Distribution and Functional Analysis of the Two Types of 8-vinyl Reductase Involved in Chlorophyll Biosynthesis in Marine Cyanobacteria

In the chlorophyll biosynthesis pathway, the 8-vinyl group of the chlorophyll precursor is reduced to an ethyl group by 8-vinyl reductase. Two isozymes of 8-vinyl reductase have been described in oxygenic photosynthetic organisms: one encoded by BciA and another by BciB. Only BciB contains an [Fe-S] cluster and most cyanobacteria harbor this form; whereas a few contain BciA. Given this disparity in distribution, cyanobacterial BciA has remained largely overlooked, which has limited understanding of chlorophyll biosynthesis in these microorganisms. Here, we reveal that cyanobacterial BciA encodes a functional 8-vinyl reductase, as evidenced by measuring the in vitro activity of recombinant Synechococcus and Acaryochloris BciA. Genomic comparison revealed that BciB had been replaced by BciA during evolution of the marine cyanobacterium Synechococcus, and coincided with replacement of Fe-superoxide dismutase (SOD) with Ni-SOD. These findings imply that the acquisition of BciA confers an adaptive advantage to cyanobacteria living in low-iron oceanic environments.

Consensus model of a cyanobacterial light-dependent protochlorophyllide oxidoreductase in its pigment-free apo-form and photoactive ternary complex

Photosynthetic organisms employ two different enzymes for the reduction of the C17 = C18 double bond of protochlorophyllide (Pchlide), yielding the chlorophyll precursor chlorophyllide. First, a nitrogenase-like, light-independent (dark-operative) Pchlide oxidoreductase and secondly, a light-dependent Pchlide oxidoreductase (LPOR). For the latter enzyme, despite decades of research, no structural information is available. Here, we use protein structure modelling, molecular dynamics (MD) simulations combined with multi-wavelength analytical ultracentrifugation (MWA-AUC) and small angle X-ray scattering (SAXS) experiments to derive a consensus model of the LPOR apoprotein and the substrate/cofactor/LPOR ternary complex. MWA-AUC and SAXS experiments independently demonstrate that the apoprotein is monomeric, while ternary complex formation induces dimerization. SAXS-guided modelling studies provide a full-length model of the apoprotein and suggest a tentative mode of dimerization for the LPOR ternary complex, supported by published cross-link constraints. Our study provides a first impression of the LPOR structural organization.

Evidence for aggregation and export of cyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone

Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export, via formation and gravitational settling of aggregates and/or consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (>0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, >53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to >53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates likely by gravitational settling and/or convective mixing. Using available pigment and ancillary data from these cruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export via formation of aggregates in this oligotrophic ocean gyre. We suggest that aggregate formation and fate should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.

Evidence for aggregation and export of cyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone

Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export, via formation of aggregates and consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (> 0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, > 53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to > 53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in-situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates. Using available pigment data and ancillary cruise data, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export in this oligotrophic ocean gyre. We suggest that this pathway should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.