Differential growth and biochemical composition of photoautotrophic and heterotrophic <Emphasis Type="Italic">Isochrysis maritima</Emphasis>: evaluation for use as aquaculture feed

The growth and biochemical composition of photoautotrophic and heterotrophic Isochrysis maritima in 50 L of Walne’s medium were compared. Heterotrophic I. maritima fed with 0.02 M glucose had a 4.6-fold higher maximum cell density (38.17 ± 0.23 × 10⁶ cells mL^?1) than photoautotrophic cells (8.29 ± 0.70 × 10⁶ cells mL^?1). The carbohydrate content was slightly higher in heterotrophic cells at all growth stages (mid-exponential, 40.8%; early stationary, 48.3%; and late stationary, 47.6%), but there was no significant effect on the protein content under either trophic condition. The total saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) were higher under heterotrophic conditions than those under photoautotrophic conditions. However, because omega-3 PUFAs are the most essential element in feed nutrition, low results for eicosapentaenoic acid (EPA) (0.28 ± 0.06%) and docosahexaenoic acid (DHA) (3.22 ± 0.26%) in the heterotrophic cells compared to the photoautotrophic cells (EPA: 0.44 ± 0.11%; DHA: 8.58 ± 0.73%) plus a low omega-3/6 PUFAs ratio (heterotrophic: 0.16–0.47; photoautotrophic: 2.60–2.88) and high value of (SFA + MUFA)/PUFA (heterotrophic: 5.50–6.81; photoautotrophic: 2.64–3.60) showed that this species is not suitable for aquaculture feed when cultivated under heterotrophic conditions.