Efficient use of bicarbonate for mass production and carbon isotopic labelling of the green alga Ulva ohnoi under natural conditions

Abstract

Mass aquaculture production of the green seaweed Ulva is advancing under the stimulus of many interesting applications. Increasing the efficiency of carbon use in Ulva mass cultures is important to improve its production and diversify biomass applications. Given the high capacity of Ulva species to use bicarbonate as a C source, mass cultures of Ulva ohnoi carried out under natural conditions and controlled pH were stoichiometrically supplied with sodium bicarbonate on demand. The specific growth rate (average 0.108 ± 0.008 day⁻¹) and the biochemical composition of U. ohnoi remained stable over three consecutive production cycles, in which C conversion from added bicarbonate into net produced biomass ranged between 88.5 ± 4.2 and 95.9 ± 0.9%. C conversion was significantly higher (p < 0.05) than conversion of nitrogen (73.4 to 87.4%) and phosphorus (68.7 to 80.2%). By enriching the total bicarbonate at 4% with H¹³CO₃⁻, the algal isotopic signature δ¹³C increased from − 7.5 ± 0.38 to 1505 ± 56‰ after 8 days of growth. Uptake of ¹³C was steady throughout the culturing period and resulted in biomass production estimates very similar to those calculated from the growth rate. The use of bicarbonate in Ulva cultures represents an important improvement in C conversion efficiency. It is also an easy alternative to produce biomass sufficiently enriched in ¹³C, which is demanded in studies on the nutritional value of algae. Reliable production estimates in mass cultures can be expected from the ¹³C uptake rate in algae with high bicarbonate utilization capacity.