Microalgal Downstream Processing: Harvesting, Drying, Extraction, Separation, and Purification 101
Sonication
Sonic waves with a frequency of around 25 kHz can generate cavitation that can implode. Cavitation
implosion creates localized shock waves and high temperature which can disrupt algae membranes.
Microalgae cultures of Thalassiosira pseudonana and Thalassiosira fluviatilis were submitted to an
ultrasonic ice/water bath during extraction with hexane for 20 min (Neto et al. 2013), yielding higher
yields of lipids than traditional extraction technique.
The sonication effect is very much dependent on the energy and duration of sonication treatment as
well as on the type of algae treated.
Optimization of the ultrasonic treatment should be considered to maximize both the energy and the
time of treatment for cell disruption as shown by Gerde et al. (2012). In that study it was found that for
the seawater heterotrophic species Schizochytrium limacinum, an energy input of 800 J/10 mL (regardless
of cell concentration) achieved optimal lipid extraction. Higher energy levels did not improve lipid yield
even though greater cell disruption was observed under microscope.
In another study, Nowotarski et al. (2012) investigated the effect of ultrasonic disruption of two
unicellular algal species, N. oculata and D. salina, with regard to release of lipids for biofuel production.
In this study, D. salina on the one hand proved to be susceptible to sonication at low algal densities with
90% reduction in cell number after 1 minute and complete cell disruption after 4 minutes of treatment.
On the other hand, for N. oculata sonication allowed declumping of algal clumps into individual cells
(20% increase in cell number), but this species turned out to be more resistant to sonication even after
16 minutes of treatment as a reduction in metabolic activity could be monitored but no cell disruption.
McMillan et al. (2013) performed disruption of N. oculata with an ultrasonic bath for 20 minutes and
evaluated the effect of the treatment by quantifying the damaged cells using direct optical microscopy
techniques. This work confirmed that liquid ultrasonic shear treatment was the least effective on this alga
as it only achieved 67.66% cell disruption. Halim et al. (2012) investigated the effect of sonication on the
microalga Chlorococcum sp. and found that ultrasonication of both low-density and high-density stock
cultures at low (65 W) and high (130 W) power levels was inefficient to disrupt the cells as observed
by microscope with an average disruption of 4.5% of the initial intact cells. However, sonication
disintegrated the microalgal colonies as it was observed that the colony diameter was reduced and this
effect was directly correlated with the acoustic power input.
Oil extraction from the microalgal biomass of two marine algal species Nannochloropsis sp. and
D. tertiolecta was performed in a Soxhlet apparatus with n-hexane as solvent. Prior to Soxhlet extraction,
the biomass was sonicated to achieve cell disruption and treated with propanol (Gouveia and Oliveira
2009) yielding oil contents of 28.7% and 16.7% for Nannochloropsis sp. and D. tertiolecta, respectively.
Neto et al. (2013) also used sonication as pretreatment of biomasses of Chlorella minutissima,
T. fluviatilis, and T. pseudonana for lipid extraction. Lipid content extracted by hexane after sonication
was found out to be similar to previous studies for the three species (15.5, 40.3, and 39.5% of DW).
However, for the diatoms, it was observed that a vortex-mixing step was necessary to rupture the frustule
during the resubmission of cell debris to hexane extraction as the lipid content only reached 8–12% of the
dry weight without vortex-mixing. The vortex-mixing step did not improve the lipid recovery from the
Chlorophyceae algae and this was explained by the cell fragility when compared to the diatoms. Thus the
study concluded that sonication pretreatment improved the yield of lipid recovery when combined with
hexane extractions, as well as with vortex-mixing for diatoms.
Efficiency of sonication is also related to the components to be extracted. Ryckebosch et al. (2012)
showed that sonication allowed 40% total lipids extraction of dry weight of P. tricornutum algae, while
only 18% extraction was achieved for non-polar lipids. Balasubramanian et al. (2013) demonstrated that
for the biomass of Nannochloropsis sp. sonication was not the most efficient method to extract lipids
compared to accelerated solvent extraction (ASE), Soxhlet extraction and homogenization as the yield of
lipids extracted was 22, 33, 30, and 28%, respectively. Similarly, lipid extraction using sonication from
Nannochloropsis sp. (Koberg et al. 2011) only afforded 18.9% biodiesel compared to the microwave
which afforded 32.8%.
Sonication
Sonic waves with a frequency of around 25 kHz can generate cavitation that can implode. Cavitation
implosion creates localized shock waves and high temperature which can disrupt algae membranes.
Microalgae cultures of Thalassiosira pseudonana and Thalassiosira fluviatilis were submitted to an
ultrasonic ice/water bath during extraction with hexane for 20 min (Neto et al. 2013), yielding higher
yields of lipids than traditional extraction technique.
The sonication effect is very much dependent on the energy and duration of sonication treatment as
well as on the type of algae treated.
Optimization of the ultrasonic treatment should be considered to maximize both the energy and the
time of treatment for cell disruption as shown by Gerde et al. (2012). In that study it was found that for
the seawater heterotrophic species Schizochytrium limacinum, an energy input of 800 J/10 mL (regardless
of cell concentration) achieved optimal lipid extraction. Higher energy levels did not improve lipid yield
even though greater cell disruption was observed under microscope.
In another study, Nowotarski et al. (2012) investigated the effect of ultrasonic disruption of two
unicellular algal species, N. oculata and D. salina, with regard to release of lipids for biofuel production.
In this study, D. salina on the one hand proved to be susceptible to sonication at low algal densities with
90% reduction in cell number after 1 minute and complete cell disruption after 4 minutes of treatment.
On the other hand, for N. oculata sonication allowed declumping of algal clumps into individual cells
(20% increase in cell number), but this species turned out to be more resistant to sonication even after
16 minutes of treatment as a reduction in metabolic activity could be monitored but no cell disruption.
McMillan et al. (2013) performed disruption of N. oculata with an ultrasonic bath for 20 minutes and
evaluated the effect of the treatment by quantifying the damaged cells using direct optical microscopy
techniques. This work confirmed that liquid ultrasonic shear treatment was the least effective on this alga
as it only achieved 67.66% cell disruption. Halim et al. (2012) investigated the effect of sonication on the
microalga Chlorococcum sp. and found that ultrasonication of both low-density and high-density stock
cultures at low (65 W) and high (130 W) power levels was inefficient to disrupt the cells as observed
by microscope with an average disruption of 4.5% of the initial intact cells. However, sonication
disintegrated the microalgal colonies as it was observed that the colony diameter was reduced and this
effect was directly correlated with the acoustic power input.
Oil extraction from the microalgal biomass of two marine algal species Nannochloropsis sp. and
D. tertiolecta was performed in a Soxhlet apparatus with n-hexane as solvent. Prior to Soxhlet extraction,
the biomass was sonicated to achieve cell disruption and treated with propanol (Gouveia and Oliveira
2009) yielding oil contents of 28.7% and 16.7% for Nannochloropsis sp. and D. tertiolecta, respectively.
Neto et al. (2013) also used sonication as pretreatment of biomasses of Chlorella minutissima,
T. fluviatilis, and T. pseudonana for lipid extraction. Lipid content extracted by hexane after sonication
was found out to be similar to previous studies for the three species (15.5, 40.3, and 39.5% of DW).
However, for the diatoms, it was observed that a vortex-mixing step was necessary to rupture the frustule
during the resubmission of cell debris to hexane extraction as the lipid content only reached 8–12% of the
dry weight without vortex-mixing. The vortex-mixing step did not improve the lipid recovery from the
Chlorophyceae algae and this was explained by the cell fragility when compared to the diatoms. Thus the
study concluded that sonication pretreatment improved the yield of lipid recovery when combined with
hexane extractions, as well as with vortex-mixing for diatoms.
Efficiency of sonication is also related to the components to be extracted. Ryckebosch et al. (2012)
showed that sonication allowed 40% total lipids extraction of dry weight of P. tricornutum algae, while
only 18% extraction was achieved for non-polar lipids. Balasubramanian et al. (2013) demonstrated that
for the biomass of Nannochloropsis sp. sonication was not the most efficient method to extract lipids
compared to accelerated solvent extraction (ASE), Soxhlet extraction and homogenization as the yield of
lipids extracted was 22, 33, 30, and 28%, respectively. Similarly, lipid extraction using sonication from
Nannochloropsis sp. (Koberg et al. 2011) only afforded 18.9% biodiesel compared to the microwave
which afforded 32.8%.