The efficiency of a tubular
photobioreactor for the extensive mass production of Chlorella
minutissima in Crete
European Network for the Dissemination of Aquaculture
RTD Information (Q5CA-2000-30105) and previously FAIR-3837, Aquaflow ref. :
TL2003-181
The
aim of this research project was to study the efficiency of a modified
closed tubular photobioreactor (a technology used for extensive massive
production of microalgae),
for year-round production of Chlorella
minutissima under the special climatic conditions of South East
Mediterranean (Greece, Crete Island). The indexes applied
for the estimation of the effectiveness of the production method were: the
daily
and seasonal variations of the (a) physicochemical (photosynthetically
active radiation- PAR, temperature, pH), and (b) biological (photosynthetic
efficiency, growth rate, chlorophyll) parameters of the produced
phytoplanktonic biomass.
The
culture system was based on the increase of biomass due to the efficient
absorption of sun radiation. The system operated with automatic control/test
for water/nutrients and CO2 supply, and continuous airflow. Chlorella minutissima (endemic species in Crete) was
cultured from September to July in six periods. The temperature of the
culture medium was maintained at the outside edge of the algae and the mean
monthly value was 16.5±2.8°C minimum and 24.7±2.9°C
maximum, due to the freezing system used. The minimum temperature reported
at early morning in winter was 11.2°C. The pH ranged 6.9 to 7.6.
The highest pH was reported during the morning and midday, indicating a high
demand in CO2.
In
Crete, high mean values of PAR (1.256±515–1.750±110μmol/m2/sec) were observed for a period of 9 consecutive months
and low values (488±670–695±352μmol/m2/sec) only from December to February.
The
initiation of productive phase, specified by the maximum level of cell
concentration, varied with the cultivation period, e.g. in summer the
culture reached the maximum number of cells per ml after 7 days, in winter
after 10 days. The duration of exponential phase was 17 days in summer and
24 days in winter. The mean cell concentration for all the cultures was more
than 200 millions cells/ml over 2μlPCV/ml. The photosynthetic performance
Fv/Fmax was above 0.7 and exceptions for summer and autumn cultures were due
to photo-inhibition under high irradiation intensity
and lack of nutrients respectively. The
total chlorophyll quantity decreased from May to October while Chl a/b
increased, showing photo-adaptation to high active photosynthetic radiation
(>1700μmol/m2/sec).
In conclusion, this
modified closed tubular photobioreactor is of high effectiveness in the climate conditions
of South East Mediterranean and with a reduced cost up to 50% compared to
other relative technologies. Alternative carbon sources and determination of
nutrient demands during the year and in relation with cell concentration
should be the next step for higher photosynthetic efficiency of the system
and therefore higher productivity and lower costs.
For more information, contact:
P. Divanach
Hellenic Centre Marine Research
Department of Aquaculture
PO Box 2214, Post of Poros
GR-71003 Iraklio Crete - Greece
Tel.: +30 810 241 892
Fax: +30 810 241 882
E-mail: divanach@imbc.gr