Academic literature on the topic 'Gracilaria gracili'

Includes bibliographical references.<br>In the past, commercial quantities of G. gracilis were harvested from Saldanha Bay until population collapses of the natural resource necessitated the need for research into alternative aquaculture programs for G. gracilis cultivation. One of the reasons for the G. gracilis population collapse was attributed to adverse conditions during summer which led to thermal stratification of the water column and subsequent nutrient limiting conditions. Inorganic nitrogen has been identified as the major nutrient factor limiting growth and production of G. gracilis populations in South Africa. Although the physiological mechanisms implemented by G. gracilis which allow adaption to low nitrogen environments have been investigated, not much is known about the molecular mechanisms which underlie these adaptions. Thus, it is necessary to elucidate the molecular basis of these adaptions in G. gracilis to complement the existing physiological data.

Cette these a consiste en une etude ecologique de deux especes de rhodophycees (gracilaria bursa-pastoris et gracilaria gracilis) dans l'etang de thau (herault, france). L'etude a consiste en suivi des population naturelles : analyse de la biomasse, de la composition pigmentaire, du taux de croissance, de la reproduction, de la composition pigmentaire de la production et de la qualite de l'agar en fonction des parametres environnementaux (temperature, salinite, lumiere, sels nutritifs, etc. ). Cette etude a montre des differences entre les deux especes avec des periodes de developpement et de reproduction bien distincts. Il existe une dominance de g. Bursa-pastoris sur g. Gracilis. Cette suprematie est illustree par une biomasse plus importante, deux periodes de croissance, la presence de plants fertiles durant toute l'annee et une grande capacite photoadaptative. Les deux especes ont aussi montre des differences quant a la production et a la qualite de l'agar. Ces deux parametres sont influences par les conditions du milieu (temperature, salinite), l'etat nutritionnel des algues (contenu interne en azote) et sont directement lies a la croissance. Les deux gracilaires presentent de fortes capacites d'adaptation au milieu lagunaire, resultats de strategies ecologiques particulieres (eurythermie, reproduction, photoadaptation, etc. ), qui expliquent leur abondance et leur perennite dans l'etang de thau.

Bibliography: leaves 86-97.<br>Low environmental nutrient concentration is the main factor limiting natural production and success of Gracilaria gracilis cultivation in Saldanha Bay, South Africa and nitrogen is the single element of all the nutrients required by seaweeds that is most frequently limiting to growth. Biomass, relative growth rate, the concentration of nitrogen in the growth medium, the mean nitrogen uptake rate of the plant and the amount of nitrogen in the thallus were determined for nitrogen enriched and nitrogen deprived G. gracilis cultures grown in the laboratory.

Includes bibliographical references.<br>The red agarophytic alga, Gracilaria gracilis, occurs naturally within Saldahna Bay, South Africa. Gracilaria species are commercially exploited for their hydrocolloid agar, valued at US$132 million per annum (FAO, 2002). G. gracilis is thus a valuable resource for South Africa in terms of generating foreign currency through export. Nitrogen limitation occurring within Saldahna Bay during the summer to autumn months, however, is considered to be the major factor preventing commercial cultivation of G. gracilis. Algae possess various survival mechanisms that generally involve the activation or repression of various gene regulatory systems. Such regulatory systems aid in preserving energetic homeostasis at a cellular level, thereby allowing maximal algal survival during adverse environmental conditions. Microarray technology is a highthroughput global gene analysis tool, allowing for the simultaneous identification of thousands of differentially expressed genes in a single assay. Combined with genomic technologies such as genetic engineering, microarray technology has the potential to identify genes that confer enhanced in vivo tolerance to a specific stress. In the current investigation, microarray technology was utilized to identify genes differentially expressed in G. gracilis in response to nitrogen limitation. A total of 39 differentially expressed genes were identified. Ofthe 35 genes that were sequenced, 13 were assigned a putative function based on sequence similarity to genes deposited in various databases. The remaining sequences corresponded to either unknown hypothetical proteins or else had no significant homology to any of the sequences in the databases. Overall, the current investigation has provided a foundation for future research into the biological role(s) of each of the identified differentially expressed genes within their true isogenic background. Future studies may thus allow for improved crop tolerance and stability, enhancing aquaculture of this valuable resource in Southern Africa.

Bibliography: pages 132-157.<br>The growth of Gracilaria gracilis (Stackhouse) Steentoft, Irvine et Farnham was examined by studying the effect of organismic determinants such as thallus length, position along the thallus and branching in a series of in situ and laboratory-based experiments. Knowledge of these factors is essential in order to maximise production from suspended seaweed rafts seeded with vegetative G. gracilis fragments. Seeding netlons with freshly collected material provided up to 30 % higher relative growth rates than seaweed maintained on the netlons for three successive months. Initial seedstock length greatly affected growth rate and yield such that 30 cm thalli fragments resulted in growth rates 14 % higher than for 10 cm fragments. This difference is suggested to be due to higher contribution of growth by lateral branches to overall biomass. Comparisons of the growth of apical and basal fragments suggested that growth takes place over the entire length of the thallus but that the apex contributes more to overall elongation than does the proximal part. The removal of apical meristems resulted in an enhanced branching frequency with production of four times as many branches as intact fragments. Evidence is also provided for severe morphological differentiation following long periods of rapid growth. These thalli have very high frequency of branching, are hollow due to the disintegration of medullary cells and are considered to be completely senescent.

Gracilaria gracilis occurs naturally in Saldanha Bay, and was important to the South African export industry as it is a source of two important types of agar, namely food grade and sugar reactive agar. However, a number of abiotic and biotic factors, such as nutrient limitation, has virtually destroyed the G. gracilis resource. An understanding of the physiological response of the alga to nutrient limitation will aid in re-establishing and sustaining G. gracilis populations. However, modelling algal physiology and growth in response to abiotic stresses such as nutrient limitation requires an understanding of the underlying metabolic processes. The present study aimed to address this by investigating nitrogen metabolism and the mechanisms regulating nitrogen metabolism in G. gracilis. This was achieved by profiling changes in gene and protein expression, and activity of two major nitrogen metabolic enzymes, nitrate reductase and glutamine synthetase. Long term culture of G. gracilis in nitrogen replete and lacking conditions indicated that nutrient limitation causes a reduction in intracellular nitrogen and nitrogen protein stores such as phycoerythrin. When various sources were introduced to the culture medium to replenish nitrogen starved G. gracilis, changes in nitrate reductase and glutamine synthetase mRNA, protein and activity seemed to be dependent on the nutrient history of the cells, intracellular and extracellular nitrogen concentrations, metabolites of nitrogen assimilation and other metabolic processes such as carbon metabolism and photosynthesis. Nutrient studies suggested that multiple G. gracilis nitrate reductase and glutamine synthetase isoforms are present and differentially regulated via transcriptional, post-trancriptional, translational and posttranslational mechanisms. Furthermore, the insensitivity of these nitrogen metabolic enzymes to ammonium inhibition and the ability to alter the GS1:GS2 activity ratio possibly represents adaptive strategies developed by G. gracilis to survive nitrogen limitation. Immunocytochemical investigations confirmed the presence of multiple nitrate reductase and glutamine synthetase isoforms. The enzymes were successfully localised to the cell wall, chloroplast and cytosol of G. gracilis. A novel finding was the immuno-localisation of glutamine synthetase to intracellular starch granules. Overall, findings in the current study have suggested multiple roles for these metabolic enzymes that include nitrogen assimilation/transport, cell wall biosynthesis and senescence. This study led to the development of a model of the metabolic changes that occur in nitrogen replete and deplete G. gracilis and provides a firm foundation for future studies of the nitrogen stress response in G. gracilis. Characterisation of the G. gracilis nitrogen stress response may ultimately revive mariculture of this commercially important alga in South Africa.