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Nutrient utilisation by Trichodesmium characterisation of molecular and physiological Processes
No full textThe activity of photosynthetic cyanobacteria capable of nitrogen (N2) fixation (diazotrophs) strongly influences oceanic primary production and global biogeochemical cycles. The niche of these organisms extends mainly across low latitude oligotrophic oceans, largely deficient in nitrate, where they introduce ‘new’ nitrogen (N) to the system. In these regions the abundant marine cyanobacterium Trichodesmium spp. accounts for a significant proportion of the fixed N flux. Despite fixation of N, the availability of phosphorus (P) and iron (Fe) remain a constraint to the activity and biogeography of diazotrophs. The genome of Trichodesmium has therefore been shaped to provide intricate adaptive strategies optimising growth under both P and Fe depletion. Characterisation of these strategies can provide information that will enhance the understanding of the organism’s biogeography in the contemporary and future ocean. In this work, molecular and physiological techniques are employed to study nutrient uptake pathways, and the metabolic response of Trichodesmium erythraeum IMS101 (Trichodesmium hereafter) to nutrient limitation. The current lack of an established system for genetic manipulation of this organism inhibits direct functional characterisation of proteins. To circumvent this, the model cyanobacteria Synechocystis sp. PCC 6803 (Synechocystis hereafter) is used as a vehicle for the heterologous expression of Trichodesmium genes. Using this technique, the suggested contribution of Trichodesmium to an emerging oceanic P redox cycle is first explored. A four-gene cluster (ptxABCD), that encodes a putative ABC transporter (ptxABC) and NAD-dependent dehydrogenase (ptxD), is demonstrated to be responsible for the organism’s ability to utilise the reduced inorganic compound phosphite. The presence and expression of this gene cluster is also confirmed in diverse field metagenomic and metatranscriptomic datasets further confirming its role in Trichodesmium species. Pathways of Fe utilisation are also investigated. Through heterologous expression the function of a currently employed Fe stress biomarker, protein Tery_3377 (IdiA), which is homologous to both Fe3+ transporters (FutA2-like) and intracellular proteins with protective function under Fe stress (FutA1-like), is elucidated. Fusing the signal sequence of this protein to GFP revealed its periplasmic localisation, and its expression in Synechocystis mutants of both futA1 and futA2 paralogues further supported involvement in Fe3+ uptake, providing evidence for its function as an Fe transporter in Trichodesmium. Finally, a physiological experiment was performed to determine the significance of direct physical contact with Saharan desert dust for acquisition of Fe by Trichodesmium. It is demonstrated that cell surface processes are fundamental in dust-Fe utilisation by this organism and transcriptomic analysis identifies a number of unique genes regulated under different Fe and dust regimes including putative cell-surface proteins not previously studied in Trichodesmium. Combined, these studies have revealed a diverse array of molecular and physiological strategies potentially employed by Trichodesmium to survive and thrive on the ephemeral supplies of nutrients encountered in oligotrophic oceans, an attribute that facilitates its significant contribution to biogeochemical cycles.<br/
Phosphite utilization by the globally important marine diazotroph Trichodesmium
No full textSpecies belonging to the filamentous cyanobacterial genus Trichodesmium are responsible for a significant fraction of oceanic nitrogen fixation. The availability of phosphorus (P) likely constrains the growth of Trichodesmium in certain regions of the ocean. Moreover, Trichodesmium species have recently been shown to play a role in an emerging oceanic phosphorus redox cycle, further highlighting the key role these microbes play in many biogeochemical processes in the contemporary ocean. Here, we show that Trichodesmium erythraeum?IMS101 can grow on the reduced inorganic compound phosphite as its sole source of P. The components responsible for phosphite utilization are identified through heterologous expression of the T. erythraeum?IMS101 Tery_0365–0368 genes, encoding a putative adenosine triphosphate (ATP)-binding cassette transporter and nicotinamide adenine dinucleotide (NAD)-dependent dehydrogenase, in the model cyanobacteria Synechocystis sp. PCC6803. We demonstrate that only combined expression of both the transporter and the dehydrogenase enables Synechocystis to utilize phosphite, confirming the function of Tery_0365-0367 as a phosphite uptake system (PtxABC) and Tery_0368 as a phosphite dehydrogenase (PtxD). Our findings suggest that reported uptake of phosphite by Trichodesmium consortia in the field likely reflects an active biological process by Trichodesmium. These results highlight the diversity of phosphorus sources available to Trichodesmium in a resource-limited ocean
Dynamics of extracellular superoxide production by Trichodesmium colonies from the Sargasso Sea
Full text linkReactive oxygen species (ROS) are key players in the health and biogeochemistry of the ocean and its inhabitants. The vital contribution of microorganisms to marine ROS levels, particularly superoxide, has only recently come to light, and thus the specific biological sources and pathways involved in ROS production are largely unknown. To better understand the biogenic controls on ROS levels in tropical oligotrophic systems, we determined rates of superoxide production under various conditions by natural populations of the nitrogen-fixing diazotroph Trichodesmium obtained from various surface waters in the Sargasso Sea. Trichodesmium colonies collected from eight different stations all produced extracellular superoxide at high rates in both the dark and light. Colony density and light had a variable impact on extracellular superoxide production depending on the morphology of the Trichodesmium colonies. Raft morphotypes showed a rapid increase in superoxide production in response to even low levels of light, which was not observed for puff colonies. In contrast, superoxide production rates per colony decreased with increasing colony density for puff morphotypes but not for rafts. These findings point to Trichodesmium as a likely key source of ROS to the surface oligotrophic ocean. The physiological and/or ecological factors underpinning morphology-dependent controls on superoxide production need to be unveiled to better understand and predict superoxide production by Trichodesmium and ROS dynamics within marine systems
Desert Dust as a Source of Iron to the Globally Important Diazotroph Trichodesmium
Full text linkThe marine cyanobacterium Trichodesmium sp. accounts for approximately half of the annual ‘new’ nitrogen introduced to the global ocean but its biogeography and activity is often limited by the availability of iron (Fe). A major source of Fe to the open ocean is Aeolian dust deposition in which Fe is largely comprised of particles with reduced bioavailability over soluble forms of Fe. We report that Trichodesmium erythraeum IMS101 has improved growth rate and photosynthetic physiology and down-regulates Fe-stress biomarker genes when cells are grown in the direct vicinity of, rather than physically separated from, Saharan dust particles as the sole source of Fe. These findings suggest that availability of non-soluble forms of dust-associated Fe may depend on cell contact. Transcriptomic analysis further reveals unique profiles of gene expression in all tested conditions, implying that Trichodesmium has distinct molecular signatures related to acquisition of Fe from different sources. Trichodesmium thus appears to be capable of employing specific mechanisms to access Fe from complex sources in oceanic systems, helping to explain its role as a key microbe in global biogeochemical cycles
Structural and functional characterisation of IdiA/FutA (Tery_3377), an iron binding protein from the ocean diazotroph Trichodesmium erythraeum
Full text linkAtmospheric nitrogen fixation by photosynthetic cyanobacteria (diazotrophs) strongly influences oceanic primary production and in turn affects global biogeochemical cycles. Species of the genus Trichodesmium are major contributors to marine diazotrophy, accounting for a significant proportion of the fixed nitrogen in tropical and subtropical oceans. However, Trichodesmium spp. are metabolically constrained by the availability of iron, an essential element for both the photosynthetic apparatus and the nitrogenase enzyme. Survival strategies in low-iron environments are typically poorly characterized at the molecular level, as these bacteria are recalcitrant to genetic manipulation. Here, we studied a homolog of the iron deficiency-induced A (IdiA)/ferric uptake transporter A (FutA) protein, Tery_3377, which has been used as an in situ iron-stress biomarker. IdiA/FutA has an ambiguous function in cyanobacteria, with its homologs hypothesized to be involved in distinct processes depending on their cellular localization. Using signal sequence fusions to GFP and heterologous expression in the model cyanobacterium Synechocystis sp. PCC 6803 we show that Tery_3377 is targeted to the periplasm by the twin-arginine translocase and can complement the deletion of the native Synechocystis ferric-iron ABC transporter periplasmic binding protein (FutA2). EPR spectroscopy revealed that purified recombinant Tery_3377 has specificity for iron in the Fe3+ state, and an X-ray crystallography determined structure uncovered a functional iron substrate-binding domain, with Fe3+ penta-coordinated by protein and buffer ligands. Our results support assignment of Tery_3377 as a functional FutA subunit of an Fe3+ ABC-transporter, but do not rule out that it also has dual IdiA function
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Transcriptional responses of Trichodesmium to natural inverse gradients of Fe and P availability
Full text linkThe filamentous diazotrophic cyanobacterium Trichodesmium is responsible for a significant fraction of marine di-nitrogen (N2) fixation. Growth and distribution of Trichodesmium and other diazotrophs in the vast oligotrophic subtropical gyres is influenced by iron (Fe) and phosphorus (P) availability, while reciprocally influencing the biogeochemistry of these nutrients. Here we use observations across natural inverse gradients in Fe and P in the North Atlantic subtropical gyre (NASG) to demonstrate how Trichodesmium acclimates in situ to resource availability. Transcriptomic analysis identified progressive upregulation of known iron-stress biomarker genes with decreasing Fe availability, and progressive upregulation of genes involved in the acquisition of diverse P sources with decreasing P availability, while genes involved in N2 fixation were upregulated at the intersection under moderate Fe and P availability. Enhanced N2 fixation within the Fe and P co-stressed transition region was also associated with a distinct, consistent metabolic profile, including the expression of alternative photosynthetic pathways that potentially facilitate ATP generation required for N2 fixation with reduced net oxygen production. The observed response of Trichodesmium to availability of both Fe and P supports suggestions that these biogeochemically significant organisms employ unique molecular, and thus physiological responses as adaptations to specifically exploit the Fe and P co-limited niche they construct
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