Date Published: July 18, 2011
Author(s): Diane Esson, Susanna A Wood, Michael A Packer.
Benthic species of algae and cyanobacteria (i.e., those that grow on surfaces), may provide potential advantages over planktonic species for some commercial-scale biotechnological applications. A multitude of different designs of photobioreactor (PBR) are available for growing planktonic species but to date there has been little research on PBR for benthic algae or cyanobacteria. One notable advantage of some benthic cyanobacterial species is that during their growth cycle they become positively buoyant, detach from the growth surface and form floating mats. This ‘self-harvesting’ capability could be advantageous in commercial PBRs as it would greatly reduce dewatering costs. In this study we compared the growth rates and efficiency of ‘self-harvesting’ among three species of benthic cyanobacteria; Phormidium autumnale; Phormidium murrayi and Planktothrix sp.. Phormidium autumnale produced the greatest biomass and formed cohesive mats once detached. Using this strain and an optimised MLA media, a variety of geometries of benthic PBRs (bPBRs) were trialed. The geometry and composition of growth surface had a marked effect on cyanobacterial growth. The highest biomass was achieved in a bPBR comprising of a vertical polyethylene bag with loops of silicone tubing to provide additional growth surfaces. The productivity achieved in this bPBR was a similar order of magnitude as planktonic species, with the additional advantage that towards the end of the exponential phase the bulk of the biomass detached forming a dense mat at the surface of the medium.
Algal productivity is up to an order of magnitude greater than that of most terrestrial crops making them a promising feedstock for many industrial processes (Packer 2009; Stephens et al. 2010). Atmospheric carbon dioxide is fixed into biomass by photosynthetic activity as part of the biogeochemical cycling of carbon. Biofuel production offers potential recycling of anthropogenically-released carbon from fossil stores. Their high productivity has therefore led to a great deal of interest in commercial-scale algal production. Despite the promise, and a good deal of hype, several hurdles remain before economic commodity-scale algal farming becomes a reality. Two significant restraints are a lack of cost-effective bioreactor technology and efficient harvesting (dewatering) techniques (Norsker et al. 2011). There are two main ways of growing algae; open ponds and enclosed bioreactors systems (Bosma et al. 2007; Heubeck and Craggs 2007; Packer 2009) both of which are generally used for planktonic species where the cells are in suspension. Very little attention has been given to developing growth systems for benthic species, those that require a surface to grow on (Mulbry et al. 2006).
Five days after inoculation, both Ph. autumnale (CAWBG26) and Planktothrix sp. (CAWBG59) had filament growth extending from the initial inoculation site across the walls of the culture bottles. By the thirteenth day of growth, all available growth surfaces were covered and there was a visible ring of filaments at the surface of the media. In addition, CAWBG26 was spreading across the media surface. CAWBG59 and CAWBG26 continued to grow across the media surface and gradually peeled from the pottle sides until complete detachment. In contrast, Ph. murrayi (CAWBG22) increased in bulk at the initial inoculation site, with very little spread of filaments across the bottom of the culture bottle.
Growth rates in cultures of planktonic species can be monitored via regular sampling and cell counts. These methods cannot be applied to benthic mat-forming species without disturbing their mat-like growth. In this study we developed a method which involved inoculating multiple 30 mL cultures with approximately 7 mg of benthic cyanobacterial strains and harvesting replicate containers regularly through a growth curve. Leflaive and colleagues (Leflaive and Ten-Hage 2009) used a similar approach with the filamentous cyanobacterium Uronema confervicolum to study allelopathic interactions in complex benthic communities. Our approach requires careful experimental design with many replicate cultures to have sufficient data points over important phases of a growth curve. Despite this difficulty it was found to work well enabling accurate biomass comparison for the work presented here.
The authors declare that they have no competing interests.