Low and variable productivity and low efficiency of mass cultures of the haptophyte Isochrysis aff. galbana (T-iso) in outdoor tubular photobioreactors

The haptophyte Isochrysis aff. galbana (T-iso) was cultured during 2 consecutive years (from spring to autumn) in outdoor horizontal tubular photobioreactors (PBRs) that had a size compatible with that recommended for hatchery service. Around one-third of the cultures carried out in these PBRs showed immediate growth after inoculation up to a maximum biomass. In all other instances a lag phase or a culture collapse were found. In the cultures that showed productivity, average growth rate, μ (d−1), productivity (g L−1 d−1) and photosynthetic efficiency, PE (%), were 0.39 ± 0.085 d−1, 0.075 ± 0.038 g L−1 d−1 and 2.51 ± 0.85%, respectively. These values were low compared to values for cultures routinely incubated indoors, which were used as inocula for the outdoor cultures, and were studied as a reference. In these 50-L acrylic columns (19.4 cm internal diameter) growth, productivity and PE were 0.91 ± 0.087 d−1, 0.076 ± 0.012 g L−1 d−1 and 13.72 ± 2.23%, respectively. Outdoor productivity of T-iso was limited to average temperatures above 15 °C, while below an average temperature of 21 °C growth rate and productivity were lower than above 21 °C. Above this temperature, solar irradiance and biomass density had a more important effect on growth rate and productivity than temperature. Analysis of our data revealed a critical value of the ratio between incident solar irradiance (MJ m−2 d−1) and cell density (106 cells ml−1) at the moment when the protective black netting was removed from the cultures. Cultures that had a ratio above 1.5 suffered an initial lag phase or collapsed, while cultures that showed direct growth had a value below 1.5. This seemed to be independent of absolute initial cell density. However, we recommend using cell densities higher than the ca. 5 × 106 cells ml−1 used in the present study. In the South of Spain this seems to be especially relevant in spring, when temperatures are relatively low in relation to incident solar irradiance. T-iso seems to be particularly sensitive to the variable conditions outdoors and we suggest that new designs of closed PBRs, specifically developed for outdoor cultivation of T-iso, try to achieve more stable and homogeneous conditions throughout the system and over the daily cycle.