Introduction
Recirculating aquaculture systems (RAS) allow minimizing the water renewal flow rate needed in fish production by increased Nitrogen (NO3) and Phosphorous concentrations. Integrating seaweed production to RAS can improve its performances and reduce soluble wastes, contributing to a more sustainable production system. Fish and seaweed production technologies must be adapted to maximize synergies and achieve a feasible integrated system. The overall objective of the IntegRAS project, funded by the Spanish government, is to optimize the design and management of a fish-seaweed IMTA-RAS system minimizing energy consumption, seawater renovation rate and improving the productivity and health of the system by selecting the most suitable seaweed species and by colonising the seaweed with probiotic microbiota. Here we show the main goals and activities of the project, together with some preliminary results.
Material and methods
Sole (Solea senegalensis) and sea-lettuce (Ulva spp.), were chosen for the study, based on the matching of ecophysiological requirements for both cultures and the commercial interest of sole, whose cultivation is nowadays entirely performed in inland facilities.
The optimization of the system is approached from a multidisciplinary perspective, including:
1) Identification and characterization of distinct Ulva species to select the more suitable for developing IMTA-RAS systems:
Molecular identification.
Comparative study of the behavior in culture conditions of the selected species.
Qualitative studies of Ulva biomass production for use as feed and food.
2) Engineering aspects related with the design of the IMTA-RAS system and the evaluation of the culture performance:
Design of seaweeds culture tanks: Analysis of several geometries in aerated and non-aerated tanks, stocking density and water renewal strategies.
Experimentation with a scale model of a Sole-Ulva IMTA -RAS system to establish the appropriate range of environmental parameters and facility management.
3) The experimental colonization of seaweeds surface with probiotic bacteria with antagonism against fish pathogens:
Selection and identification of epiphytic bacteria with antagonistic effect.
Colonization of Ulva and validation of the probiotic effect against pathogenic bacteria in fish cultures.
Results and discussion
The preliminary results obtained from the activities performed so far are summarized below:
1) It is very difficult to identify the different species of Ulva only by morphological characters and therefore molecular identification has been used. Until now, five distinct Ulva species: U. australis, U. fasciata, U. ohnoi, U. pseudocurvata y U. rigida have been identified from field sampling. These species are now being cultivated showing a great variability in the development of the different species in the same environmental conditions. As a main physical parameter, the influence of the quality and quantity of light on the growth of these species is being assessed, in order to establish the light saturation thresholds for each species.
2) Rotating velocity of algae inside the tanks has been related with the aeration rate and with the water inlet impulse in aerated and non-aerated tanks respectively. A lineal relationship has been observed between water inlet impulse force and rotating velocity. The minimal water impulse and aeration rate needed to start algae rotation has been related with stocking density for several tank geometries and the energy cost estimated.
Water from a RAS, with concentrations in N (NO3) over 50 mg/l, P over 5 mg/l, and CO2 over 10 mg/l, has been used to feed algae tanks. Nutrient uptake, pH and CO2 concentrations have been determined with several water renewal strategies in aerated and non- aerated seaweed tanks, showing the last the lower pH values and higher CO2 concentrations.
3) Samples of U. australis and U. rigida were taken in Galicia (NW Spain) from two intertidal locations and their epiphytic bacterial communities analyzed.. Both algae species showed a similar bacterial load. 245 different bacterial colonies were isolated from algae, 8 showing strong antagonistic activity against V. anguillarum. Sequencing revealed 99% similarity to Pseudomonas sp. and with 99% similarity to Agarivorans sp. for two strains isolated only from U. rigida. Strains with 100% similarity to Phaeobacter gallaeciensis and Ph. inhibens were isolated both from U. australis and U. rigida. The antagonistic Phaeobacter strains will be used in experimental colonization studies of Ulva cultures.
Similar studies are being carried out with other species of Ulva, and a comparative analysis of the evolution of epiphytic bacterial communities of the different Ulva cultures is being conducted.
Acknowledgements
This work was funded by Spanish Ministerio de Economia y Competitividad (AGL2013-41868-R).