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Marine primary producers are largely dependent on and shape the Earth's climate, although their relationship with climate varies over space and time. The growth of phytoplankton and associated marine primary productivity in most of the modern global ocean is limited by the supply of nutrients, including the micronutrient iron. The addition of iron via episodic and frequent events drives the biological carbon pump and promotes the sequestration of atmospheric carbon dioxide (CO2) into the ocean. However, the dependence between iron and marine primary producers adaptively changes over different geological periods due to the variation in global climate and environment. In this review, we examined the role and importance of iron in modulating marine primary production during some specific geological periods, that is, the Great Oxidation Event (GOE) during the Huronian glaciation, the Snowball Earth Event during the Cryogenian, the glacial–interglacial cycles during the Pleistocene, and the period from the last glacial maximum to the late Holocene. Only the change trend of iron bioavailability and climate in the glacial–interglacial cycles is consistent with the Iron Hypothesis. During the GOE and the Snowball Earth periods, although the bioavailability of iron in the ocean and the climate changed dramatically, the changing trend of many factors contradicted the Iron Hypothesis. By detangling the relationship among marine primary productivity, iron availability and oceanic environments in different geological periods, this review can offer some new insights for evaluating the impact of ocean iron fertilization on removing CO2 from the atmosphere and regulating the climate. [ABSTRACT FROM AUTHOR]

The water delivery of pipelines in mountainous areas is faced with problems such as large elevation difference and high water head. In order to meet the downstream water use requirements, energy dissipation and pressure reduction measures need to be set in the downstream area. At present, the most economical and practical measure is orifice energy dissipation. The orifice plate has simple structure, low manufacturing cost, high energy dissipation rate and can well meet the working requirements. Thus it is more and more widely used in throttling and pressure reduction. There are many studies on single orifice and multi-stage orifice plates, but there are too few studies on porous orifice plates. In order to explore the changes of flow field characteristics and energy dissipation rate after setting porous orifice in pressurized pipeline, global structured grid and global unstructured grid are adopted for comparative analysis in this paper. In addition, pressure-based type and transient is adopted in the solver setup. So as to truly reflect the flow characteristics, gravity is considered. Besides, the Realizable k-ε model is used to simulate the flow field and energy dissipation rate of five different types of orifice plates and the wall function method are used to deal with the boundary conditions. The results show that: The results of using global unstructured grid and the Realizable k-ε model are in good agreement with the experimental results, and the maximum relative error is 6.81%. The research on the flow field characteristics behind different porous orifice plates shows that when the water flows through the orifice plate, the flow movement state changes under the influence of its narrow flow. After passing through the orifice plate, the pressure decreases rapidly, and a vortex area is formed in the back, and the length of the reflux area formed by different types of orifice plates is different. Research findings on energy dissipation rate of different porous orifice plates shows that the energy dissipation rate of porous orifice is closely related to the opening rate and Re. The smaller the opening rate is, the better the energy dissipation effect is, The highest energy dissipation rate is 97.1%; The dissipation rate increases rapidly with the increase of the Re and then slowly increases, The maximum increase is 7%. Through dimensional analysis and curve fitting, the empirical formula of energy dissipation rate of porous plate is obtained, The relative error between the calculated value and the measured value is controlled within ± 6%. [ABSTRACT FROM AUTHOR]

Summary: In oligotrophic oceans, low bioavailability of Fe is a key factor limiting primary productivity. However, excessive Fe in cells leads to the Fenton reaction, which is toxic to cells. Cyanobacteria must strictly maintain intracellular Fe homeostasis. Here, we knocked out a series of genes encoding efflux systems in Synechocystis sp. PCC 6803, and found eight genes that are required for high Fe detoxification. Unexpectedly, the HlyBD‐TolC efflux system plays an important role in the adaptation of Synechocystis under Fe‐deficient conditions. Mutants of HlyD and TolC grew worse than the wild‐type strain under low‐Fe conditions and showed significantly lower intracellular Fe contents than the wild‐type strain. We excluded the possibility that the low Fe sensitivity of the HlyBD‐TolC mutants was caused by a loss of the S‐layer, the main extracellular protein secreted via this efflux system. Inactivation of the HlyD protein influenced type IV pili formation and direct inactivation of type IV pili related genes affected the adaptation to low‐Fe conditions. HlyBD‐TolC system is likely involved in the formation of type IV pili and indirectly influenced Fe acquisition. Our findings suggest that efflux system in non‐siderophore‐producing cyanobacteria can facilitate Fe uptake and help cells adapt to Fe‐deficient conditions via novel pathways. [ABSTRACT FROM AUTHOR]

Summary: Cyanobacteria are globally important primary producers and nitrogen fixers with high iron demands. Low ambient dissolved iron concentrations in many aquatic environments mean that these organisms must maintain sufficient and selective transport of iron into the cell. However, the nature of iron transport pathways through the cyanobacterial outer membrane remains obscure. Here we present multiple lines of experimental evidence that collectively support the existence of a novel class of substrate‐selective iron porin, Slr1908, in the outer membrane of the cyanobacterium Synechocystis sp. PCC 6803. Elemental composition analysis and short‐term iron uptake assays with mutants in Slr1908 reveal that this protein is primarily involved in inorganic iron uptake and contributes less to the accumulation of other metals. Homologues of Slr1908 are widely distributed in both freshwater and marine cyanobacteria, most notably in unicellular marine diazotrophs. Complementary experiments with a homologue of Slr1908 in Synechococcus sp. PCC 7002 restored the phenotype of Synechocystis knockdown mutants, showing that this siderophore producing species also possesses a porin with a similar function in Fe transport. The involvement of a substrate‐selective porins in iron uptake may allow cyanobacteria to tightly control iron flux into the cell, particularly in environments where iron concentrations fluctuate. [ABSTRACT FROM AUTHOR]