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Sulfur metabolism in microalgae
No full textLa concentrazione media di SO4
2- in oceano è 28-30 mmol L-1. Essa ha subito notevoli variazioni
nel corso della storia della Terra e potrebbe aver facilitato la radiazione delle alghe della linea rossa
(alghe con chl a+c quali diatomee, dinoflagellate e coccolitoforidi) a discapito di quelle della linea
verde (alghe con chl a+b) e dei cianobatteri. È ancora da chiarire se gli enzimi responsabili
dell’assimilazione del SO4
2- negli organismi fotosintetici abbiano svolto un ruolo nel controllare
l’adattamento delle alghe alle variazioni di concentrazione del SO4
2-.
In accordo con studi precedenti, la mia tesi dimostra che diverse specie algali possono
acclimatarsi/adattarsi a diverse concentrazioni di SO4
2-. Anche se la disponibilità di SO4
2- non ha
provocato cambiamenti drastici nella composizione cellulare, l’attività degli enzimi coinvolti
nell’assimilazione riduttiva del SO4
2- è stata sensibilmente modificata, soprattutto quella dell’ATP
solforilasi (ATPS), suggerendo che la disponibilità di SO4
2- sia un fattore importante nel controllo
delle risposte intracellulari a breve termine mediate da induzione/inibizione delle attività
enzimatiche.
Lo studio delle sequenze di ATPS ha rivelato che contrariamente ad altri organismi, quelle algali
hanno molti residui di cisteina. La posizione di alcuni di essi è conservata, sebbene vi sia una
notevole differenza tra alghe eucariotiche e α-cianobatteri, e β-cianobatteri. Per verificare se
l’ATPS potesse essere redox regolata in modo diverso in questi gruppi, l’attività dell’ATPS di sette
specie è stata testata dopo trattamento con agenti riducenti (ditiotreitolo) o ossidanti (trans-4,5-
diidrossi-1,2-ditiano) dei gruppi tiolici delle cisteine. Le cisteine dell’ATPS parzialmente purificata
da Synechocystis sp. (β-cianobatteri) e T. pseudonana (eucarioti/α-cianobatteri) sono state poi
identificate mediante spettrometria di massa dopo alchilazione dei gruppi sulfidrilici con
iodoacetamide.
I risultati mostrano che il numero e la posizione delle cisteine nelle ATPS algali sono fattori
importanti per la regolazione della loro attività
Direct and indirect influence of sulfur availability on phytoplankton evolutionary trajectories
No full textThe sulfate facilitation hypothesis suggests that changes in ocean sulfate concentration influenced the rise to dominance of phytoplankton species of the red lineage. The mechanistic reasons for this phenomenon are not yet understood. We started to address this question by investigating the differences in S utilization by algae of the green and red lineages and in cyanobacteria cultured in the presence of either 5 mmol L-1 (approximately equivalent to Paleozoic ocean concentrations) or 30 mmol L- 1 (corresponding to post-Mesozoic/ extant concentrations) sulfate. The activities of the main enzymes involved in SO42-- assimilation changed in response to changes in growth sulfate
concentration. ATP sulfurylase showed different kinetics in the various taxa, with an especially odd behavior for the dinoflagellate. Sulfate availability had a modest effect on cell organic composition.
Species-specific differences in the use of some elements were instead obvious in algae grown in the presence of different sulfate concentrations, overall confirming that algae of the red lineage do better at high sulfate than algae of the green lineage. The increase in sulfate concentration may thus have had an impact on phytoplankton radiation both through changes in their enzymatic machinery and through indirect repercussion on elemental usage
Diversity and regulation of ATP sulfurylase in photosynthetic organisms
No full textATP sulfurylase (ATPS) catalyzes the first committed step in the sulfate assimilation pathway,
the activation of sulfate prior to its reduction. ATPS has been studied in only a few model
organisms and even in these cases to a much smaller extent than the sulfate reduction and
cysteine synthesis enzymes. This is possibly because the latter were considered of greater
regulatory importance for sulfate assimilation. Recent evidences (reported in this paper)
challenge this view and suggest that ATPS may have a crucial regulatory role in sulfate assimilation, at least in algae.
In the ensuing text, we summarize the current knowledge on ATPS, with special attention to the
processes that control its activity and gene(s) expression in algae. Special attention is given to
algae ATPS proteins. The focus on algae is the consequence of the fact that a comprehensive
investigation of ATPS revealed that the algal enzymes, especially those that are most likely
involved in the pathway of sulfate reduction to cysteine, possess features that are not present in
other organisms. Remarkably, algal ATPS proteins show a great diversity of isoforms and a high
content of cysteine residues, whose positions are often conserved. According to the occurrence
of cysteine residues, the ATPS of eukaryotic algae is closer to that of marine cyanobacteria of
the genera Synechococcus and Prochlorococcus and is more distant from that of freshwater
cyanobacteria. These characteristics might have evolved in parallel with the radiation of algae in
the oceans and the increase of sulfate concentration in seawater
Redox regulation of ATP sulfurylase activity in microalgae.
No full textATP sulfurylase (ATPS) catalyzes the first step of sulfur assimilation in photosynthetic organisms. An ATPS
type A is mostly present in freshwater cyanobacteria, with four conserved cysteine residues. Oceanic
cyanobacteria and most eukaryotic algae instead, possess an ATPS-B containing seven to ten cysteines;
five of them are conserved, but only one in the same position as ATPS-A. We investigated the role of
cysteines on the regulation of the different algal enzymes.We found that the activity of ATPS-B from four
different microorganisms was enhanced when reduced and decreased when oxidized. The LC-MS/MS
analysis of the ATPS-B from the marine diatom Thalassiosira pseudonana showed that the residue Cys-
247 was presumably involved in the redox regulation. The absence of this residue in the ATPS-A of
the freshwater cyanobacterium Synechocystis sp. instead, was consistent with its lack of regulation. Some
other conserved cysteine residues in the ATPS from T. pseduonana and not in Synechocystis sp.were
accessible to redox agents and possibly play a role in the enzyme regulation. Furthermore, the fact that
oceanic cyanobacteria have ATPS-B structurally and functionally closer to that from most of eukaryotic
algae than to the ATPS-A from other cyanobacteria suggests that life in the sea or freshwater may have
driven the evolution of ATPS
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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