Complete genome sequence of Candidatus Ruthia magnifica

The hydrothermal vent clam Calyptogena magnifica (Bivalvia: Mollusca) is a member of the Vesicomyidae. Species within this family form symbioses with chemosynthetic Gammaproteobacteria. They exist in environments such as hydrothermal vents and cold seeps and have a rudimentary gut and feeding groove, indicating a large dependence on their endosymbionts for nutrition. The C. magnifica symbiont, Candidatus Ruthia magnifica, was the first intracellular sulfur-oxidizing endosymbiont to have its genome sequenced (Newton et al. 2007). Here we expand upon the original report and provide additional details complying with the emerging MIGS/MIMS standards. The complete genome exposed the genetic blueprint of the metabolic capabilities of the symbiont. Genes which were predicted to encode the proteins required for all the metabolic pathways typical of free-living chemoautotrophs were detected in the symbiont genome. These include major pathways including carbon fixation, sulfur oxidation, nitrogen assimilation, as well as amino acid and cofactor/vitamin biosynthesis. This genome sequence is invaluable in the study of these enigmatic associations and provides insights into the origin and evolution of autotrophic endosymbiosis.     

退耕对民勤绿洲土壤碳氮循环关键微生物及功能基因的影响

采用时空互代法,以巴丹吉林沙漠东南缘民勤绿洲不同退耕年限样地土壤为研究对象,利用微生物宏基因组测序技术,以KEGG数据库碳固定、氮代谢途径为工具,研究长期退耕对参与区域土壤碳固定和氮代谢途径的主要微生物群落组成及其功能基因变化的影响。试验共设置9个退耕年限梯度样地：未退耕耕地、退耕1年样地、退耕2年样地、退耕4年样地、退耕8年样地、退耕13年样地、退耕20年样地、退耕30年样地和退耕40年样地。结果表明：退耕明显改变了碳固定、氮代谢土壤微生物和功能基因丰度,细菌在碳固定和氮代谢两个过程中均起到主导作用;还原三羧酸循环途径、还原乙酰辅酶A途径以及3-羟基丙酸/4-羟基丁酸循环途径等为研究区土壤微生物主要碳固定途径,厌氧氨氧化途径、硝酸盐异化还原途径、硝酸盐同化还原途径、反硝化途径以及硝化途径等为研究区土壤微生物主要氮代谢途径;芽单胞菌属（Gemmatimonas）、未分类绿弯菌属（unclassified_Chloroflexi）和链霉菌属（Streptomyces）等是区域土壤微生物碳固定主要菌属,氮代谢则以腈基降解菌属（Nitriliruptor）、芽单胞菌属（Gemmatimonas）、土壤红杆菌属（Solirubrobacter）、未分类绿弯菌属（unclassified_Chloroflexi）和链霉菌属（Streptomyces）等为主;Gemmatirosa、未分类绿弯菌属（unclassified_Conexibacter）、未分类念珠菌门（unclassified_Candidatus Rokubacteria）、Gaiella和Geminicoccus等5个属分类土壤微生物可作为研究区退耕地碳固定途径标记性微生物种群,coxL.cutL和ACO.acnA等是研究区退耕地土壤微生物碳固定途径主要响应功能基因;腈基降解菌属（Nitriliruptor）、未分类念珠菌门（unclassified_Candidatus Rokubacteria）、Geminicoccus、未分类绿弯菌属（unclassified_Conexibacter）、土壤红杆菌属（Solirubrobacter）、未分类酸杆菌门（unclassified_Acidobacteria）和红色杆菌属（Rubrobacter）等7个属分类土壤微生物可作为研究区退耕地氮代谢途径标记性微生物种群,GDH2和E1.4.7.1是研究区退耕地土壤微生物氮代谢途径主要响应功能基因。该结果对于明确退耕影响下民勤绿洲土壤碳氮循环过程具有重要意义。     

High genetic similarity between two geographically distinct strains of the sulfur-oxidizing symbiont 'Candidatus Thiobios zoothamnicoli'

The giant marine ciliate Zoothamnium niveum ( Ciliophora, Oligohymenophora ) is obligatorily covered by a monolayer of putative chemoautotrophic sulfur-oxidizing (thiotrophic) bacteria. For Z. niveum specimens from the Caribbean Sea it has been demonstrated that this ectosymbiotic population consists of only a single pleomorphic phylotype described as Candidatus Thiobios zoothamnicoli. The goal of our study was to identify and phylogenetically analyse the ectosymbiont(s) of a recently discovered Z. niveum population from the Mediterranean Sea, and to compare marker genes encoding key enzymes of the carbon and sulfur metabolism between the two symbiont populations. We identified a single bacterial phylotype representing the ectosymbiont of Z. niveum from the Mediterranean population showing 99.7% 16S rRNA gene (99.2% intergenic spacer region) similarity to the Caribbean Z. niveum ectosymbiont. Genes encoding enzymes typical for an inorganic carbon metabolism [ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO)] and for sulfur metabolism (5'-adenylylsulfate reductase, dissimilatory sulfite reductase) were detected in both symbiotic populations. The very high amino acid sequence identity (97–100%) and the high nucleic acid sequence identity (90–98%) of these marker enzymes in two geographically distant symbiont populations suggests that the association of Z. niveum with Cand . Thiobios zoothamnicoli is very specific as well as temporally and spatially stable.     

"Candidatus Endobugula glebosa," a specific bacterial symbiont of the marine bryozoan Bugula simplex

The bryozoans Bugula neritina and Bugula simplex harbor bacteria in the pallial sinuses of their larvae as seen by electron microscopy. In B. neritina, the bacterial symbiont has been characterized as a gamma-proteobacterium, "Candidatus Endobugula sertula." "Candidatus E. sertula" has been implicated as the source of the bryostatins, polyketides that provide chemical defense to the host and are also being tested for use in human cancer treatments. In this study, the bacterial symbiont in B. simplex larvae was identified by 16S rRNA-targeted PCR and sequencing as a gamma-proteobacterium closely related to and forming a monophyletic group with "Candidatus E. sertula." In a fluorescence in situ hybridization, a 16S ribosomal DNA probe specific to the B. simplex symbiont hybridized to long rod-shaped bacteria in the pallial sinus of a B. simplex larva. The taxonomic status "Candidatus Endobugula glebosa" is proposed for the B. simplex larval symbiont. Degenerate polyketide synthase (PKS) primers amplified a gene fragment from B. simplex that closely matched a PKS gene fragment from the bryostatin PKS cluster. PCR surveys show that the symbiont and this PKS gene fragment are consistently and uniquely associated with B. simplex. Bryostatin activity assays and chemical analyses of B. simplex extracts reveal the presence of compounds similar to bryostatins. Taken together, these findings demonstrate a symbiosis in B. simplex that is similar and evolutionarily related to that in B. neritina.     

Effect of Japanese Paramecium bursaria extract on photosynthetic carbon fixation of symbiotic algae

To investigate the relationship between the Japanese Paramecium bursaria host and its symbiont, we studied the effect of a host cell-free extract on carbon fixation and photosynthate release of the symbiont. The host extract enhanced symbiotic algal carbon fixation about 3-fold at an increased concentration; however, release of photosynthate hardly changed. Since the enhancing effect was not affected by elimination of carbon dioxide from the host extract, the existence of a host factor that stimulates algal carbon fixation was made clear. The host factor is a heat-stable, low molecular weight substance. In relation to the pH dependence, the extract improved carbon fixation at acidic and neutral pH and showed almost no effect at pH 9.0. Therefore, the stimulation of carbon fixation by the host factor is unlikely to be caused by intracellular pH change. The extract also improved carbon fixation of several Chlorella species, symbiotic and free-living, and apparently exhibited no species specificity. Therefore, the host seems to regulate the photosynthesis of the symbiont via a specific compound.     

In situ abundance and carbon fixation activity of distinct anoxygenic phototrophs in the stratified seawater lake Rogoznica

Sulphide-driven anoxygenic photosynthesis is an ancient microbial metabolism that contributes significantly to inorganic carbon fixation in stratified, sulphidic water bodies. Methods commonly applied to quantify inorganic carbon fixation by anoxygenic phototrophs, however, cannot resolve the contributions of distinct microbial populations to the overall process. We implemented a straightforward workflow, consisting of radioisotope labelling and flow cytometric cell sorting based on the distinct autofluorescence of bacterial photopigments, to discriminate and quantify contributions of co-occurring anoxygenic phototrophic populations to in situ inorganic carbon fixation in environmental samples. This allowed us to assign 89.3% ± 7.6% of daytime inorganic carbon fixation by anoxygenic phototrophs in Lake Rogoznica (Croatia) to an abundant chemocline-dwelling population of green sulphur bacteria (dominated by Chlorobium phaeobacteroides), whereas the co-occurring purple sulphur bacteria (Halochromatium sp.) contributed only 1.8% ± 1.4%. Furthermore, we obtained two metagenome assembled genomes of green sulphur bacteria and one of a purple sulphur bacterium which provides the first genomic insights into the genus Halochromatium, confirming its high metabolic flexibility and physiological potential for mixo- and heterotrophic growth.     

Linking hydrothermal geochemistry to organismal physiology: physiological versatility in Riftia pachyptila from sedimented and basalt-hosted vents

Much of what is known regarding Riftia pachyptila physiology is based on the wealth of studies of tubeworms living at diffuse flows along the fast-spreading, basalt-hosted East Pacific Rise (EPR). These studies have collectively suggested that Riftia pachyptila and its chemoautotrophic symbionts are physiologically specialized, highly productive associations relying on hydrogen sulfide and oxygen to generate energy for carbon fixation, and the symbiont's nitrate reduction to ammonia for energy and biosynthesis. However, Riftia also flourish in sediment-hosted vents, which are markedly different in geochemistry than basalt-hosted systems. Here we present data from shipboard physiological studies and global quantitative proteomic analyses of Riftia pachyptila trophosome tissue recovered from tubeworms residing in the EPR and the Guaymas basin, a sedimented, hydrothermal vent field. We observed marked differences in symbiont nitrogen metabolism in both the respirometric and proteomic data. The proteomic data further suggest that Riftia associations in Guaymas may utilize different sulfur compounds for energy generation, may have an increased capacity for energy storage, and may play a role in degrading exogenous organic carbon. Together these data reveal that Riftia symbionts are far more physiologically plastic than previously considered, and that--contrary to previous assertions--Riftia do assimilate reduced nitrogen in some habitats. These observations raise new hypotheses regarding adaptations to the geochemical diversity of habitats occupied by Riftia, and the degree to which the environment influences symbiont physiology and evolution.     

CO2 uptake and fixation by endosymbiotic chemoautotrophs from the bivalve Solemya velum

Chemoautotrophic symbioses, in which endosymbiotic bacteria are the major source of organic carbon for the host, are found in marine habitats where sulfide and oxygen coexist. The purpose of this study was to determine the influence of pH, alternate sulfur sources, and electron acceptors on carbon fixation and to investigate which form(s) of inorganic carbon is taken up and fixed by the gamma-proteobacterial endosymbionts of the protobranch bivalve Solemya velum. Symbiont-enriched suspensions were generated by homogenization of S. velum gills, followed by velocity centrifugation to pellet the symbiont cells. Carbon fixation was measured by incubating the cells with (14)C-labeled dissolved inorganic carbon. When oxygen was present, both sulfide and thiosulfate stimulated carbon fixation; however, elevated levels of either sulfide (>0.5 mM) or oxygen (1 mM) were inhibitory. In the absence of oxygen, nitrate did not enhance carbon fixation rates when sulfide was present. Symbionts fixed carbon most rapidly between pH 7.5 and 8.5. Under optimal pH, sulfide, and oxygen conditions, symbiont carbon fixation rates correlated with the concentrations of extracellular CO(2) and not with HCO(3)(-) concentrations. The half-saturation constant for carbon fixation with respect to extracellular dissolved CO(2) was 28 +/- 3 microM, and the average maximal velocity was 50.8 +/- 7.1 micromol min(-1) g of protein(-1). The reliance of S. velum symbionts on extracellular CO(2) is consistent with their intracellular lifestyle, since HCO(3)(-) utilization would require protein-mediated transport across the bacteriocyte membrane, perisymbiont vacuole membrane, and symbiont outer and inner membranes. The use of CO(2) may be a general trait shared with many symbioses with an intracellular chemoautotrophic partner.     

Candidatus Sodalis melophagi sp. nov.: phylogenetically independent comparative model to the tsetse fly symbiont Sodalis glossinidius

Bacteria of the genus Sodalis live in symbiosis with various groups of insects. The best known member of this group, a secondary symbiont of tsetse flies Sodalis glossinidius, has become one of the most important models in investigating establishment and evolution of insect-bacteria symbiosis. It represents a bacterium in the early/intermediate state of the transition towards symbiosis, which allows for exploring such interesting topics as: usage of secretory systems for entering the host cell, tempo of the genome modification, and metabolic interaction with a coexisting primary symbiont. In this study, we describe a new Sodalis species which could provide a useful comparative model to the tsetse symbiont. It lives in association with Melophagus ovinus, an insect related to tsetse flies, and resembles S. glossinidius in several important traits. Similar to S. glossinidius, it cohabits the host with another symbiotic bacterium, the bacteriome-harbored primary symbiont of the genus Arsenophonus. As a typical secondary symbiont, Candidatus Sodalis melophagi infects various host tissues, including bacteriome. We provide basic morphological and molecular characteristics of the symbiont and show that these traits also correspond to the early/intermediate state of the evolution towards symbiosis. Particularly, we demonstrate the ability of the bacterium to live in insect cell culture as well as in cell-free medium. We also provide basic characteristics of type three secretion system and using three reference sequences (16 S rDNA, groEL and spaPQR region) we show that the bacterium branched within the genus Sodalis, but originated independently of the two previously described symbionts of hippoboscoids. We propose the name Candidatus Sodalis melophagi for this new bacterium.     

Complete genome sequence of "Candidatus Tremblaya princeps" strain PCVAL, an intriguing translational machine below the living-cell status

The sequence of the genome of "Candidatus Tremblaya princeps" strain PCVAL, the primary endosymbiont of the citrus mealybug Planococcus citri, has been determined. "Ca. Tremblaya princeps" presents an unusual nested endosymbiosis and harbors a gammaproteobacterial symbiont within its cytoplasm in all analyzed mealybugs. The genome sequence reveals that "Ca. Tremblaya princeps" cannot be considered an independent organism but that the consortium with its gammaproteobacterial symbiotic associate represents a new composite living being.     

Bacterial symbiosis maintenance in the asexually reproducing and regenerating flatworm Paracatenula galateia

Bacteriocytes set the stage for some of the most intimate interactions between animal and bacterial cells. In all bacteriocyte possessing systems studied so far, de novo formation of bacteriocytes occurs only once in the host development, at the time of symbiosis establishment. Here, we present the free-living symbiotic flatworm Paracatenula galateia and its intracellular, sulfur-oxidizing bacteria as a system with previously undescribed strategies of bacteriocyte formation and bacterial symbiont transmission. Using thymidine analogue S-phase labeling and immunohistochemistry, we show that all somatic cells in adult worms - including bacteriocytes - originate exclusively from aposymbiotic stem cells (neoblasts). The continued bacteriocyte formation from aposymbiotic stem cells in adult animals represents a previously undescribed strategy of symbiosis maintenance and makes P. galateia a unique system to study bacteriocyte differentiation and development. We also provide morphological and immunohistochemical evidence that P. galateia reproduces by asexual fragmentation and regeneration (paratomy) and, thereby, vertically transmits numerous symbiont-containing bacteriocytes to its asexual progeny. Our data support the earlier reported hypothesis that the symbiont population is subjected to reduced bottleneck effects. This would justify both the codiversification between Paracatenula hosts and their Candidatus Riegeria symbionts, and the slow evolutionary rates observed for several symbiont genes.     

The eroded genome of a Psychotria leaf symbiont: hypotheses about lifestyle and interactions with its plant host

Several plant species of the genus Psychotria (Rubiaceae) harbour Burkholderia sp. bacteria within specialized leaf nodules. The bacteria are transmitted vertically between plant generations and have not yet been cultured outside of their host. This symbiosis is also generally described as obligatory because plants devoid of symbionts fail to develop into mature individuals. We sequenced for the first time the genome of the symbiont of Psychotria kirkii in order to shed some light on the nature of their symbiotic relationship. We found that the 4?Mb genome of Candidatus Burkholderia kirkii (B.?kirkii) is small for a Burkholderia species and displays features consistent with ongoing genome erosion such as large proportions of pseudogenes and transposable elements. Reductive genome evolution affected a wide array of functional categories that may hinder the ability of the symbiont to be free-living. The genome does not encode functions commonly found in plant symbionts such as nitrogen fixation or plant hormone metabolism. Instead, a collection of genes for secondary metabolites' synthesis is located on the 140?kb plasmid of B.?kirkii and suggests that leaf nodule symbiosis benefits the host by providing protection against herbivores or pathogens.     

Swapping symbionts in spittlebugs: evolutionary replacement of a reduced genome symbiont

Bacterial symbionts that undergo long-term maternal transmission experience elevated fixation of deleterious mutations, resulting in massive loss of genes and changes in gene sequences that appear to limit efficiency of gene products. Potentially, this dwindling of symbiont functionality impacts hosts that depend on these bacteria for nutrition. One evolutionary escape route is the acquisition of a novel symbiont with a robust genome and metabolic capabilities. Such an acquisition has occurred in an ancestor of Philaenus spumarius, the meadow spittlebug (Insecta: Cercopoidea), which has replaced its ancient association with the tiny genome symbiont Zinderia insecticola (Betaproteobacteria) with an association with a symbiont related to Sodalis glossinidius (Gammaproteobacteria). Spittlebugs feed exclusively on xylem sap, a diet that is low both in essential amino acids and in sugar or other substrates for energy production. The new symbiont genome has undergone proliferation of mobile elements resulting in many gene inactivations; nonetheless, it has selectively maintained genes replacing functions of its predecessor for amino-acid biosynthesis. Whereas ancient symbiont partners typically retain perfectly complementary sets of amino-acid biosynthetic pathways, the novel symbiont introduces some redundancy as it retains some pathways also present in the partner symbionts (Sulcia muelleri). Strikingly, the newly acquired Sodalis-like symbiont retains genes underlying efficient routes of energy production, including a complete TCA cycle, potentially relaxing the severe energy limitations of the xylem-feeding hosts. Although evolutionary replacements of ancient symbionts are infrequent, they potentially enable evolutionary and ecological novelty by conferring novel metabolic capabilities to host lineages.     

Interaction of the coccolithophore Gephyrocapsa oceanica with its carbon environment: response to a recreated high-CO2 geological past

Coccolithophores have played a key role in the carbon cycle since becoming dominant in the Cretaceous ocean, and their influence depends fundamentally on how they interact with their external carbon environment. Because the photosynthetic carbon-fixing enzyme Rubisco requires high levels of CO(2) for effective catalysis, coccolithophores are known to induce carbon concentrating mechanisms (CCMs) to raise the level of dissolved inorganic carbon (DIC) in an 'internal pool'. The ocean carbon system has varied greatly over the geological past, suggesting that coccolithophore interactions with that external carbon environment will have changed in parallel. The widespread present-day coccolithophore Gephyrocapsa oceanica was acclimated here to a geological scale change in the seawater carbon system (five times higher DIC and alkalinity). Significant acclimation in response to the external carbon environment was demonstrated by a fourfold increase in the K(m) substrate concentration requirement for half-maximum photosynthetic carbon fixation rates (suggesting that CCMs were down-regulated when ambient carbon was more available). There was, however, no difference in growth rate, morphology or calcification, suggesting that calcification is not coupled to photosynthesis as one of the CCMs induced here and that productivity (growth rate and calcification) is not carbon-limited under representative present-day conditions. Beyond the kinetic parameters of photosynthesis, the only other indication of changed cell physiology seen was the increased fractionation of carbon isotopes into organic matter. These findings demonstrate that G. oceanica changes its carbon-use physiology to maintain consistent photosynthetic carbon fixation in concert with different levels of ambient DIC without changing its morphology or calcification.     

Bacterial endosymbiont localization in Hyalesthes obsoletus, the insect vector of Bois noir in Vitis vinifera

One emerging disease of grapevine in Europe is Bois noir (BN), a phytoplasmosis caused by "Candidatus Phytoplasma solani" and spread in vineyards by the planthopper Hyalesthes obsoletus (Hemiptera: Cixiidae). Here we present the first full characterization of the bacterial community of this important disease vector collected from BN-contaminated areas in Piedmont, Italy. Length heterogeneity PCR and denaturing gradient gel electrophoresis analysis targeting the 16S rRNA gene revealed the presence of a number of bacteria stably associated with the insect vector. In particular, symbiotic bacteria detected by PCR with high infection rates in adult individuals fell within the "Candidatus Sulcia muelleri" cluster in the Bacteroidetes and in the "Candidatus Purcelliella pentastirinorum" group in the Gammaproteobacteria, both previously identified in different leafhoppers and planthoppers. A high infection rate (81%) was also shown for another symbiont belonging to the Betaproteobacteria, designated the HO1-V symbiont. Because of the low level of 16S rRNA gene identity (80%) with the closest relative, an uncharacterized symbiont of the tick Haemaphysalis longicornis, we propose the new name "Candidatus Vidania fulgoroideae." Other bacterial endosymbionts identified in H. obsoletus were related to the intracellular bacteria Wolbachia pipientis, Rickettsia sp., and "Candidatus Cardinium hertigii." Fluorescent in situ hybridization coupled with confocal laser scanning microscopy and transmission electron microscopy showed that these bacteria are localized in the gut, testicles, and oocytes. As "Ca. Sulcia" is usually reported in association with other symbiotic bacteria, we propose that in H. obsoletus, it may occur in a bipartite or even tripartite relationship between "Ca. Sulcia" and "Ca. Purcelliella," "Ca. Vidania," or both.     

Stable carbon isotopic fractionations associated with inorganic carbon fixation by anaerobic ammonium-oxidizing bacteria

Isotopic analyses of Candidatus "Brocadia anammoxidans," a chemolithoautotrophic bacterium that anaerobically oxidizes ammonium (anammox), show that it strongly fractionates against (13)C; i.e., lipids are depleted by up to 47 per thousand versus CO(2). Similar results were obtained for the anammox bacterium Candidatus "Scalindua sorokinii," which thrives in the anoxic water column of the Black Sea, suggesting that different anammox bacteria use identical carbon fixation pathways, which may be either the Calvin cycle or the acetyl coenzyme A pathway.     

A delta13C-based carbon flux model for the hydrothermal vent chemoautotrophic symbiosis Riftia pachyptila predicts sizeable CO(2) gradients at the host-symbiont interface

The chemoautotrophic symbiosis Riftia pachyptila has extremely 13C-enriched delta13C values. Neither isotopic discrimination by the RubisCO enzyme of their bacterial endosymbionts, nor the delta13C value of CO2 at their hydrothermal vent habitat, suffice to explain biomass delta13C values in this organism, which range from - 9 to - 16 per thousand. However, these 13C-enriched delta13C values are consistent with the presence of 13C-enriched CO2 within the symbiont cytoplasm. Such a 13C-enriched pool of CO2 is expected when the rate of CO2 fixation by RubisCO, which fixes 12CO2 more rapidly than 13CO2, approaches the rate of exchange between intracellular and extracellular CO2 pools. Rapid CO2 fixation rates will also generate concentration gradients between these two pools. In order to estimate the size of these concentration gradients, an equation was derived, which describes the delta13C of tubeworm biomass in terms of the size of the CO2 gradient between the hydrothermal vent environment and the symbiont cytoplasm. Using mass balance equations for CO2 exchange and fixation by the symbionts and the tubeworm host, this model predicts that a CO2 concentration gradient of up to 17-fold between the symbiont cytoplasm and the environment is sufficient to explain even the most 13C-enriched R. pachyptila biomass. This model illustrates how both physical and enzymatic factors can act to influence the delta13C of intracellular CO2, which, in turn, highlights the danger of assigning a carbon fixation pathway to an autotroph based solely on its biomass delta13C value.     

Genome sequence of an ammonia-oxidizing soil archaeon, "Candidatus Nitrosoarchaeum koreensis" MY1

Ammonia-oxidizing archaea are ubiquitous microorganisms which play important roles in global nitrogen and carbon cycle on earth. Here we present the high-quality draft genome sequence of an ammonia-oxidizing archaeon, "Candidatus Nitrosopumilus koreensis" MY1, that dominated an enrichment culture of a soil sample from the rhizosphere. Its genome contains genes for survival in the rhizosphere environment as well as those for carbon fixation and ammonium oxidation to nitrite.     

Azolla: Botany,physiology, and use as a green manure

This is a comprehensive review of literature pertaining to the aquatic fern Azolla and its nitrogen-fixing algal symbiont, Anabaena azollae. The preceding decade has witnessed an explosive growth in research on Azolla, and hopefully this paper will facilitate those efforts. The paper is broken into three major categories: botany, physiology and biochemistry, and agriculture. The botany section includes a world distribution map and reference tables for the 6 Azolla species, and includes the first review of literature on Anabaena azollae. The physiology and biochemistry section covers the range of topics from environmental factors to life processes and nitrogen fixation. Tables on the effect of growth regulators and on the rate of nitrogen fixation measured by acetylene reduction are presented. The agriculture section draws extensively from literature published in the People’s Republic of China and in the Democratic Republic of Vietnam. The major focus of this section is on the history and management practices for Azolla cultivation as a green manure for rice. The effect of weed suppression, use as a fish food and animal fodder, and the insects and diseases of Azolla are also discussed.