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Despite a growing interest in microalgae and cyanobacteria as potential sources of nutrients in aquafeeds, little information is presently available on their nutritive value for carnivorous fish species. The aim of this study was to evaluate chemical composition and nutrient digestibility of a panel of microalgae and cyanobacteria dried biomasses (MACB), using rainbow trout (Oncorhynchus mykiss W.) as a fish model. Nine test diets were obtained by mixing 80 parts of a reference diet, added with 20 g/kg of acid insoluble ash as an indigestible marker, to 20 parts of each of the following dried whole-cell biomass: Arthrospira platensis, Nostoc sphaeroides, two strains of Chlorella sorokiniana, Nannochloropsis oceanica, Tisochrysis lutea, Phaeodactylum tricornutum, Porphyridium purpureum and Tetraselmis suecica. The digestibility measurements were conducted with rainbow trout (52.4 ± 1.5 g) kept in six tank units each including three 60-L vessels singularly stocked with 12 fish and fitted with a settling column for faecal recovery. Per each diet, faeces were collected over three independent 10-day periods. Apparent digestibility coefficients (ADCs) of dry matter, crude protein (CP), organic matter and gross energy (GE) of single MACB were calculated by difference relative to those of the reference diet. The MACBs had heterogeneous chemical composition (CP, from 20 to 69%; Lipid, 5-27%; GE, 12.5--22.6 MJ/kg dry matter basis) reflecting their overall biodiversity. Most of them can be considered as virtually good sources of minerals and trace elements and exhibit an essential amino acid profile comparable or even better than that of soybean meal commonly used in fish feeds with P. purpureum showing the best protein profile. The digestibility results obtained with rainbow trout allowed ranking the MACBs into two major groups. A first one, including C. sorokiniana, N. oceanica and T. suecica, resulted in markedly lower (P < 0.05) crude protein and energy ADC (64-73%; 51-59%, respectively) compared to a second group including P. purpureum, T. lutea and cyanobacteria (CP-ADC, 83-88%; GE-ADC, 74-90%) while P. tricornutum resulted in intermediate values. Overall, the present study confirms the consistently reported role of cell-wall structure/composition in affecting accessibility of nutrients to digestive enzyme. Based on the overall outcomes, only T. lutea and cyanobacteria actually meet the requirements for being used as protein sources in aquafeeds provided their mass production becomes more feasible and cost-effective, hence attractive for the feed-mill industry in the near future.

Recently, the insects meal has been released by the European Union as an ingredient in aquafeeds and only seven insect species have been allowed to be used for these preparations. To avoid misleading results, a suitable method for the detection of these insect species in food/feed, would be highly desirable. In this paper, we present an innovative, sensitive, specific and low-cost Point-of-Care apparatus, based on DNA probes for the detection of one of the most popular insects used in aquafeed preparation, Hermetia illucens (HI), commonly known as black soldier fly. The DNA probes adopted for this genosensor have been designed to enable a very specific insect recognition which were derived from DNA primers. The DNA primers were first tested with qPCR. Forty different samples (animal species and common aquafeed ingredients) have been tested to check the possible cross-reactivity, confirming the adequate primers specificity. The primers have been used in a practical application, like the detection of HI in complex matrices of practical aquafeeds. The DNA probes derived by these primers have been then applied to implement an Organic Light Emitting Diode (OLED) based genosensor. The developed genosensor has been tested with HI DNA directly extracted from the insect, without any DNA amplification; a good test sensitivity has been obtained, being 0.75 ng/μl its detection (LOD) limit. Then, the genosensor has been tested with the DNA extracted from the same aquafeeds used in the qPCR experiments. The results obtained by these two methods have then been compared giving very good consistency and both allowed to detect HI meal up to a minimum threshold of 5 g/100gr feed.