Expanded algal cultivation can reverse key planetary boundary transgressions

Substrate properties as controlling parameters in attached algal cultivation

There is growing interest in attached algae cultivation systems because they could provide a more cost- and energy-efficient alternative to planktonic (suspended algae) 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 biofuels. New algal biorefinery pathways have assessed the commercial potentials of algal biomass beyond biofuel production and placed more emphasis on value-added products from that biomass. Therefore, algal strain selection criteria and biomass cultivation methods need to be updated to include additional strains 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 review explores the effect of substrate chemical and topographical characteristics on the cultivation of attached algae. It also highlights the importance of considering algal community structure and attachment mechanisms in investigating attached algae systems using the example of filamentous algae found in algal turf scrubber (ATS™) systems. KEY POINTS : • Attached algal cultivation is a promising alternative to planktonic cultivation. • Performance increase results from tuning surface qualities of attachment substrates. • Attachment adaptation of periphytic algae has innate potential for cultivation.

Carbon limitation in hypereutrophic, periphytic algal wastewater treatment systems

Global eutrophication degrades water quality in freshwater ecosystems and limits the availability of freshwater for human consumption. While current wastewater treatment facilities (WWTF) remove pathogens and pollutants, many US WWTF continue to discharge nutrients that contribute to eutrophication. Traditional nutrient removal technologies can effectively reduce eutrophication risk, but can have unintended negative consequences on human and environmental health. Alternatives, such as algae-based treatment systems, improve the sustainability of the nutrient recovery process by producing biomass that can be converted to biofuel. However, research is needed to increase the productivity of algal treatment systems to improve their economic viability. Because algae in wastewater treatment systems are grown in wastewater rich in nutrients, the algae could become limited by dissolved inorganic carbon. This hypothesis was tested in 1.2 m long recirculating floways (n = 8 for each treatment/control) by quantifying algal dry mass and wastewater nutrient concentrations in 3 independent experiments: (1) carbon dioxide gas infused vs air infused control; (2) hydrochloric acid acidified vs neutralized solution control vs no chemical addition control; and (3) sodium bicarbonate addition vs no chemical control. Results showed increases in algal biomass after 18 days in wastewater augmented with dissolved inorganic carbon (carbon dioxide or sodium bicarbonate). In contrast, maintaining wastewater at near neutral pH with hydrochloric acid reduced algal productivity relative to controls. Nutrient reductions generally paralleled algal biomass increases except in the bicarbonate addition experiment. These findings provide evidence for the importance of carbon limitation in algal wastewater treatment floways. These results could help explain why carbon dioxide infusions stimulate algae in treatment systems. Furthermore, these results suggest that algae in nutrient enriched, sun-exposed streams (e.g., agricultural ditches or urbanized streams) may become carbon limited during peak periods of productivity. These findings could have important implications for ecosystems undergoing eutrophication as atmospheric carbon dioxide concentrations continue to rise.

Socioeconomic prospects of a seaweed bioeconomy in Sweden

Seaweed cultivation is a large industry worldwide, but production in Europe is small compared to production in Asian countries. In the EU, the motivations for seaweed farming may be seen from two perspectives; one being economic growth through biomass production and the other being the provisioning of ecosystem services such as mitigating eutrophication. In this paper, we assess the economic potential of large-scale cultivation of kelp, Saccharina latissima, along the Swedish west coast, including the value of externalities. The findings suggest that seaweed farming has the potential of becoming a profitable industry in Sweden. Furthermore, large-scale seaweed farming can sequester a significant share of annual anthropogenic nitrogen and phosphorus inflows to the basins of the Swedish west coast (8% of N and 60% of P). Concerning the valuation of externalities, positive values generated from sequestration of nitrogen and phosphorus are potentially counteracted by negative values from interference with recreational values. Despite the large N and P uptake, the socioeconomic value of this sequestration is only a minor share of the potential financial value from biomass production. This suggests that e.g. payment schemes for nutrient uptake based on the socioeconomic values generated is not likely to be a tipping point for the industry. Additionally, seaweed cultivation is not a cost-efficient measure in itself to remove nutrients. Policy should thus be oriented towards industry development, as the market potential of the biomass will be the driver that may unlock these bioremediation opportunities.

Vetiver and Dictyosphaerium sp. co-culture for the removal of nutrients and ecological inactivation of pathogens in swine wastewater

Swine wastewater poses chemical and biological risks because it contains high concentrations of ammonia and diverse species of pathogens. Herein, a vetiver-Dictyosphaerium sp. co-culture for the rapid removal of ammonia and the effective inactivation of pathogens was developed. Plants and microalgae benefited mutually and co-utilized the nutrients in the wastewater in the co-culture. The pathogens were inactivated by reactive oxygen species that were released by the microalgae as well as the supersaturated concentrations of dissolved oxygen in the enclosed bioreactor. In a greenhouse experiment, the time required for wastewater NH4-N to decrease from 102 mg L-1 to 5 mg L-1 was 65.5 days, 34.2 days, and 13.3 days in the plant culture, the algal culture, and the plant-algal co-culture, respectively. Among the 35 detected genera of bacteria, the operational taxonomic units for 31 tended to decrease with culture time in the plant-algal co-culture. Additionally, certain bacteria (e.g., Escherichia spp.) were completely removed by day 9 or 15, and the aerobic phototrophic bacterium Erythromicrobium spp. became most abundant on day 15 in the plant-algal co-culture. Important positive interactions that were observed between plants and microalgae included co-utilization of the nutrients, wastewater acidification through plant root respiration and algal growth with reduced ammonia toxicity, algal depletion of bicarbonate and alleviation of bicarbonate toxicity to plants, and release of oxygen from algal photosynthesis and plant growth with reduced hypoxic stress.