Les sels nutritifs et la lumiere sont les facteurs environnementaux cles qui determinent les taux de biomasse et la productivite d'un systeme d'aquaculture. La disponibilite de la lumiere est dependante du choix du site d'aquaculture. Les sels nutritifs sont controlables sur un site et des manipulations de teneurs en sels nutritifs peuvent controler la production de biomasse, la productivite, les epiphytes et les quantites de certains produits du metabolisme. Le carbone, l'azote et le phosphore sont les elements nutritifs majeurs qui sont utilises a grande echelle en aquaculture. Cette revue est axee sur l'azote et la physiologie de l'azote chez les algues marines. Des concepts importants comme l'absorption des sels nutritifs, l'assimilation et les reponses a l'addition d'elements nutritifs, les formes d'azote et les niveaux critiques d'azote dans les tissus sont presentes en relation avec leur application en aquaculture. Pour des especes particulieres cultivees dans des sytemes d'aquaculture, il est important de connaitre le rapport C:N:P de l'algue en conditions de saturation en sels nutritifs. Cette information permet de rationaliser les apports de C, N, et P. Les taux d'absorption varient considerablement avec les facteurs physiques, chimiques et biologiques. En particulier, la lumiere et la temperature influent sur les taux d'absorption de sels nutritifs, mais des recherches supplementaires sont necessaires pour comprendre leurs effets. L'absorption de nitrates par certaines algues est dependante de la lumiere (periodicite circadienne) alors que l'absorption d'ammonium est moins dependante de la lumiere. Les taux de croissance a partir de differentes sources d'azote (NO 3 - , NH 4 + , uree ou combinaisons de ces formes) doivent etre evalues. Une large variete de facteurs biologiques comme les variations individuelles, l'histoire de la nutrition, le type de tissu, les stades du cycle de vie et l'âge, le rapport surface:volume du thalle et les changements morphologiques comme la production de poils hyalins peuvent influer sur les taux d'absorption d'azote. Certaines algues marines presentent trois phases d'absorption des elements particulierement limitant (NH 4 + ) en situation de carence azotee. Le taux d'absorption par a coup est superieur au taux de croissance et ainsi l'algue peut compenser son deficit en azote. Le taux d'absorption par a coup peut etre utilise pour reduire la competition avec les epiphytes par additions pulsees de NH 4 + la nuit. Les epiphytes peuvent aussi etre controles par privation des algues en azote pendant plusieurs jours, afin que la teneur en azote des tissus (N stocke) decroisse jusqu'a un niveau critique au dessous duquel le taux de croissance diminue.

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Nutrient physiology of seaweeds: application of concepts to aquaculture

Agar, a gelatinous polysaccharide in the cell wall of many red algal species, is widely used as a gelling, thickening and stabilizing agent. The commercial value of seaweed is judged by their agar content and gel quality. Seaweed materials with higher agar yield and better gelling properties are desired due to the growing demand for agar in the global market. Agar biosynthesis in seaweeds is affected by genetic variations, developmental stages and environmental conditions, while different agar extraction techniques can also affect the yield and quality of agar. In this paper, the effects of different physiological states of seaweed, abiotic and biotic factors, seaweed storage and agar extraction techniques on the agar yield and gelling characteristics, are reviewed. This information is important as a guide for marine aquaculture of potential agarophytes and the possible effects of climate change on the stock of this natural resource.

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Factors affecting yield and gelling properties of agar

Abstract Potosynthetic performance in response to UV radiation was measured in the red alga Gelidium pulchellumTurner (Kützing) from northern Portugal in the laboratory. The experiments basically consisted of exposures to three different UV radiation conditions (PAR + UV-A + UV-B; PAR + UV-A and PAR alone) using artificial lamps and cut off foils followed by incubations at low irradiance of white light to determine the recovery capacity. The effects of two different PAR backgrounds (105 and 480 μmol photon m−2 s−1) and two growth temperatures (15 and 25 °C) as additional factors were also assessed. The optimal quantum yield of fluorescence (Fv/Fm) decreased after 12 h exposure to PAR + UV (equivalent to a weighted UV dose closed to 150 kJ m−2; from photoinhibition of PSII electron transport action spectrum). The electron transport was also impaired by UV radiation. The PAR background was a key factor determining the UV responses in this species: thalli exposed to high PAR background (480 μmol m−2 s−1) exhibited a greater degree of photoinhibition and a slower recovery than plants irradiated by 105 μmol m−2 s−1. On the other hand, an incubation temperature of 15 °C exacerbated the effect of UV radiation on photosynthesis (increased photoinhibition) followed by a slowing down in the recovery kinetic compared to samples incubated at 25 °C. These results suggest that repair processes may be stimulated with increasing growth temperature.

Effects of UV radiation and temperature on photosynthesis as measured by PAM fluorescence in the red alga Gelidium pulchellum (Turner) Kutzing

Abstract The effects of phosphorus (P) and nitrogen (N) enrichments, and N:P ratios on nutrient and carrageenan contents in Chondrus crispus Stackhouse were investigated using a factorial block experiment design. After a variable preconditioning period until they were markedly depleted in P and N, plants were cultivated for 5 weeks in seawater enriched with different concentrations of P (0, 1.5, 3, 4.5, 6 μM wk−1) and N (12.5, 20, 40 μM wk−1). Tissue total P content increased significantly with P enrichment up to 4.5 μM wk−1. Variations in tissue total N content were more the reflection of seasonal variations of the initial nutrient content than due to experimental enrichment. Carrageenan production was optimized in the range of 20 μM N wk−1 and 1.5 to 3 μM P wk−1, and of 40 μM N wk−1 and 1.5 μM P wk−1, or higher, enrichments. No clear correlation was found between N:P ratio and carrageenan content. Dissolved inorganic N (DIN) in seawater remained low at all N and P enrichments; however, dissolved inorganic P (DIP) concentrations increased sharply at an enrichment of 4.5 μM P wk−1 and greater when algal tissues were becoming P saturated. Hence, determining the proper combinations of P and N enrichments, not N:P ratios, is of major importance for aquaculture systems for three reasons: 1) optimizing carrageenan production, 2) minimizing the direct cost of nutrients, and 3) minimizing the levels of DIP and DIN in effluents and, therefore, the indirect cost of nutrients due to their treatment to avoid excessive enrichment of coastal waters.

Factorial Study of the Effects of Phosphorus and Nitrogen Enrichments on Nutrient and Carrageenan Content in Chondrus crispus (Rhodophyceae) and on Residual Nutrient Concentration in Seawater

Investigation of light conditions suitable for cultivation of Gelidium pulchellum (Turner) Kurtz was performed under controlled laboratory conditions at 20 °C and in the range of irradiance of 10–430 μmol photons m−2 s−1. Growth, measured as fresh weight increment, increased with irradiance up to 130 μmol m−2 s−1 and no significant photoinhibition was observed up to 430 μmol m−2 s−1. Maximum growth rate (10.0% day−1) was obtained at 130–240 μmol m−2 s−1 under continuous light and aeration. The effect of irradiance on agar yield and quality was assessed. Agar yield varied from 31 to 38.6% of the algal dry weight, and variation was not related to irradiance. However, the yield of agar molecules soluble at 95 °C increased with increasing irradiance. A similar trend was found for sulphate content in both series o f extracts, at 95 and 121 °C. On the contrary, the molecular weight and the degree of methylation of agar molecules in the 95 °C extracts decreased with increasing light intensity. As a consequence of the variations in sulphate content, molecular weight and molecular weight distribution, the gel strength was considerably lower at high light intensity. Starch content varied from 0.9 to 7.7% of the algal dry weight, and apparently was not related with irradiance.

The effect of light on growth and agar content of Gelidium pulchellum (Gelidiaceae, Rhodophyta) in culture

Phosphate concentration of the growth medium was found to affect the growth rate and agar yield of a clone of Gelidium robustum grown in the laboratory. To study differences in growth we used phosphate concentrations from 0 to 200 µM. To determine the effect of phosphate on agar yield and its properties we used concentrations from 0 to 20 µM. Growth rates generally increased with increasing phosphate concentration, with the highest growth rate (21% d-1) obtained at 150 µM. Agar yield as percentage of fresh weight was highest (10%) in the algae grown with low phosphate concentrations, but agar yield as percentage of dry weight was highest(43%) at 20 µM of phosphate. Gel strength increased with phosphate concentration with a maximum of 160 g m-2 for 0.75% gels for the cultures at 20 µM. Melting and gelling temperatures of the gels were also affected by phosphate concentration of the growth medium. Starch yield was highest in algae grown in low phosphate concentrations.

Ray Lewis

Papers

The effect of phosphate concentration on growth and agar content of Gelidium robustum (Gelidiaceae, Rhodophyta) in culture