Two macroalgae (Gracilaria sp., 36% of crude protein (CP) and 1% of crude lipid (CL) and Ulva sp., 15% CP and 1% CL) and three microalgae (Nannochloropsis sp., 36% CP and 11% CL; Chlorella sp., 56% CP and 10% CL and Tetraselmis sp., 27% CP and 7% CL) were subjected to two technological process (physic and enzymatic) and dried.
Microscopy observation and the determination of soluble protein (Bradford method) of samples were used to verify and confirm the algae cell wall disruption after processing. For determination of protein apparent digestibility coefficient (ADCs) in European sea bass, a commercial-based extruded diet with 49% CP and 13% CL was used as reference diet (REF) and 1% chromic oxide was added as inert marker. The test diets were obtained by replacing 30% of the REF diet by algae biomass, either before (whole alga) or after physic (PHY) or enzymatic (ENZ) processing. Homogeneous groups of 12 European seabass (62 ± 8.6g), subjected to a natural photoperiod were kept in a recirculating salt water system (salinity 35‰, 21 ± 1.8oC) with a Guelph system to collect feces. Each diet was randomly assigned to three tanks, for replication of results. The apparent digestibility coefficients (ADCs) of the experimental diets were calculated according to Maynard et al. (1979) and the protein ADCs of the test ingredients were estimated according to NRC (2011).
Both disruption methods promoted cell desegregation and increased the soluble protein fraction (SPF). The enzymatic process was very effective in Nannochloropsis sp. resulting in an increase in the SPF value from 0% in the non-processed to 4.3% in the processed counterpart. In the case of Chlorella sp. and Gracilaria sp., the physic process also increased SPF value from 0% in the non-processed algae to 5.1% and 2.5%, respectively, in the processed counterparts.
Among microalgae, the diet containing 30% ENZ Nannochloropsis sp. showed the highest protein ADC that was similar to the REF diet (>90%); concerning ingredients ADC, the highest protein ADC value was also observed for ENZ- Nannochloropsis (>80%). Unprocessed microalgae were generally well digested (>70%), having the Chlorella sp. the highest protein ADC value. Nannochloropsis sp. protein ADC can be further improved after enzymatic processing. The diet containing 30% PHY Gracilaria sp. showed the highest protein ADC that was similar to the REF diet (>90%); the highest protein ADC value among test ingredients was also observed for PHY- Gracilaria (>90%). Gracilaria sp. is a better protein source than Ulva sp. to be incorporated in sea bass diets, irrespectively of the processing, and can be further improved by physic processing.
