Optimising
the production of micrioalgae rich in essential fatty acids
Aquaflow
Technical Leaflet 2003-135
European
Network for the Dissemination of Aquaculture RTD Information
(Q5CA-2000-30105) and previously FAIR-3837, URL: http://www.aquaflow.org/
T-ISO (Isochrysis aff. galbana)
is a microalgal strain widely used as live food in larviculture of fish,
shrimp and mollusc species. The success of the strain is based on its
advantageous fatty acid spectrum, combined with its relative ease of
culture. Apart from culture temperature, the light regime is critical, since
it determines the amount of microalgae produced per day, as well as its
fatty acid composition. The objective of this study was to determine the
optimal combination of Photon Flux Densities (PFD) and photoperiod for T-ISO
biomass production, and, secondly, to establish light regimes favouring the
production of highly unsaturated fatty acids (HUFA’s) in T-ISO.
Tests were run under three
different photoperiods (24:0, 16:08 and 12:12-h L/D), combined with three
PFD (120, 220 and 460 µmol photons m-2.s-1) at 25°C.
Sampling took place at both the exponential and post-exponential population
growth phases.
Results showed that
specific growth rate of T-ISO maximised with an increase of the total PFD
supplied per day. Under continuous light, cell size of T-ISO (cell dry
weight and cell volume) correlated positively to PFD with a further increase
when the cells were transiently light-limited in the post-exponential phase.
In contrast, cell dry weight under discontinuous light increased only at
subsaturated PFD, with a significant decrease in the post-exponential phase.
Cell size did not correlate to cell dry weight under discontinuous light,
revealing an intracellular density change. As a result, biomass yield (mg.l-1)
and productivity (mg.l-1 day-1) had different patterns
for continuous and discontinuous light, whereas cell yield (cells.l-1)
and cell productivity (cells.l-1 day-1) were simply a
function of total PFD per day.
This study also reports
the fatty acid profiles of T-ISO at these combinations of photoperiod and
PFD. In general, fatty acid proportions were affected by a strong
interaction of L:D x PFD, resulting in metabolic changes difficult to be
modelled. Fatty acid differences were located in the n-3 fraction with the
n-6 content being rather constant. The ratios n-3/n-6 and DHA (22:6n-3) over
EPA (20:5n-3) at 24:0 or 12:12-h were optimal for fish and shellfish
nutrition: according to literature, n-3/n-6 and DHA/EPA ratios should be
higher than 2, and well above 1, resp. The 16:08-h regime, especially at low
PFD, produced inadequate ratios.
The production of n-3 HUFA
in T-ISO is essentially influenced by the total photon flux available per
day. The capacity of the strain for storing lipids is limited under the
conditions tested; consequently, the fatty acid content follows the biomass
yield and productivity pattern. Hence, in the context of aquaculture a light
regime of 12:12-h and a PFD within the photolimitation-photoinhibition range
offers advantages for the culture of T-ISO. If the high investment can be
substantiated, continuous cultures under 24:0-h L/D at the same PFD range
could serve to further optimise ptroduction using advanced photobioreactors.
This research was part of
a collaborative GOA project on Larval Nutrition, sponsored by the Belgian
Ministry of Science Policy, run by the Laboratory of Aquaculture &
Artemia Reference Center and the Laboratory of Environmental Toxicology
& Aquatic Ecology, both Ghent University, Belgium.
For more information:
Ioannis Tzovenis
Laboratory of Aquaculture & Artemia Reference Center
Ghent University
Rozier 44, B9000 Gent - Belgiu
Tel.:+32
9 264 37 54; Fax:
+32 9 264 41 93
E-mail: itzoveni@eexi.gr