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Subsurface chlorophyll maximum layers (SCMLs) are nearly ubiquitous in stratified water columns and exist at horizontal scales ranging from the submesoscale to the extent of oligotrophic gyres. These layers of heightened chlorophyll and/or phytoplankton concentrations are generally thought to be a consequence of a balance between light energy from above and a limiting nutrient flux from below, typically nitrate (NO3). Here we present multiple lines of evidence demonstrating that iron (Fe) limits or with light colimits phytoplankton communities in SCMLs along a primary productivity gradient from coastal to oligotrophic offshore waters in the southern California Current ecosystem. SCML phytoplankton responded markedly to added Fe or Fe/light in experimental incubations and transcripts of diatom and picoeukaryote Fe stress genes were strikingly abundant in SCML metatranscriptomes. Using a biogeochemical proxy with data from a 40-y time series, we find that diatoms growing in California Current SCMLs are persistently Fe deficient during the spring and summer growing season. We also find that the spatial extent of Fe deficiency within California Current SCMLs has significantly increased over the last 25 y in line with a regional climate index. Finally, we show that diatom Fe deficiency may be common in the subsurface of major upwelling zones worldwide. Our results have important implications for our understanding of the biogeochemical consequences of marine SCML formation and maintenance.

Coral reefs are known to have extremely high rates of primary production. However, common geochemical methods for determining bulk rates of reef metabolism cannot distinguish which organisms are responsible for primary production. Here we used a "bottom-up" approach to estimate the contribution of diverse primary producers including hard corals, octocorals, and algae to gross primary production on coral reefs by scaling up taxon-specific rates by the abundance of those taxa in the environment. Chamber-based production rates of the dominant primary producers were obtained as a function of irradiance, the primary short-term driver of photosynthesis. These rates were then combined with annotated three-dimensional (3D) reconstructions of reef sections and a simple light field model to estimate reef-scale gross and net primary production rates over time. At a degraded reef in the Florida Keys octocorals and algae were the main producers, but at a more intact site a scleractinian coral (Acropora palmata) was the most important producer. As a validation of the approach, rates of primary production estimated using the "bottom-up" approach were compared with in situ eddy covariance fluxes. The daily integrated rates agreed within 16%, though maximal production was ~ 35% lower in the "bottom- up" approach likely due to under-representation of octocorals and macroalgae in the 3D reconstructions. The "bottom up" approach yielded results that were largely consistent with the in situ measurements of primary production and irradiance with the significant benefit of providing taxon-specific and spatially-explicit primary production rates. [ABSTRACT FROM AUTHOR]

Membrane permeabilities to CO2 and HCO3− constrain the function of CO2 concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO2 but effectively impermeable to HCO3−. Here, CO2 and HCO3− membrane permeabilities were measured using an 18O‐exchange technique on two species of haptophyte algae, Emiliania huxleyi and Calcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO2 (0.006–0.02 cm · s−1) but minimally permeable to HCO3−. Increased temperature and CO2 generally increased CO2 membrane permeabilities in both species, possibly due to changes in lipid composition or CO2 channel proteins. Changes in CO2 membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO2 were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret 18O‐exchange data assumes that carbonic anhydrase, which catalyzes 18O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate 18O‐exchange data and then inferring plasma‐membrane CO2 permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO2 permeabilities are minimal estimates but should be quite accurate under most conditions. [ABSTRACT FROM AUTHOR]

Wave functions for one-electron diatomic molecules such as H[sup +][sub 2] and HeH[sup 2+] are analyzed by Bader's atoms in molecules method. The locations of the degenerate axial critical points in the electron density generated from sigma states arising from the n = 1 - 6 united atom manifolds are shown to correspond well with the boundaries of domains obtained solely from a classical description of the electron motion. The relationship clarifies the connection between the atomic and molecular regimes of classical trajectories and the quantum description of the states. In particular, the classical transition from atomic to molecular character roughly corresponds to the appearance of a critical point in the electron density located on the internuclear axis between the nuclei. The global aspects of the relationship between the classical and quantum descriptions helps to demonstrate the classical framework of the quantum picture. [ABSTRACT FROM AUTHOR]