Investigating the effect of substrate properties on attached algal cultivation

Abstract

There is growing interest in attached algae cultivation systems because they could provide a more cost and energy-efficient alternative to suspended (planktonic) microalgae cultivation systems for many applications. However, attached growth systems have been far less studied than planktonic systems and have largely emphasized algae strains of most interest for biofuel production. Currently, through the introduction and exploration of new algal biorefinery pathways, commercial potentials of algal biomass are being assessed beyond biofuel production. More emphasis is being placed on the production of value-added products from the biomass while coupled with water remediation. Therefore, algal strain selection criteria and biomass cultivation methods need to be updated for improved efficiency. One possible way of improving attached cultivation systems is through engineering substrate surface characteristics to boost algal adhesion and enable strain selective algal colonization and growth. This work has explored the effect of substrate chemical and topographical characteristics on the attached cultivation of algae with a focus on periphytic filamentous algae. There is a need to understand the practical implications of the substrate surface characteristics and algae-substrate interactions on attached cultivation, especially in the case of less explored groups such as periphytic filamentous algae. Periphyton-based systems and the filamentous algae that often dominate them are known for their superior wastewater treatment capabilities and have shown promise as a source of biomass. These algae can form extensive three-dimensional networks of filament growing attached to a substrate (often through specialized attachment mechanisms), differentiating their attached growth from prostrate microalgal biofilms. Yet, lack of knowledge about their substrate preferences and lack of controlled studies tailored for the attached cultivation of these species in a scale appropriate to their community structure is an obstacle to utilizing these algae for commercial applications. This work aimed to fill the knowledge gaps about periphyton-based algae systems by designing an intermediate scale framework for studying the attached cultivation and substrate effects of periphytic filamentous algae. This was done through the design and optimization of a flow way photobioreactor for the cultivation of filamentous algae Stigeoclonium tenue on polylactic acid (PLA) substrate surfaces. The photobioreactor design and the cultivation protocols developed were employed to study the effect of substrate chemical and topographical characteristics on the attached growth of Stigeoclonium tenue. The results of this study showed the effectiveness of simple millimeter-scale surface topographical features in the shape of concave hemispheres in significantly increasing the amount of biomass cultivated in a given period of time without increasing the footprint of the cultivation area. Additionally, investigating the material-related effects revealed that substrate chemical composition effects are more prominent at the early stages of adhesion. This work has also attempted to develop and modify surface characterization methods for finding zeta potential and surface energy of green periphytic filamentous algae like Stigeoclonium tenue through contact angle and streaming potential measurements, respectively. Characterization of physicochemical characteristics of the surface enabled modeling of the algae-substrate interactions for this algae through the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach. Overall, this work provides fundamental insights into understanding and engineering attached cultivation systems for periphytic algae, which are naturally advantageous for attached growth and remediation applications. In addition to the primary research topic, this work also presented results on the development and implementation of engineering activities focused on water quality and water quality assessment for K-12 students and college freshmen and assessed and showed the positive impact of the activities on engaging the students and introducing engineering as an altruistic career path.