September | October 2013

The potential of microalgae meals in compound feeds for aquaculture

The International magazine for the aquaculture feed industry

International Aquafeed is published six times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2013 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058

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Intensive production of mainly car-nivorous fish has resulted in fish feedscontaining high levels of fishmeal andfish oil, with Europe requiring around

1.9 million tonnes a year.Although this useoffishmealwasinitiallytherecyclingofwastefrom fishing through theuseofbycatchandtrimmings, due to the rapid developmentofaquaculturethisrelianceonfishmealandfishoil is environmentallyunsustainable.Thishasresulted in other sources of fish feed beinginvestigated.This literature reviewwill focuson microalgae; the composition in termsof nutritional quality, the current methodsof production and associated costs alongwith potential future uses such as feed inaquaculture.

Algae overviewMarine algae are distributed from the

polar regions to tropical seas innutrient richand poor environments. Algae are photoau-

totrophs and are characterised by their lackofroots,leavesandpresenceofchlorophylla.Theyrangeinsizefrommicroscopicindividualcells calledmicroalgae to seaweeds that canbegreaterthan30minlength(Qin2012).

Marine microalgae are the dominantprimary producers in aquatic systems andaccountforasimilarlevelofcarbonfixationasterrestrialplants(40-50%)butrepresentonly1 percent of the planetary photosynthetic

The potential of microalgae meals in compound feeds for aquacultureby Nathan Atkinson, MSc Sustainable Aquaculture Systems student, Fish Nutrition and Aquaculture Health Group, Plymouth University, United Kingdom

table 1: amino acid profile of different algae as compared with conventional protien sources and the WHo/Fao (1973) reference pattern (g per 100 protein)

Source Ile leu Val lys Phe tyr Met Cys try thr ala arg asp Glu Gly His Pro Ser

WHo/Fao 4.0 7.0 5.0 5.5 6.0 3.5 1.0

egg 6.6 8.8 7.2 5.3 5.8 4.2 3.2 2.3 1.7 5.0 - 6.2 11.0 12.6 4.2 2.4 4.2 6.9

Soybean 5.3 7.7 5.3 6.4 5.0 3.7 1.3 1.9 1.4 4.0 5.0 7.4 1.3 19.0 4.5 2.6 5.3 5.8

Chlorella vulgaris 3.8 8.8 5.5 8.4 5.0 3.4 2.2 1.4 2.1 4.8 7.9 6.4 9.0 11.6 5.8 2.0 4.8 4.1

Dunaliella bardawil 4.2 11.0 5.8 7.0 5.8 3.7 2.3 1.2 0.7 5.4 7.3 7.3 10.4 12.7 5.5 1.8 3.3 4.6

Scenedesmus obliquus 3.6 7.3 6.0 5.6 4.8 3.2 1.5 0.6 0.3 5.1 9.0 7.1 8.4 10.7 7.1 2.1 3.9 4.2

arthrospira platensis 6.7 9.8 7.1 4.8 5.3 5.3 2.5 0.9 0.3 6.2 9.5 7.3 11.8 10.3 5.7 2.2 4.2 5.1

aphanizomenon sp. 2.9 5.2 3.2 3.5 2.5 - 0.7 0.2 0.7 3.3 4.7 3.8 4.7 7.8 2.9 0.9 2.9 2.9

Figure 1: Percentages (dry weight basis) of protein, lipid and carbohydrate in microalgae. The range of values is shown by range bars (Brown 1997)

14 | InternatIOnal AquAFeed | September-October 2013

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September-October 2013 | InternatIOnal AquAFeed | 15

64 | InternatIOnal AquAFeed | September-October 2013

biomass (Stephenson2011). Microalgaearesometimes directly consumed by humans ashealthsupplementsduetothishighnutritionalvalueandabundance(Dallaire2007)butthisisrelativelyrare.

Ascarnivorous fish ingestalgaeasa foodsource (Nakagawa 1997) there has been amove toutilise them for fish feed.Currently30 percent of the world algal production isused for animal feed (Becker 2007) but theuse in aquaculture is mainly for larval fish,molluscs and crustaceans (FAO 2009a). Asmentionedabove,thefishmealandoiluseinaquaculture is unsustainable and algae havethepotentialtoreducethisdependence.Thisis due to the algae being photosynthetic sothey have the ability to turn the sun’s hugeamountofenergy,120,000TWofradiation,intoprotein,lipidsandnutrients.Moreenergyfrom the sun hits the surface of the earthin one hour than the energy used in oneyearand this isahugeamountofuntapped,sustainableenergycanbeexploitedbyalgae.This is a relatively new areaof researchbuthasmanypositiveaspects thatgive ita largeamountofpotentialforfutureuse.

MicroalgaeThe term ‘microalgae’ is often used to

referspecificallytoeukaryoticorganisms,bothfromfreshwaterandmarineenvironmentsbutcanincludeprokaryotessuchascyanobacteria(Stephenson 2011). Microalgal productionhas received someattention recentlydue toitspotentialuseasabiofuel(Slocomb2012),use in animal feed, human consumption andrecombinant protein technology (Becker,2007; Potvin and Zhang 2010; Williams andLaurens, 2010). This has resulted in a hugeamount of knowledge and research into

microalgae andresulted in reviewsbeing publishedabout specific sub-jectssuchasgeneticengineering of algae(Qin 2012), poten-tial use as biofuel(Demirbas 2011)and novel methodsto measure suchimportant com-ponents such asprotein (Slocomb2012).

This interest andknowledge in thearea has allowedaquaculture toessentiallypiggybackthe research beingperformed by thebiodiesel industryand even act syner-gistically with it byconsuming the by-products produced(Ju 2012). Currentlymicroalgae havebeen used in aqua-culture as foodadditives, fishmealand oil replace-ment, colouring ofsalmonids, inducingbiological activitiesand increasing thenutritional value ofzooplankton whichare fed to fish lar-vaeandfry(Dallaire2007).

Although thebiodiesel industryhas been conduct-inga largeamountof research, thishas mainly beenfocused towardsspecies that havehigh lipid contentswhereas species inaquaculture mustbe of appropriatesize for ingestionand be read-ily digested. Theymust also haverapidgrowthrates,be able to be cul-tured on a massscale, be robustenough to copewith fluctuations

table 2: oil contents of some microalgae (Demirbas 2007)

Microalgae oil content (wt% of dry

basis)

Botryococcus braunii 25-75

Chlorella sp. 28-32

Crypthecodinium cohnii 20

Cylindrotheca sp. 16-37

Dunaliella primolectra 23

Isochrysis sp. 25-33

Monallanthus salina >20

nannochloris sp. 20-35

nannochlorosis sp. 31-68

neochloris oleoabundans 35-54

nitzschia sp. 45-47

Phaeodactyhum tricornutum 20-30

Schizochytrium sp. 50-77

tetraselmis sueica 15-23

14 | InternatIOnal AquAFeed | September-October 2013 September-October 2013 | InternatIOnal AquAFeed | 15

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in temperature, light and nutrients andhaveagoodnutrientcomposition(Brown2002).

Varying nutritional valuesThe nutritional value of any algal species

dependson its cell size, digestibility, produc-tion of toxic compounds and biochemi-cal composition. This, along with differencesamong species and method of production,explains the variability in the amount ofprotein, lipids and carbohydrates, which are12-35 percent, 7.2-23 percent, and 4.6-23percentrespectively(FAO2009a)(Figure1).

This levelof fluctuationcanbe influencedbythecultureconditions(Brownet al.,1997)butrapidgrowthandhighlipidproductioncanbeachievedbystressingtheculture.

Protein Mostofthefigurespublishedinthelitera-

tureontheconcentrationofalgalproteinsarebased on estimations of crude protein andas other constituents of microalgae such asnucleic acids, amines, glucosamides and cellwallmaterialswhichcontainnitrogen;thiscan

resultinanoverestimationofthetrueproteincontent(Becker2007).

Thenon-proteinnitrogencanbeupto12percentinScenedesmus obliquus,11.5percentin Spirulina and6percent inDunaliella. Evenwith thisoverestimation thenutritional valueof the algae is high with the average qual-itybeingequal, sometimeseven superior, toconventional plant proteins (Becker 2007)(Table1).

The amino acid composition of theprotein is similarbetweenspeciesand isrelativelyunaffectedbythegrowthphaseandlightconditions(Brownet al.,1993a,

b). Aspartate andglutamate occur inthe highest concen-trations (7.1-12.9%)whereas cysteine,methionine, tryp-tophanandhistidineoccur in the lowestconcentrations(0.4-3.2%) with otheramino acids rangingfrom (3.2-13.5%)(Brown1997).

LipidsThelipidsinmicro-

algalcellshaverolesasboth energy storage

molecules and in the forma-tion of biological membranesand can be as high as 70percent dry weight in somemarine species (Stephenson2011) (Table2).Under rapidgrowth conditions these lipidlevelscandropto14-30per-cent dry weight, which is alevel more appropriate foraquaculture. These lipids arecomposedofpolyunsaturatedfatty acids such as docosa-hexaenoicacid(DHA),eicos-apentaenoic acid (EPA) andarachidonicacid(AA)(Brown2002) and in high concen-trations; most species havepercentagesofEPAfrom7-34percent(Brown2002)(Figure2).

These fatty acids are highly sought afterandastheycurrentlycannotbesynthesisedin a laboratory and are usually obtainedthrough fish oil and are a limiting factor invegetable oils such as palm, soybean andrapeseed oil use in aquaculture. The fattyacid composition is associated with lightintensity, culture media, temperature andpH.Appropriatemeasuresandcontrol,alongwith the suitable selection of a species, isnecessarytoproducealgaewiththedesiredlipidlevelandcomposition.

VitaminsMicroalgaealsocontainvitaminswhichcan

bebeneficialtothehealthoftheconsumerbutvary greatly between species (Brown 2002).This variation is greatest for ascorbic acid(VitaminC),which varies from1-16mggdryweight(Brown&Miller,1992),butothervita-minstypicallyshowa2-4xdifferencebetweenspecies(Brownet al.,1999)(Figure3).

Despite the variation in vitamin C all thespecies would provide an adequate supply tocultured animals which are reported to onlyrequire0.03-0.2mgg-1ofthevitaminintheirdiet(DurveandLovell,1982).Howevereveryspeciesofalgaehad lowconcentrationsofat leastonevitamin(DeRoeck-Holtzhaueret al.,1991)soacarefulselectionofamixedalgaldietwouldbenecessarytoprovideallthevitaminstoculturedanimalsfeedingdirectlyonmicroalgae.

Algae in aquacultureTheuseofalgaeasanadditive inaqua-

culturehas received a lotof attentionduetothepositiveeffect ithasonweightgain,increased triglyceride and protein deposi-tion in muscle, improved resistance todisease, decreased nitrogen output intotheenvironment,increasedfishdigestibility,physiological activity, starvation toleranceandcarcassquality(Becker,2004;Fleurence2012). Li (2009) showed that the additionofdriedmicroalgaeinthediet,albeitatlowconcentrations 1.0-1.5 percent, resulted inincreasedweightgainofthechannelcatfish(Ictalurus punctatus) along with improv-ing the feed efficiency ratio and levels ofpoly-unsaturatedfattyacids.Ganuza(2008)showed that algal oil canbe an alternativesourceofDHA (docosahexaenoic acid) tofishoilingiltheadseabream(Sparus aurata)microdietsalthoughitdidnotallowforthecomplete substitution of fisheries productsduetothelowEPA(eicosapentaenoicacid)levelsinthespeciesofalgaeused.

These were at relatively low-level inclu-sions; at greater levels it can have a detri-mentaleffect.At12.5percentinclusionalgaecausedreducedgrowthperformancesinrain-bowtroutandat25percentand50percentthissubstitutionoffishfeedcausednutritionaldeficiencies that led to decreased growth,feedefficiencyandbodylipids(Dallaire2007).

Levels of algal inclusion of 15 percentand30percentalsoreducedfeedintakeandgrowth rate in Atlantic cod (Walker 2011).AsAtlanticcodareknowntohavearobustdigestivesystemitwassuggestedthatthiswasduetoreducedpalatabilitywhichcouldbeanissueforalgaluseinaquaculture.

High levels of inclusion does not causesuchnegativeeffects inallspeciesraised inaquaculture, 50 percent replacement didnot have a negative effect on shrimp (Ju2012), but is generally experienced amongfinfish.

Figure 2: Average percentage compositions of the long-chain PUFAs docosahexaenoic acid (DHA; 226n-3),

eicosapentaenoic acid (EPA; 20:5n-) and arachidonic acid (20;4n-6) of microalgae commonly used in aquaculture. Data compiled from over 40 species from laboratory of

CSIRO Marine Research.

Figure 3: Concentrations of different vitamins in microalgae in µg g-1. Graph adapted from Brown

2002 with data collected from Seguineau et al., 1996 and Brown et al., 1999

16 | InternatIOnal AquAFeed | September-October 2013

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September-October 2013 | InternatIOnal AquAFeed | 17

Algae production The production of algae, in particular

microalgae, is a rapidly developing industrydue to the biofuel research that is currentlytakingplace.Theannualworldproductionofall species isestimatedtobe10,000tyear-1(Richmond,2004)withthemainlimittopro-duction currently being the cost. ProductioncostsarecurrentlyrangefromUS$4-300perkg dry weight (FAO 2009a) depending onthe type of production method employed(Table3).

There has been a shift away from typicalsystemssuchasoutdoorpondsandracewaysto large-scale photobioreactors which have amuchhighersurfaceareatovolumeratioandcould potentially reduce the production cost(Brown2002).Thesephotobioreactors couldyield19,000-57,000litresofmicroalgaloilperacreperyear,whichisover200timestheyieldfromthebestperformingvegetableoils(Chisti2007), and reduce the cost of algal oil from$1.81to$1.40perlitre(Demirbas2011).

However, for algal oil to be competitivewithpetrodiesel, itshouldbelessthan$0.48perlitre.Thisisachievablethrougheconomiesofscale(Demirbas2011)andwouldmakeitacheapandsustainableoil fortheaquacultureindustry.Therearealsootherdevelopmentssuch as increasing the specific activity of theenzyme RUBISCO which would increaseyields, transgenic studies, increasing the pro-

portion of photo protective pigments whichwouldimprovethelight-dependantreactionsand selecting for algae with small antennaswhich is fundamental toachievinghighyieldsinbiomassdensecultures(Stephenson2011).This research is essential as the productioncosts of microalgae are still too high tocompetewith traditional protein sources foraquaculture(Becker2007).

Benefits and obstaclesAlgae have a great potential for use in

sustainableaquacultureastheyarenotonlyasourceofprotein, lipidsandhaveothernutri-tional qualities but they are phototrophic soproducethesedirectlyfromsunlight.Producing100 tons of algal biomass also fixes roughly183tonsofcarbondioxidewhichhasobviousimplicationsinthisperiodofclimatechange.

The production does not always requirefreshwater, compete for fertile land and arenot nutritionally imbalanced with regard totheaminoacidcontentlikesoybean.

Therearestillsomeobstaclessuchasthepowder-likeconsistencyofthedriedbiomassand applications to feed manufacture, theproduction costs and pests and pathogensthat will effect large scale algal cultivationsustainability(Hannonet al.,2010),whichisanareathatlittleisknownabout.

Therestillneedstobemanyfeedingtrialsas the majority of research has focused on

improving the nutritional value of rotifersand not as algae as a potential replacementof fishmealandfishoil.There isalso interestintostoringalgalpasteswhichhaveextendedshelf life (2-8weeks)or theuseof defattedmicroalgaemealfromthebiodieselindustry.

Theuseofalgaeinaquacultureisapromis-ing and young area of research and whencompared to agriculture,which has increasedcropproductivityby138percentina50yearperiod,itdemonstratesthegreatpotentialthatalgaehas.

Referencesavailableonrequest

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