Intermittent Mixing Research Articles

Development and temperature gradient online monitoring of a vehicular rotary solid‐state bioreactor: a novel device for large‐scale preparation of Aspergillus niger spore inoculum

AbstractBACKGROUNDMechanical cultivation of Aspergillus niger spores at industrial scales as the fermentationinoculum in citric acid plants is difficult to achieve: in solid‐state cultivation, intense mechanical agitation or continuous mixing to reduce damage to the filamentous microbes is not feasible, but inadequate mixing or static cultivation would cause serious inner‐bed temperature gradients. It is therefore necessary to develop novel bioreactors coordinating the conflict between microbe protection and heat transfer, by globally investigating the inner‐bed temperature profile, an investigation that has not been reported upon previously.RESULTSA novel vehicular rotary solid‐state bioreactor was developed combining the characteristics of both a vertical packed bed and a horizontal rotating drum. Moderate and intermittent rotary mixing based on the characteristic posture‐shifting function ensured high spore yields of 3.1 × 1012 per batch (22‐cm‐thick bed, using a rich medium) and 1.3 × 1012 per batch (15‐cm‐thick bed, using a poor medium and two‐stage temperature control strategy). The two‐stage temperature control strategy effectively depressed the upper limit of bed temperature under 37.0 °C. A 16‐point temperature measuring system provided online 4D data of the culture bed including time, temperature, radius and height, globally displaying the inner‐bed temperature gradients. A cylindrical air distributor benefits inner bed radial air flow distribution, but did not affect the spore yield and average moisture content of the solid culture.CONCLUSIONSThe novel devices and methods realized large‐scale production of A. niger spores by coordinating moderate mixing and inner bed temperature field monitoring and control; they also possessed application value for other filamentous microbes. © 2019 Society of Chemical Industry

Optimizing mixing strategy to improve the performance of an anaerobic digestion waste-to-energy system for energy recovery from food waste

This study investigates the effect of different mixing strategies on anaerobic digestion of food waste in order to make the anaerobic digestion waste-to-energy process more energy efficient. Results showed that intermittent mixing is an alternative strategy to continuous mixing or unmixing for high efficient biogas production and energy saving. Through computational fluid dynamics modelling of fluid flow in anaerobic digesters, the mixing time was optimized to 2 mins/hr, at which point the reaction mixture is almost entirely homogeneous. The results of the model was validated with the experimental data. At an organic loading rate of 2.4 g VSFW/L/day, the semi-continuously mixed reactor maintains a higher specific methane yield of 437 mL CH4/g VSFW in comparison with the other controls. In addition, the semi-continuously mixed reactor has a larger proportion of Bacteroides and Methanocuelles. Based on the results of the bench scale experiment, the energy balance of the hybrid AD waste-to-energy system was simulated and evaluated. The calculated AD efficiency of the semi-continuously mixed reactor is 74.4%, which is higher than the AD efficiencies of the continuously mixed (66.9%) and unmixed reactors (14.9%). The energy balance investigated the electricity generated and the net heat output generated, in addition to self-sustaining and meeting the energy requirements of the various AD processes investigated. Based on the analysis, it was found the semi-continuous mixing is more energy efficient and sustainable to generate sufficient biogas output for the energy system to provide a net positive heat and electricity output.

Novel method for sustainable and selective separation of PVC and PET by the homogeneous dissociation of H2O2 using ultrasonication

This paper presents a one-step selective separation of polyvinyl chloride (PVC) from PVC/PET mixture based on hydrophilicity building on the PVC surface using H2O2/ultrasonication. After the combined treatment, the decrease of PVC contact angle (from 87.2° to 71.5°) is consistent with the increase in hydrophilic functional groups that is evidenced by Fourier transform infrared and X-ray photoelectron spectroscopy results on the PVC surface. The H2O2/ultrasonic treatment generates oxidizing agent and increases hydrophilicity on the PVC surface, which allows to selectively separate the treated PVC by its submerging on the reactor bottom. Meanwhile, the treated PET is easily floated off because it is not affected by the combined treatment and still maintains the hydrophobic surface. The combined treatment of H2O2 and ultrasonic irrigation obtains 100% purity and recovery of the PVC separation under the optimum conditions. The optimized separation conditions are H2O2 concentration 3%, ultrasonic irrigation time 30 min and temperature 30 °C, floating agent concentration 0.4 mg/L and intermittent mixing at 50 rpm. Reusing of H2O2 is also feasible to save cost and environmental benefits. The combination of ultrasonication and H2O2 is an effective and inexpensive method for PVC separation to improve plastic recycling quality.

Bioremediation of Oil Contaminated Soils by Washing under Static Conditions (Research Note)

In this research five different solutions were used to wash an oil contaminated soil with the initial TPH (Total Petroleum Hydrocarbon) content of 46 g kg-1 (grams of TPH per kg of dry soil). The solutions were a basal mineral medium (solution I), the basal mineral medium inoculated with petroleum degrading microorganisms (solution II), the basal mineral medium inoculated with a culture of petroleum degrading microorganisms and supplemented with a commercial washing powder (solution III), the basal mineral medium inoculated with the microorganisms and supplemented with Tween 80 (solution IV), and the basal mineral medium inoculated with the microorganisms and supplemented with sucrose (Solution V). Washing was performed by saturation of the soil with the solutions in columns and intermittent mixing. Solutions IV and V performed better than the others, giving more than 90% TPH removal in two months. In a separate experiment TPH removal was monitored as a function of time during washing. Solution V reduced the TPH content of the soil more rapidly than the others, with more than 70% TPH removal in 28 days. Addition of sand particles to the soil for the purpose of better mixing resulted in marginal positive effects.

Design and Operation of a Pilot-Scale Packed-Bed Bioreactor for the Production of Enzymes by Solid-State Fermentation.

In this review, we describe our experience in building a pilot-scale packed-bed solid-state fermentation (SSF) bioreactor, with provision for intermittent mixing, and the use of this bioreactor to produce pectinases and lipases by filamentous fungi. We show that, at pilot scale, special attention must be given to several aspects that are not usually problematic when one works with laboratory-scale SSF bioreactors. For example, it can be a challenge to produce large amounts of inoculum if the fungus does not sporulate well. Likewise, at larger scales, the air preparation system needs as much attention as the bioreactor itself. Sampling can also be problematic if one wishes to avoid disrupting the bed structure. In the fermentations carried out in the pilot bioreactor, when the substrate bed contained predominantly wheat bran, the bed shrank away from the walls, providing preferential flow paths for the air and necessitating agitation of the bed. These problems were avoided by using beds with approximately 50% of sugarcane bagasse. We also show how a mathematical model that describes heat and water transfer in the bed can be a useful tool in developing appropriate control schemes. Graphical Abstract.

Investigation of the effect of intermittent minimal mixing intensity on methane production during anaerobic digestion of dairy manure

Mixing plays an important role in maintaining the solids suspended and the fermenting liquid homogeneous throughout the digester, which helps to improve methane production. The pilot-scale digester with working volume of 1 m3 was mixed with a top-driven impeller. Three mixing intensities, 50, 100 and 150 rpm were minimally intermittent mixed once a day for 5 min under mesophilic temperature conditions (35 ± 0.3 °C) with average total solids content of 14.1% and hydraulic retention time of 30 days. The results show that 100 rpm mixing intensity outperformed the other two, indicating that mixing intensity threshold exists and beyond which methane production is negatively affected. However, 50 rpm was regarded as the economical mixing intensity even though dead zones were recorded. Minimally intermittent mixing once a day was enough to maintain anaerobic digestion process and performance efficiency for optimum methane production. Computational fluid dynamics (CFD) results agreed with the experiment, demonstrating that 100 rpm mixing intensity was sufficient to homogenize the digester content. Further, CFD was used to predict the mixing time in the digester.

Solid state fermentation of mixed substrate for l-asparaginase production using tray and in-house designed rotary bioreactor

l-asparaginase cover a broad spectrum of industrial application like food, biosensor and chemotherapeutic drug. In the present work, l-asparaginase production was carried out using tray bioreactor and in-house designed, fabricated rotary bioreactor using previously optimized tri-substrate mixture using Aspergillus tubingensis IBBL1. l-asparaginase production by tray bioreactor was performed in temperature controlled chamber. Enzyme production was also performed in newly designed and fabricated rotary bioreactor at room temperature and effect of variables such as substrate loading, mixing events and particle size were evaluated. Maximum l-asparaginase activity of 20.58 U/gds was observed using tray bioreactor with bed height of 1 cm and by applying intermittent mixing strategy at 120 h of fermentation. In case of in-house designed rotary bioreactor, l-asparaginase activity of 19.96 U/gds was obtained with 1.5 kg substrate loading, 3 min delay time of the drum and on rotation for 30 s. These findings demonstrate that in-house designed rotary bioreactor was easy to handle, and has shown comparable results with tray bioreactor. At optimized conditions growth kinetics were studied in tray and rotary bioreactor using various growth model equations. Correlation between growth dynamics and l-asparaginase production was explained by means of Leudeking-Piret model. Results indicate that under the optimized conditions, l-asparaginase can be produced in bulk quantities.

Large-scale cultivation of Spirulina in a floating horizontal photobioreactor without aeration or an agitation device.

A low-cost floating photobioreactor (PBR) without the use of aeration and/or an agitation device, in which carbon was supplied in the form of bicarbonate and only wave energy was utilized for mixing, was developed in our previous study. Scaling up is a common challenge in the practical application of PBRs and has not yet been demonstrated for this new design. To fill this gap, cultivation of Spirulina platensis was conducted in this study. The results demonstrated that S. platensis had the highest productivity at 0.3molL-1 sodium bicarbonate, but the highest carbon utilization (104 ± 2.6%) was obtained at 0.1molL-1. Culture of Spirulina aerated with pure oxygen resulted in only minor inhibition of growth, indicating that its productivity will not be significantly reduced even if dissolved oxygen is accumulated to a high level due to intermittent mixing resulting from the use of wave energy. In cultivation using a floating horizontal photobioreactor at the 1.0m2 scale, the highest biomass concentration of 2.24 ± 0.05gL-1 was obtained with a culture depth of 5.0cm and the highest biomass productivity of 18.9gm-2day-1 was obtained with a depth of 10.0cm. This PBR was scaled up to 10m2 (1000L) with few challenges; biomass concentration and productivity during ocean testing were little different than those at the 1.0m2 (100L) scale. However, the larger PBR had an apparent carbon utilization efficiency of 45.0 ± 2.8%, significantly higher than the 39.4±0.9% obtained at the 1m2 scale. These results verified the ease of scaling up floating horizontal photobioreactors and showed their great potential in commercial applications.

Enhancing the bioproduction of value-added aroma compounds via solid-state fermentation of sugarcane bagasse and sugar beet molasses: Operational strategies and scaling-up of the process

Bioproduction of generally recognized as safe (GRAS) products starting with low-cost raw materials has become significant in the biorefinery concept. Thus, the solid-state fermentation (SSF) of agro-industrial residues using GRAS strains appears as alternative to obtain aroma compounds. Here, the SSF of the mixture sugarcane bagasse/sugar beet molasses was used for producing a mixture of value-added fruit-like compounds. The study aimed to enhance the production and ester selectivity evaluating three operational strategies at three scales (0.5, 4.5 and 22 L) using non-sterilized residues. While the average total volatile production was 120 mgVol per gram of dry substrate (g−1ITS), fed-batch operation promoted the highest increases in the ester content up to 57 mgEst g−1ITS, an 88 and 59% more than in the static-batch and intermittent mixing modes respectively. Alternative operational strategies have compensated the scale-up adverse effects in the bioproduction, moving towards a sustainable large-scale application in a circular economy scheme.

Radio frequency heating as a disinfestation method against Corcyra cephalonica and its effect on properties of milled rice

Radio frequency (RF) heating has the potential to be developed as an alternative non-chemical disinfestation method. In contrast to microwave (MW), RF exhibited higher penetration depth, which helps RF to be a useful technique in disinfesting packaging foods. A 3 kW, 27.12 MHz RF system was used to validate the practical of radio frequency technology for rice moth (Corcyra cephalonica) control in milled rice. Rice samples were placed in the polystyrene bag and moved at a speed of 0.8 m/min, and heated in the RF system with intermittent mixing. Four electrode gaps and five sample thicknesses were chosen to confirm the optimal conditions of RF treatment. The results showed that the sample thickness of 15 mm and electrode gap of 40 mm could provide the optimum heating rate for rice. Mortality of each stage (adult, larva, egg) of C. cephalonica increased with increasing heating temperature and reached 100% while RF heated 180 s (45.8 °C), 300 s (56.9 °C), and 420 s (70 °C), respectively. No C. cephalonica was determined in the samples during 45 days storage incubation period at RF treatment to 70 °C. There were no significant differences between control and RF treated samples in quality parameters (moisture, protein, fat, gelatinization, and sensory attributes). Therefore, RF treatment may provide a practical and effective method for disinfesting milled rice without affecting product quality.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave–turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes.

Investigation of a new double-stage aerobic-anoxic continuous-flow cyclic baffled bioreactor efficiency for wastewater nutrient removal

Nitrogen and phosphorus are among the potential pollutants of receptive water sources entering into these water sources via sewage, which are not sufficiently treated. The purpose of this study is to investigate the efficiency of a new two-stage aerobic-anoxic continuous-flow baffled cycling reactor (CFBCR) to reduce nitrogen and phosphorus load from wastewater. Therefore, a double-stage baffled reactor was used in which the second part was integrated with the settling part causing the sludge to be spontaneously returned to the second reservoir. Additionally, the effect of different concentrations of chemical oxygen demand (COD) of 400–800 mg/L, ammonia of 40–60 mg/L, phosphate of 12–20 mg/L, internal rate of return of 100–200% and the retention time of 18–30 h was investigated. Furthermore, to investigate the performance of this reactor, four phases with different aeration and mixing conditions were designed. The percentage of ammonia removal with influent concentration of 40 mg/L in phase 2 with intermittent mixing and one-hour aeration time was 98.7%; effluent nitrate average concentration was 8.4 mg/L NO3-N, and phosphate removal percent was 83%. The best nutrient removal efficiency was with the retention time of 24 h and internal return rate of 150%. In conclusion, CFBCR reactor with continuous influent and effluent and reduction of the need for sludge return, has the potential to be applied to remove nutrients from wastewater.

Observations of submeso motions and intermittent turbulent mixing across a low level jet with a 132‐m tower

The coexistence of wave‐like submeso motions and anisotropic intermittent turbulence in a night‐time stable boundary layer is investigated. Submeso motions of different characteristics and amplitudes interact with each other. These interactions may lead to intermittent turbulence production which alters the turbulent structure of the stable boundary layer. On the other hand, the production and transfer of turbulence affect the delicate balance of submeso motions. In this work, sonic anemometer data collected at 11 levels in southeastern Brazil have been used to study a case of a nocturnal boundary layer at a coastal site. The absence of forcing at the synoptic scale allows the development of a breeze circulation on which a low‐level jet of moderate intensity (4 m s−1) and low height (about 50 m) takes place. The jet evolution is coupled with dirty waves, while its full development is associated with gravity waves driven by a strong vertical temperature gradient. The layer centred at the low‐level jet nose is characterized by horizontal meandering and very weak turbulence intensity. The air far below and above the low‐level jet maximum experiences bursts of significant increase of the turbulence intensity, showing a three‐layer structure. The oscillations of the horizontal wind components exhibit the same frequency as the temperature oscillations, suggesting that the presence of an adequate temperature horizontal gradient is one of the fundamental drivers of the meandering phenomenon. The considered night has been studied by means of the Eulerian auto‐correlation functions for the detection of the meandering hours and their oscillation time‐scales, and by means of the continuous Morlet wavelet function for the detection of the gravity waves and the characterization of their spatial time‐scales and temporal evolution.

Early warning of limit-exceeding concentrations of cyanobacteria and cyanotoxins in drinking water reservoirs by inferential modelling

An early warning scheme is proposed that runs ensembles of inferential models for predicting the cyanobacterial population dynamics and cyanotoxin concentrations in drinking water reservoirs on a diel basis driven by in situ sonde water quality data. When the 10- to 30-day-ahead predicted concentrations of cyanobacteria cells or cyanotoxins exceed pre-defined limit values, an early warning automatically activates an action plan considering in-lake control, e.g. intermittent mixing and ad hoc water treatment in water works, respectively. Case studies of the sub-tropical Lake Wivenhoe (Australia) and the Mediterranean Vaal Reservoir (South Africa) demonstrate that ensembles of inferential models developed by the hybrid evolutionary algorithm HEA are capable of up to 30days forecasts of cyanobacteria and cyanotoxins using data collected in situ. The resulting models for Dolicospermum circinale displayed validity for up to 10days ahead, whilst concentrations of Cylindrospermopsis raciborskii and microcystins were successfully predicted up to 30days ahead. Implementing the proposed scheme for drinking water reservoirs enhances current water quality monitoring practices by solely utilising in situ monitoring data, in addition to cyanobacteria and cyanotoxin measurements. Access to routinely measured cyanotoxin data allows for development of models that predict explicitly cyanotoxin concentrations that avoid to inadvertently model and predict non-toxic cyanobacterial strains.

Lithium Depletion in Solar-like Stars: Effect of Overshooting Based on Realistic Multi-dimensional Simulations

Abstract We study lithium depletion in low-mass and solar-like stars as a function of time, using a new diffusion coefficient describing extra-mixing taking place at the bottom of a convective envelope. This new form is motivated by multi-dimensional fully compressible, time-implicit hydrodynamic simulations performed with the MUSIC code. Intermittent convective mixing at the convective boundary in a star can be modeled using extreme value theory, a statistical analysis frequently used for finance, meteorology, and environmental science. In this Letter, we implement this statistical diffusion coefficient in a one-dimensional stellar evolution code, using parameters calibrated from multi-dimensional hydrodynamic simulations of a young low-mass star. We propose a new scenario that can explain observations of the surface abundance of lithium in the Sun and in clusters covering a wide range of ages, from ∼50 Myr to ∼4 Gyr. Because it relies on our physical model of convective penetration, this scenario has a limited number of assumptions. It can explain the observed trend between rotation and depletion, based on a single additional assumption, namely, that rotation affects the mixing efficiency at the convective boundary. We suggest the existence of a threshold in stellar rotation rate above which rotation strongly prevents the vertical penetration of plumes and below which rotation has small effects. In addition to providing a possible explanation for the long-standing problem of lithium depletion in pre-main-sequence and main-sequence stars, the strength of our scenario is that its basic assumptions can be tested by future hydrodynamic simulations.

Feasibility of thermophilic anaerobic processes for treating waste activated sludge under low HRT and intermittent mixing

Thermophilic anaerobic digestion (AD) arises as an optimized solution for the waste activated sludge (WAS) management. However, there are few feasibility studies using low solids content typically found in the WAS, and that consider uncommon operational conditions such as intermittent mixing and low hydraulic retention time (HRT). In this investigation, a single-stage pilot reactor was used to treat WAS at low HRT (13, 9, 6 and 5 days) and intermittent mixing (withholding mixing 2 h prior feeding). Thermophilic anaerobic digestion (55 °C) was initiated from a mesophilic digester (35 °C) by the one-step startup strategy. Although instabilities on partial alkalinity (1245–3000 mgCaCO3/L), volatile fatty acids (1774–6421 mg/L acetic acid) and biogas production (0.21–0.09 m3/m3reactor.d) were observed, methanogenesis started to recover in 18 days. The thermophilic treatment of WAS at 13 and 9 days HRT efficiently converted VS into biogas (22 and 21%, respectively) and achieved high biogas yield (0.24 and 0.22 m3/kgVSfed, respectively). Intermittent mixing improved the retention of methanogens inside the reactor and reduced the washout effect even at low HRT (<9 days). The negative thermal balance found was influenced by the low solids content in the WAS (2.1% TS) and by the heat losses from the digester walls. The energy balance and economic analyses demonstrated the feasibility of thermophilic AD of WAS in a hypothetical full-scale system, when the heat energy could be recovered from methane in a scenario of higher solids concentration in the substrate (>5% TS).

Size partitioning and mixing behavior of trace metals and dissolved organic matter in a South China estuary

The Jiulong River estuary, located in the southeastern China, suffered from metal pollution due to industrial effluent releases. Mixing of effluent and estuarine water may have significantly affected the size distribution of trace metals and their environmental fate. In the present study, colloidal size distribution of organic matter and selected metals were quantified using asymmetric flow field-flow fractionation (AF4) and ICP-MS. We demonstrated a dominance of dissolved metals in the 1–10kDa fraction, and metals such as Cu, Zn, Ni, Co, Pb, Cd and Mn were mostly regulated by terrestrial fulvic acid. The larger inorganic colloids played a limited role, although Fe reduction was likely to affect the size partitioning of colloidal Mn. The holding pond represented a source of trace metals and chromophoric and humic-like dissolved organic matter to the estuary. Scavenging or removal behavior became evident following the intermittent mixing, and the small sized colloidal organic complexes were responsible for binding and stabilizing trace metals. Variations in particle size distributions indicated different sources, fates and geochemical controls of the metals. Our results highlighted the impacts of both natural and anthropogenic processes on the transformation of trace metals among phases in this dynamic estuary system.

Effect of mixing events on the production of a thermo-tolerant and acid-stable phytase in a novel solid-state fermentation bioreactor

Abstract Mixing constitutes a critical design parameter in solid-state fermentation (SSF) bioreactor and its effect on heat and water transport, and microbial growth in substrate bed can significantly influence overall productivity. Effect of mixing events on the production of a thermo-tolerant and acid stable phytase, by Rhizopus oryzae, was studied in a novel SSF bioreactor, using optimized growth medium containing wheat bran and linseed oil cake (1:1) as main substrate. A critical mixing phase was identified, in the absence of which fungal growth led to the onset of heat accumulation and subsequent bed drying. The tensile strength of hyphal bonds between two substrate particles, at this critical phase, was estimated and related to the mixing intensity in the bioreactor that resulted in an optimum working value of 15 rpm. Effect of mixing time on bioreactor performance was also investigated where a 3 min mixing duration, at every 6 h, increased biomass and phytase productivity to 2.2- and 4.5-fold, respectively, in comparison to packed bed bioreactor (PBR). The proposed bioreactor system with intermittent mixing gave superior performance than PBR and tray bioreactor, in terms of maximum bed temperature, axial bed temperature and biomass gradient, average bed moisture content, biomass and phytase productivity.

Low-frequency and high-frequency oscillatory winds synergistically enhance nutrient entrainment and phytoplankton at fronts

When phytoplankton growth is limited by low nutrient concentrations, full-depth-integrated phytoplankton biomass increases in response to intermittent mixing events that bring nutrient-rich waters into the sunlit surface layer. Here, it is shown how oscillatory winds can induce intermittent nutrient entrainment events and thereby sustain more phytoplankton at fronts in nutrient-limited oceans. Low frequency (i.e. synoptic to planetary scale) along-front wind drives oscillatory cross-front Ekman transport, which induces intermittent deeper mixing layers on the less dense side of fronts. High-frequency wind with variance near the Coriolis frequency resonantly excites inertial oscillations, which also induce deeper mixing layers on the less dense side of fronts. Moreover, we show that low and high frequency winds have a synergistic effect and larger impact on the deepest mixing layers, nutrient entrainment, and phytoplankton growth on the less dense side of fronts than either high-frequency winds or low frequency winds acting alone. These theoretical results are supported by two-dimensional numerical simulations of fronts in an idealized nutrient-limited open-ocean region forced by low and high frequency along-front winds. In these model experiments, higher-amplitude low-frequency wind strongly modulates and enhances the impact of the lower-amplitude high-frequency wind on phytoplankton at a front. Moreover, sensitivity studies emphasize that the synergistic phytoplankton response to low and high frequency wind relies on the high-frequency wind just below the Coriolis frequency. This article is protected by copyright. All rights reserved.

Planktonic equatorial diversity troughs: fact or artifact? Latitudinal diversity gradients in Radiolaria.

In contrast to the classical notion of an increasing biodiversity from the poles to the equator, a number of studies concluded that the diversity of marine species is highest at the middle latitudes, and decreases at the equator. Using a worldwide database critically compiled from 72 surveys (307 species, 4,807 water column and surface sediment samples), we analyzed the latitudinal gradients in species richness (LGSR) of a highly diversified group of marine holoplanktonic protists, the polycystine Radiolaria. Species richness values were corrected for uneven sample coverage and sample size, and contrasted with gradients in 11 environmental variables. Radiolarian species richness decreases from the equator to the poles both in the water column and in the surface sediments and is tightly coupled with temperature throughout the entire thermal range of marine waters. In the tropical Pacific Ocean, a conspicuous east-west gradient in diversity is also associated with temperature. Globally, diversity is negatively correlated with mean annual concentrations of nutrients (N, P, Si) and chlorophyll a. Disagreements with results reported for many other oceanic plankton may stem from the reduction of 3D distributional patterns onto 2D or 1D spaces, to the intermittent mixing of Subtropical and Subpolar species at the middle latitudes, and to a Mid-Domain Effect. The fact that radiolarian LGSR do not show this drop at the equator is partly due to methodological and database-related differences, and probably also in part a reflection of taxon-specific traits.