Wild Mixed Culture Microalgae Biomass from UI Agathis Small Lake Harvested Directly using an Ultrasound Harvesting Module as Biofuel Raw Material

A follow-up study on the use of ultrasonic sound for harvesting a mixed culture of microalgae from a small lake for biofuel feedstock has been carried out. Observations were made at the Agathis small lake of Universitas Indonesia during the rainy season. The aim of the study was to determine the microalgae community members in Agathis with the potential for lipid production and to determine the lipid concentration in the mixed culture biomass from Agathis, harvested using an ultrasound harvesting module (UHM). The results showed that there were 11 species of microalgae from 10 genera and four classes. Nine of the 11 species of microalgae found in Agathis have the ability to produce lipids. The total biomass obtained from water samples from all stations in Agathis was 25.18 g, and the average lipid content of the biomass of Agathis water samples in March 2020 was 55.5%.

Lipid content; Microalgae; Mixed culture; UI Agathis small lake; Ultrasound Harvesting Module (UHM)

Biofuels are energy sources made from newly grown biomass (plants, animal by-products, or microorganisms). There are four generations of biofuels based on the biomass sources used, their limitations as a renewable energy source, and their technological advances. Based only on the biomass sources used, there are three generations of biofuels, i.e., the first generation uses biomass that is also a food source, the second uses biomass from non-food sources, and the third uses biomass from microorganisms. Meanwhile, the fourth generation of biofuels is more focused on genetically modified microorganisms (Alalwan et al., 2019; Mat Aron et al., 2020).

Third-generation biofuels use algae as an energy source (Singh et al., 2011; Cercado et., 2018). Microalgae can be grown using sewage, wastewater, and salt water, such as from oceans or salt lakes. Microalgae lipids can compete with petroleum in production costs if the percentage of lipid produced by microalgae is about 60% of the biomass (Shen et al., 2009).

One of the biggest obstacles to the production of renewable fuels using microalgae is the need for more water than cultivated plants (Singh et al., 2011). Several ways to overcome microalgae culture have been carried out by developing photobioreactors (Rizaldi et al., 2019; Santoso et al., 2020). However, the problem of the need for water is still an obstacle. To cope with the significant water needs, one of the water sources that can be used is wastewater. Wastewater can be obtained continuously and contains many nutrients. Nutrient levels in wastewater cause many microalgae to grow within (Dalrymple et al., 2013).

A mixed culture is a culture in which the inoculum always consists of two or more organisms. Mixed cultures may consist of a known species to the exclusion of all others, or they may consist of a mixture of unknown species. Several researchers have used mixed cultures as a tool to study biochemical and ecological interactions or to culture new microorganisms (Little et al., 2008). Mixed cultures can occur naturally in nature, such as in lakes (Sathish and Sims, 2012). One of the small lakes in the Universitas Indonesia (UI) area is Agathis (Direktorat Umum dan Fasilitas Universitas Indonesia, 2009).

In 2003, 2006, and 2011, 16 microalgae genera were found in Agathis (Prihantini and Wardhana, 2016). Of those 16 genera of microalgae, 10 have been studied, and their lipid contents are known (Mendes et al., 2006; Prabakaran and Ravindran, 2011; Mousavi et al., 2018; Khan et al., 2020). The study showed that most of the microalgae from Agathis could produce lipids. However, the overall ability of the Agathis microalgae community to produce lipids as a mixed culture was only studied modestly in the dry season. The Agathis microalgae community in the dry season is known to produce lipids, the percentage of which is 23% by weight of the biomass (Maulana et al., 2021).

Seasonal changes can affect the state of cultured microalgae, as they lead to changes in temperature and daily light intensity (Br?nmark and ??Hansson, 2017). Microalgae that experience growth inhibition due to environmental stress cause decreased protein production and increase lipid storage. This phenomenon causes less microalgae biomass to be produced with more lipid content per g of biomass (Karima et al., 2018).

The lipid extraction yield obtained is highly dependent on the method of harvesting the microalgae biomass, whether by filtration and flocculation (Fasaei et al., 2018). In addition, there is a method with the working principle of using ultrasonic waves, i.e., an ultrasound harvesting module (UHM). The UHM uses ultrasonic waves from two sources facing each other to collect microalgae cells in water. Microalgae cells collected due to ultrasonic waves will settle due to gravity. The UHM method is also still in the experimental stage (Ardiansyah et al., 2020).

    The results of the research are as follows. In total, 11 species of microalgae were identified in Agathis in the rainy season. The microalgae community in Agathis was dominated by Eudorina sp., with an average dominance percentage of 90.652%. The average theoretical lipid content of the biomass obtained from Agathis is 3.1893%, and the lipid content of the biomass from the Agathis water sample exceeded the theoretical lipid content of 55.5%. The lipid content of the biomass from the Agathis water samples is insufficient for industrial scale, but it has the potential to be used as a more adequate lipid source. The development and improvement of the UHM will be carried out for use in field-scale harvesting.

    This work was funded by the grant of Hibah PDUPT 2021 from the Ministry of Research and Technology/National Research and Innovation Agency (Kementerian Riset dan Teknologi/BRIN) Indonesia to Dr. Nining Betawati Prihantini, M.Sc., grant number: NKB-178/UN2.RST/HKP.05.00/2021.

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