Comp. Biochem. Physiol. VoL 105B,No. 2, pp. 369-373, 1993 .0305-0491/93 $6.00+ 0.00 Printed in Great Britain PergamonPress Ltd THE BIOCHEMICAL COMPOSITION OF TWO DIATOMS AFTER DIFFERENT PRESERVATION TECHNIQUES BEATRIZCORDEROESQUIVEL,DOMENICO VOLTOLINA LOBINA and FRANCISCOCORREASANDOVAL Centro de Investigaci6n Cientifica y de Educaci6n Superior de Ensenada, Departamento de Acuicultura. Av. Espinoza # 843.A.P.2732, Ensenada, Baja California, M6xico (Fax 011-52-667-4-48-80) (Received 27 October 1992; accepted 27 November 1992) Abstract--1. Air- and freeze-dryingcaused an approximately 70% loss of total lipids, while freezing left unchanged the proximate composition of the two diatoms Chaetoceros sp. and Phaeodactylum tricornutum. 2. Loss of total organics after two months of storage was approximately 20% for air- and freeze-dried algae, but lower (10 and 5% for Phaeodactylum tricornutum and Chaetoceros sp., respectively) in the samples stored in a commercial freezer. 3. The amino acid profiles are similar to those generallyreported for algal single-cell protein. Both algae may be considered a good source of dietary protein for aquaculture since they contain all the amino acids considered essential for fish and shrimp, including methionine. 4. Short-chain (14 and 16 C) fatty acids are 90% or more in Chaetoceros, while Phaeodactylum has more than 30% of 18 to 22 C. Unsaturates are approximately 50% for Chaetoceros and vary between 44 and 63% in Phaeodactylum, depending on preservation and storage. 5. Storage caused an increase in the percentages of the essential fatty acids 18:2 and 18: 3. INTRODUCTION The traditional diet for many cultured aquatic organisms are live microalgae. For this, aquacultur- ists need large-scale microalgal cultures, which add considerably to the costs of production and involve an additional factor of risk compared to artificial food. For these reasons, the feasibility of using alternative diets, such as micropelleted artifi- cial food or microalgae preserved with differ- ent techniques, has been under investigation for several years. In spite of the advances in artificial feed formulation and presentation (Jones et al., 1974; Chu et al., 1982), researchers have been focussing their attention more on preserved microal- gae, since they are the natural diet of the organisms to be fed. Frozen concentrated cultures have been used as main diet or as a complement to live ones in shellfish cultures (Sommer et al., 1990) and freeze-, spray- or air-dried or simply refrigerated algal con- centrates have been suggested by many authors for some mollusks and crustaceans (Brown, 1972; Laing et al., 1990; Nell and O'Connor, 1991; Laing and Millican, 1992). The main concern in all these studies is the fact that the biochemical composition of mi- croalgae might be altered by the preservation and storage techniques, which in turn could affect the survival and growth of the organisms feeding on them. The present study examines the proximate and biochemical composition of two diatoms which have been used successfully for aquaculture in Mexico, and how these may be affected by three different preser- vation techniques. MATERIALS AND METHODS The algae used are the clonal strains CH-X-I (Chaetoceros sp.) and PH-T-1 (Phaeodactyum tricor- Durum Bohlin) of CICESE's collection (Voltolina et al., 1991). Biomass was produced in semi-continu- ous 15 1 cultures kept in steady-state by 50% daily dilutions. Lighting was continuous, with a concen- tration of ~ 120 gEm -2 s -1 and was provided by two Cool White and two Daylight 40 W fluorescent tubes for each set of three carboys. Daily yields were concentrated by centrifugation at 2000 rpm for 15 miD, and the concentrate divided into three equal volumes for preservation: by air-drying in a convection oven at 30 +__ I°C, by freezing at -20°C, and by freeze-drying in a Labconco freeze-dryer after pre-freezing at -20°C. Freeze- and air-dried algae were stored with silica gel in ziplock plastic bags and the frozen ones were kept in a commercial freezer at - 20°C. Proximate analyses were run in triplicate on com- posite samples between one and three days after preservation, and after about two months of storage. Fatty and amino acid profiles are only available for air- and freeze-dried samples, freshly preserved and after two months of storage for fatty acids and for two months old samples for amino acids. Total and ash-free dry weights were determined as in Sorokin (1973). Samples were dried to constant weight in a convection oven at 60°C and ashed at 470°C in a muffle furnace. Protein content was determined according to Lowry et al. (1951), as described by Malara and Charra (1972a) and modified by Farber (1986); 369