires knowing what aspect of variability biological processes respond to. We conducted four experiments (ranging from 3 to 22 days) with different variability regimes (pHT 7.4-8.1) assessing the impact of diel fluctuations in carbonate chemistry on the early development of the mussel Mytilus galloprovincialis. Larval shell growth was consistently correlated to mean exposures, regardless of variability regimes, indicating that calcification responds instantaneously to seawater chemistry. Larval development was impacted by timing of exposure, revealing sensitivity of two developmental processes: development of the shell field, and transition from the first to the second larval shell. Fluorescent staining revealed developmental delay of the shell field at low pH, and abnormal development thereof was correlated with hinge defects in D-veligers. This study shows, for the first time, that ocean acidification affects larval soft-tissue development, independent from calcification. Multiple developmental processes additively underpin the teratogenic effect of ocean acidification on bivalve larvae. These results explain why trochophores are the most sensitive life-history stage in marine bivalves and suggest that short-term variability in carbonate chemistry can impact early larval development.

The selective antigen-presenting cell capacity of activated B lymphocytes in HLA-II-restricted responses of CD4+ T lymphocytes.

We examined the role of antigen-presenting B lymphocytes using panels of antigen-specific CD4+8-T-lymphocyte clones (TLC). All 19 TLC showed a class II major histocompatibility complex-encoded (HLA-II) restricted proliferation to antigen presented by antigen-presenting cells (APC) from the monocyte fraction of peripheral blood. Only six TLC were effectively activated by antigen presented by autologous B lymphocytes activated by EBV transformation. This failure of B lymphocytes was not due to: (i) a high degree of cell surface sialic acid; (ii) a low expression of the cell surface proteins HLA-II, ICAM-1 or LFA-3 that restrict antigen presentation; (iii) lack of secretion of the cytokine IL-1 or other soluble factors that may be required as secondary signals; or (iv) induction of incomplete T-cell activation resulting in the production of growth factor interleukin-2 (IL-2) or the expression of receptors for IL-2 only. These data suggest the involvement of another cell surface interaction in antigen presentation acting besides the interactions known so far.