Abstract:
Lipids from microalgae are renewable source of biofuels. Large amounts of algal
biomass are produced under optimal growth conditions with relatively low lipid
content. However, they have potential to accumulate high amount of lipids under
stress conditions. This study was aimed to enhance the lipid content of Chlorella
sp. by providing stresses such as deficiency of nitrogen and zinc and excessive
salinity during cultivation. Bold’s Basal Media (BBM) was used as the culture
media. Two control sets were used; one set cultured by adding appropriate
amount of BBM and other set cultured by adding 3 times of BBM. Five sets each
of three treatments and two controls were maintained in batch reactors
arranged randomly. Light intensity was kept constant. Photo period cycle was
followed to optimize microalgae growth. Air spargers were applied for uniform
mixing. From the beginning, all samples were grown under the same conditions
with adding appropriate amount of BBM except two batches; Zn stressed batch
which was maintained without adding Zn from the beginning and the second
control which was added with 3 times of BBM. After 7 days, stresses were
introduced to the remaining batches except controls and Zn stressed batch. N
stressed set was maintained without adding N. Salinity stress set was treated
with extra amount of NaCl (2 M). One set from each treatment/ control was
harvested on 4th, 8th, 11th, 14th and 17th day of cultivation. Maximum optical
density was achieved after 14 days. Harvested biomass was dried and lipids
were extracted from dried biomass using chloroform: methanol (1:1) mixture.
Lipid content of stressed samples increased, compared to control samples
(highest was 16.9%). Zn stressed samples achieved highest lipid percentage
(highest was 23.3%). After certain period of time (14th day), lipid content of
stressed samples decreased. Gas chromatographic analysis of the lipid identified
the major fatty acids such as C11:0 (4.6%), C18:0 (35%), C18:2 (30%) and C21:0
(6%). Therefore, lipid content can be increased significantly by providing
stresses during cultivation and zinc deficiency was found to be more effective
than other stress conditions studied to enhance the lipid content of Chlorella sp.
