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The photosynthetic amoeba, Paulinella provides a recent (ca. 120 Mya) example of primary plastid endosymbiosis. Given the extensive data demonstrating host lineage‐driven endosymbiont integration, we analysed nuclear genome and transcriptome data to investigate mechanisms that may have evolved in Paulinella micropora KR01 (hereinafter, KR01) to maintain photosynthetic function in the novel organelle, the chromatophore. The chromatophore is of α‐cyanobacterial provenance and has undergone massive gene loss due to Muller's ratchet, but still retains genes that encode the ancestral α‐carboxysome and the shell carbonic anhydrase, two critical components of the biophysical CO2 concentrating mechanism (CCM) in cyanobacteria. We identified KR01 nuclear genes potentially involved in the CCM that arose via duplication and divergence and are upregulated in response to high light and downregulated under elevated CO2. We speculate that these genes may comprise a novel CO2 delivery system (i.e., a biochemical CCM) to promote the turnover of the RuBisCO carboxylation reaction and counteract photorespiration. We posit that KR01 has an inefficient photorespiratory system that cannot fully recycle the C2 product of RuBisCO oxygenation back to the Calvin‐Benson cycle. Nonetheless, both these systems appear to be sufficient to allow Paulinella to persist in environments dominated by faster‐growing phototrophs. [ABSTRACT FROM AUTHOR]

Of the six elements incorporated into the major polymers of life, phosphorus is the least abundant on a global scale [E. Anders, M. Ebihara, Geochim. Cosmochim. Acta 46, 2363–2380 (1982)] and has been described as the “ultimate limiting nutrient” [T. Tyrrell, Nature 400, 525–531 (1999)]. In the modern ocean, the supply of dissolved phosphorus is predominantly sustained by the oxidative remineralization/recycling of organic phosphorus in seawater. However, in the Archean Eon (4 to 2.5 Ga), surface waters were anoxic and reducing. Here, we conducted photochemical experiments to test whether photodegradation of ubiquitous dissolved organic phosphorus could facilitate phosphorus recycling under the simulated Archean conditions. Our results strongly suggest that organic phosphorus compounds, which were produced by marine biota (e.g., adenosine monophosphate and phosphatidylserine) or delivered by meteorites (e.g., methyl phosphonate) can undergo rapid photodegradation and release inorganic phosphate into solution under anoxic conditions. Our experimental results and theoretical calculations indicate that photodegradation of organic phosphorus could have been a significant source of bioavailable phosphorus in the early ocean and would have fueled primary production during the Archean eon. [ABSTRACT FROM AUTHOR]

Author(s): Antonietta Quigg (corresponding author) [1]; Zoe V. Finkel [1]; Andrew J. Irwin [1]; Yair Rosenthal [1, 2]; Tung-Yuan Ho [5]; John R. Reinfelder [3]; Oscar Schofield [1, 4]; Francois [...]

Author(s): John R. Reinfelder (corresponding author) [1]; Anne M. L. Kraepiel [2]; François M. M. Morel [3] Nearly 50 years ago, inorganic carbon was shown to be fixed in microalgae [...]

Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation. [ABSTRACT FROM AUTHOR]

Mercury (Hg) bound to fine aerosols (PM2.5-Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM2.5 samples collected between Sept. 15th and Oct. 16th, 2015 from Beijing, China, among which 26 were collected during the daytime (between 8:00 a.m. and 6:30 p.m.) and 30 during night (between 7:00 p.m. and 7:30 a.m.). The results show that diel variation was statistically significant (p < 0.05) for Hg content, Δ199Hg and Δ200Hg, with Hg content during the daytime (0.32 ± 0.14 μg g-1) lower than at night (0.48 ± 0.24 μg g-1) and Δ199Hg and Δ200Hg values during the daytime (mean of 0.26 ‰ ± 0.40 ‰ and 0.09 ‰ ± 0.06 ‰, respectively) higher than during the nighttime (0.04 ‰ ± 0.22 ‰ and 0.06 ‰ ± 0.05 ‰, respectively), whereas PM2.5 concentrations and δ202Hg values showed insignificant (p > 0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and: (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM2.5 (negative), (iv) sunshine durations (positive), and (v) ozone concentration (positive) observed for consecutive day-night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM2.5-Hg. [ABSTRACT FROM AUTHOR]

The mechanisms of carbon concentration in marine diatoms are controversial. At low CO2, decreases in O2 evolution after inhibition of phosphoenolpyruvate carboxylases ( PEPCs), and increases in PEPC transcript abundances, have been interpreted as evidence for a C4 mechanism in Thalassiosira pseudonana, but the ascertainment of which proteins are responsible for the subsequent decarboxylation and PEP regeneration steps has been elusive., We evaluated the responses of T. pseudonana to steady-state differences in CO2 availability, as well as to transient shifts to low CO2, by integrated measurements of photosynthetic parameters, transcript abundances and quantitative proteomics., On shifts to low CO2, two PEPC transcript abundances increased and then declined on timescales consistent with recoveries of Fv/ Fm, non-photochemical quenching ( NPQ) and maximum chlorophyll a-specific carbon fixation ( Pmax), but transcripts for archetypical decarboxylation enzymes phosphoenolpyruvate carboxykinase ( PEPCK) and malic enzyme ( ME) did not change. Of 3688 protein abundances measured, 39 were up-regulated under low CO2, including both PEPCs and pyruvate carboxylase ( PYC), whereas ME abundance did not change and PEPCK abundance declined., We propose a closed-loop biochemical model, whereby T. pseudonana produces and subsequently decarboxylates a C4 acid via PEPC2 and PYC, respectively, regenerates phosphoenolpyruvate ( PEP) from pyruvate in a pyruvate phosphate dikinase-independent (but glycine decarboxylase ( GDC)-dependent) manner, and recuperates photorespiratory CO2 as oxaloacetate ( OAA). [ABSTRACT FROM AUTHOR]

Author(s): John R. Reinfelder (corresponding author) Reinfelder et al . reply Johnston et al . are not convinced that a C[sub.4] photosynthetic pathway exists in microalgae. The essential feature of [...]

Methylmercury (MeHg), a neurotoxic substance that accumulates in aquatic food chains and poses a risk to human health, is synthesized by anaerobic microorganisms in the environment. To date, mercury (Hg) methylation has been attributed to sulfateand iron-reducing bacteria (SRB and IRB, respectively). Here we report that a methanogen, Methanospirillum hungatei JF-1, methylated Hg in a sulfide-free medium at comparable rates, but with higher yields, than those observed for some SRB and IRB. Phylogenetic analyses showed that the concatenated orthologs of the Hg methylation proteins HgcA and HgcB from M. hungatei are closely related to those from known SRB and IRB methylators and that they cluster together with proteins from eight other methanogens, suggesting that these methanogens may also methylate Hg. Because all nine methanogens with HgcA and HgcB orthologs belong to the class Methanomicrobia, constituting the late-evolving methanogenic lineage, methanogenic Hg methylation could not be considered an ancient metabolic trait. Our results identify methanogens as a new guild of Hg-methylating microbes with a potentially important role in mineral-poor (sulfate- and iron-limited) anoxic freshwater environments. [ABSTRACT FROM AUTHOR]