NEW PROCESS FOR MICROALGAE CULTIVATION FOR HATCHERY/NURSERY APPLICATION

Microalgae production is essential in  shellfish  hatchery/nursery.  Among species that are cultivated, Tetraselmis  and Isochrysis are common strains in shellfish structures. The aim of the study was to test ultrafiltered seawater to cultivate/protect those microalgae at industrial scale and compare the growth obtained with classical seawater

Two microalgae species, Isochrysis lutea  (T-iso)  and Tetraselmis suecica (Tetra) were cultivated in 300 L tanks in two water qualities: seawater with common treatments (prefiltration, UV, filtration 5 µm, 0.22 µm and a second UV) and ultrafiltered seawater. This  last water was produced with an ultrafiltration unit able to treat 20 m³.d^-1 , equipped with PES hollow fibre membranes (pore size = 0.02 µm).

Every day, a volume ( between 50 and 150  L) was extracted from tanks to feed oysters and cultures and were completed to 300 L with targeted water.

Except water treatments, the 4 cultures were grown  in the same conditions (temperature, light, culture medium and  volume of microalgae extracted) . Physico-parameters of cultures and waters were controlled and microalgae concentrations were measured using spectrophotometer method. Tests were realized for 3 months with 2 cultures of Tetra and 4 cultures of T-iso

In the case of T-iso, experiments highlighted an impact of water quality on cell density (Figure 1) . Indeed,  in ultrafiltered water,  microalgae concentrations were  higher  for 3 tests from 6 to 30 %. For Tetra, no significative difference in concentration was observed. However, ultrafiltration led to a better protection of cultures  towards parasites  because no predator was observed  with  tetra microalgae contrary to control cultures where ciliates (Figure 2), for instance, where detected.

To conclude, the study showed that ultrafiltered seawater was adapted for microalgae culture. Moreover, advantages of this process compared to common treatments were put in light: (i) for T-iso, ultrafiltration lead to a higher growth, (ii) a better removal of parasites is obtained and (iii) a reduction of water treatment steps is realized (from 5 to 2 steps) . Now, microalgae are only cultivated in ultrafiltered seawater and the same conclusions were made for another microalgae specie, a diatom, Thalassosira weissflogii : high protection and better production.