There was no significant difference in growth between the N
stressed cultures and the control cultures. Final biomass concentration
was similar in the N stressed (0.66 ± 0.04 g L1
) and control
cultures (0.59 ± 0.06 g L1
). In the N stressed cultures, N
was depleted on day 5 and this resulted in carbohydrate accumulation
and spontaneous settling of the filaments. The settling effi-
ciency increased to 93 ± 1% and the carbohydrate content was
about 74 ± 3% on day 9 in the N stressed culture. On day 9, there
was almost no settling in the control culture and the carbohydrate
content remained low (13 ± 1%). The accumulation of carbohydrates
under N stress has also been observed in previous
studies. Arthrospira responds to N stress by reducing protein synthesis
(which requires large amounts of N) and redirecting photosynthesis
products towards production of storage carbohydrates
(Hasunuma et al., 2013). Recent evidence suggests Arthrospira
may even convert proteins into storage carbohydrates during N
stress (Deschoenmaeker et al., 2014; Hasunuma et al., 2013). A
previous study has also observed spontaneous flocculation of
Arthrospira, but under P stress rather than N stress (Markou
et al., 2012).