Continuous production of Chaetoceros calcitrans in a system suitable for commercial hatcheries

• Continuous culture of Chaetoceros calcitrans is feasible in a simple photobioreactor.
• Continuously produced C. calcitrans is equal to batch-produced C. calcitrans.
• Design and operation of a continuous C. calcitrans culture is described.

Chaetoceros calcitrans is a small, fast-growing diatom with a high nutritional value for filter feeders. It is often used in hatcheries, particularly in the production of bivalve spat. It is widely produced in batch culture systems ranging from carboys to plastic bags to high volume tanks. Continuous culture of C. calcitrans has generally not been successful due to frequent crashes of the culture. We report the continuous culture of C. calcitrans in a hanging plastic bag system that is suitable for use in commercial hatcheries. Runs of continuous production lasted up to 125 d, with an average bag life of 24.4 d. Crashes occurred at irregular intervals in individual bags and caused the cell concentration to drop by up to three orders of magnitude. Cultures recovered over a few days following a crash. The temporary reduction of production due to crashes in individual bags was compensated by a 30% higher production than required. At dilution rates of 1.0–1.3 volume exchanges d− 1 the system produced 7–13 × 106 cells mL− 1 d− 1. The cell diameter mode as determined by a Coulter Counter was 2.8–3.0 μm. The cells had the typical flat rectangular box shape with well-developed setae. The culturable bacterial population of the algae culture was 105–107 colony forming units mL− 1. Genetic integrity of C. calcitrans and monospecific status of the continuous culture were maintained over 7 sequential bag-to-bag transfers during a 125-day run. The fatty acid profiles of continuous and batch cultures of C. calcitrans were similar. Growth and survival of Greenshell™ mussel (Perna canaliculus) larvae were identical when fed with C. calcitrans batch culture or continuous culture delivered directly from the harvest line. When continuous culture was first collected over 24 h, diluted and then delivered to the rearing tanks over 24 h, larval growth was markedly slower (24 d to reach settlement competency compared to 21 d for the other two treatments); however the subsequent settlement success of competent larvae was not statistically different between the three diets (‘Carboy’, ‘Continuous/Harvest line’, ‘Continuous/Feed-out bin’; 33.9–35.5% settled). Concomitant to the differences in larval performance we observed a difference in larval fatty acid profile: lower-performing larvae had a markedly lower proportion of mono-unsaturated fatty acids and a markedly higher proportion of poly-unsaturated fatty acids. Lower performance and different fatty acid profiles of larvae fed the ‘Continuous/Feed-out bin’ diet appeared to be caused by unidentified changes that occurred during storage of the C. calcitrans culture.