Morales 1985 Aquacultural-Engineering

7/28/2019 Morales 1985 Aquacultural-Engineering

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A q u a c u l t u r a l E n g i n e er in g 4 (1985) 257-270

H arvest ing Marine M icroaigae Species by ChitosanFlocculationJ . M o r a l e s , J . d e la N o i i e * a n d G . P i c a r d ?

Centre de recherche en nutrition , *D6partement de biologic, tD 6p arte m en t desciences et technologic des aliments, Universit6 Laval, Qu6bec, Canada G 1K 7P 4

A B S T R A C TMa r ine m ic roa lgae a re s ti l l an im po r tan t l arva l f e ed ing source . On e o f themos t p romi s ing harves t i ng t e chn iques o f t he a lgae p roduced appears t obe chem ica l f i occu la t i on . We repo r t r e su l ts ob ta ined w i th ch i t o san f i occu -l a ti o n o f f iv e m a r i n e s p e c i es o f m i cr o al ga e o f i m p o r t a n c e t o m a r i c u l tu r e(Skeletonema costatum, Dunaliella tertiolecta, Thalassiosira nordenskoldii,Chlorella sp . and Thalassionema sp .) . Th e a lgae were grow n in the labora-to ry i n 20 - l i t e r ba t ch cu l t u re s under nor ma l cond i t i ons i n a rt i fi c ia l seawa ter .W i t h o u t p H c o n t ro l , a 1 0 0 % f i o c c u l a t io n e f f i c ie n c y w a s re a c h e d a t f a i r l yh igh ch i t o san concen t ra t i ons (above 4 0 m g l it e r- I) . When the f ina l p Hw a s a d j u s t e d t o a r o u n d 7 . 8 - 8 . 0 , a 1 0 0 % f l o c c u l a t io n e f f i c ie n c y w a so b t a i n e d w i t h c h i to s a n c o n c e n t r a t i o n s o f 4 0 m g li t er - I o r m o r e . H o w e v e r ,wh en p H was ad jus t ed t o a roun d 7 o r le ss , p r io r t o ch i t o san add i t i on f o rS. costatum a n d Chlorella sp ., t he concen t ra t i on o f ch i t o san requ i red t oo b t a i n a 9 5 - 1 0 0 % f l o c c u l a t i o n e f f i c i e n c y w a s r e d u c e d t o 2 0 m g l it e r x f o rChlorella a n d 2 m g l i t e r -1 fo r S. costatum. The resu l t s are d iscussed in thel ig h t o f t h e c u r r e n t l y a c c e p t e d t h e o r ie s o n f l o c c u la t i o n .

I N T R O D U C T I O NT h e c o m m e r c i a l c u l t u r e o f m a r i n e o r g a n i s m s is a n in c r e a si n g ly i m p o r -t a n t i n d u s t r y . H o w e v e r , o n e o f th e b o t t l e n e c k s s t il l r e m a i n i n g f o r i tsc o m p l e t e d e v e l o p m e n t is t h e l i m i t e d n u m b e r o f h a t c h er i es a n dn u r s e ri e s a r o u n d t h e w o r l d . In m a n y c a s e s t h e s u c c e s s o f re a r in g d e p e n d su p o n m i c r o a l g a e t h a t a r e r e q u i r e d f o r l a rv a l f e e d i n g . T h i s i s p a r t i c u l a r l yt r u e f o r b i v a lv e s ( G o l d m a n a n d R y t h e r , 1 9 7 6 ; W i l so n , 1 9 7 8 ; P e r s o o n e

257Aquacu l tu ra l Eng ineer ing 0 1 4 4 - 8 6 0 9 /8 5 /$ 0 3 .3 0 - E l s ev i e r A p plied S c iencePublishers Ltd, England, 1985. Printed in Great Britain


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258 J. Morales, J. de la Noiie, G. Picard

a n d C l a u s , 1 9 8 0 ) , c r u s t a c e a n s ( C o o k a n d M u r p h y , 1 9 6 9 ; M o c k a n dM u r p h y , 1 9 71 ; B a r d a c h e t a l . , 1 9 7 2 ; S h i g e n o , 1 9 7 5 ; H a n s o n a n d G o o d -w i n , 1 9 7 7 ; N e w , 1 9 8 2 ) a n d l a r v a e o f f l a t f i s h e s ( H o w e l l , 1 9 7 9 ; S c o t ta n d M i d d l e t o n , 1 9 7 9 ; G a t e s o u p e a n d L u q u e t , 1 9 8 1 ).

T h e i n t e n s i v e c u l t u r e o f m a r i n e p h y t o p l a n k t o n i s a n a p p e a l i n ga l t e r n a t i v e f o r n u r s e r y o p e r a t i o n s , b u t r e q u i r e s a s o l u t i o n t o t h e p r o b l e mo f h a r v e s ti n g t h e m i c ro a l g a e p r o d u c e d .

D u r i n g t h e l as t 2 0 y e a r s , a l a rg e n u m b e r o f h a r v e s t i n g m e t h o d s h a v eb e e n d e v e l o p e d ( M o h n , 1 9 8 0 ) . U n f o r t u n a t e l y , a l l t h e s e m e t h o d s s t i l lp r e s e n t e c o n o m i c a l o r t e c h n i c a l d r a w b a c k s ( e. g . u n f e a s i b i l i t y o f s c a li n gu p s o m e m e t h o d s , h i g h e n e r g y c o st , t o x i c i t y p r o b le m s c a u s e d b y t h ec h e m i c a l p r o d u c t s u s e d a s f lo c c u l a n t s , e t c .) . H o w e v e r , a m o n g al lh a r v es t in g m e t h o d s , c h e m i c a l fl o c c u l a t i o n is p r o b a b l y o n e o f t h e m o s tp r o m i s i n g .

C h e m i c a l f l o c c u l a t io n t e c h n i q u e s w e r e u s e d a t t h e b e g i n n i n g b yw a s t e w a t e r i n v e s t i g a t o rs t o e l i m i n a t e s u s p e n d e d s o l id s a s w e l l a sm i c r o b e s o r m i c r o a l g ae f r o m t h e f i n al e f f l u e n t (I v es , 1 9 5 9 ; T e n n e y a n dS t u m m , 1 9 65 ; T e n n e y e t a l . , 1 9 6 9 ; M c G a r r y , 1 9 7 0 ; M c G a r r y a n dT o n g k a s a m e , 1 9 71 ; T i l t o n e t a l . , 1 9 7 2 ) .A l ar ge n u m b e r o f c h e m i c a l p r o d u c t s h a v e b e e n u s e d as f l o cc u l a n t s;T a b l e 1 s u m m a r i ze s t h e m o s t e x t e n si v e ly te s t e d . A m o n g t h e m , t h o s ew i t h a h i g h e r e f fe c t iv e n e s s a re a l u m a n d s o m e c a t i o n i c p o l y e l e c t r o ly t e s( T e n n e y a n d S t u m m , 1 9 65 ; T e n n e y e t a l . , 1 9 6 9 ; M c G a r r y , 1 9 7 0 ; M c -G a r r y a n d T o n g k a s a m e , 1 9 7 1 ; M o r a i n e e t a l . , 1 9 8 0 ; V iv ie r s an d V anV u u r e n , 1 98 1 ). U n f o r t u n a t e l y , al g al b i o m a s s h a r v e s t e d b y a l u m c o n t a i n sa h ig h c o n c e n t r a t i o n o f a l u m i n i u m a n d t h e r e a r e s o m e i n d i c a ti o n s t h a tc a t i o n i c p o l y e l e c t r o l y t e s ar e h a z a r d o u s f o r h u m a n h e a l t h ( D o d d , 1 9 7 9 ).T o a v o i d s u c h p r o b l e m s , c h i t o s a n , a n a t u r a l p r o d u c t , h a s b e e n u s e d b yI n d i a n i n v e s t i g a t o r s ( N i g a m e t a l . , 1 9 8 0 ; V e n k a t a r a m a n e t a l . , 1 9 8 0 ;B e c k e r a n d V e n k a t a r a m a n , 1 9 8 2 ) . T h e i r r e s u l t s h a v e b e e n c o n f i r m e dm o r e r e c e n t l y b y L a v o i e a n d d e l a N o iS e ( 1 9 8 3 ) i n C a n a d a .

A l l t h e a u t h o r s w o r k e d o n f r e s h w a t e r m i c ro a l g a e , e x c e p t f o r s o m ep r e l i m i n a r y r e s u lt s o b t a i n e d b y L a v o i e a n d d e la N o iS e ( 1 9 8 3 ) w i t hP h a e o d a c t y l u m t r i c o r n u t u m . W e h a v e t h e r e f o r e u n d e r t a k e n s y s t e m a t i cs t u d i e s w i t h m a r i n e s p e c i e s o f i m p o r t a n c e f o r a q u a c u l t u r e . W e r e p o r th e r e t h e r e s u lt s o b t a i n e d w i t h f iv e o f t h e m u s in g a m o d i f i c a t i o n o f t h eC a n a d i a n p r o c e d u r e ( L a v o i e a n d d e l a N o t i e , 1 9 8 3 ) . T h e i m p o r t a n c eo f c h i t o s a n c o n c e n t r a t i o n a n d p H a d j u s t m e n t s t r a t e g y f o r m i c r o a l g a lf l o c c u l a t i o n w i ll b e s h o w n .


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Floccu la t i on o f mar ine m icroa lgae w i th ch i t o sanT A B L E 1Flocculating Agents Successfullyused for Algae Recovery

259

F l o c c u l a n t C o n c e n t r a ti o n R e f e r e n c e(rag l i ter -1)

Alum 75-100 McGarry (1970)Alum 125-170 Van Vuuren andVan Vuuren (1965)Lime 300-400 Van Vuuren andVan Vuuren (1965)FeCI3 30 lves (1959)Cationic polyelectrolytes 50-100 Tilton e t a l. (1972)Cationic polyamine (Dow C-31) 3 Tenney et al . (1969)Ca(OH)2 100 Nigame t a l. (1980)Chitosan 50 Nigame t a l. (1980)Alum 70 Lavoie e t a l. (1984)Chitosan 30 Lavoiee t a l. (1984)

MATERIAL AND METHODSA l g a eThe species used were the following:

S k e l e t o n e m a c o s t a t u m (Thalassiosiraceae, Bacillariales)D u n a l i e l l a t e r t i o l e c t a (Dunaliellaceae, Volvocales)T h a l a s s i o s i r a n o r d e n s k o l d i i (Thalassiosiraceae, Bacillariales)C h l o r e l l a sp. (Oocystaceae, Chlorococcales)T h a l a s s i o n e m a sp. (Fragilaviaceae, Baccillariales).

Algal batch cultures were grown in 20-liter Pyrex carboys on amedium derived from that proposed by De Pauw e t a l . (1980), which isitself a modification of tha t of Walne (1956). Table 2 shows the compo-sition and concentra tions of the algal growth medium.

Artificial seawater was prepared with 'Instant Ocean' to give a salinityof o31 ~o. Algal cultures were maintained in a thermos tat ted room at20C for 6-7 days. Under these conditions, pH varied without controlbetween 8.7 and 9.9 for all cultures. Light intensity was 30000 lux


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260 J . M o r a l e s , J . d e l a N o l l e , G . P i c a r dT A B L E 2

Algal Cul ture M edium (M odif ied a f te r De Pauw e t a l . , 1980)FeSO 4.7 H20 0 .278 g l i te r -1MnC 12.4 H2 0 0-47 g l i ter -1NaH2PO4.2 H2 0 5 .0 g l i te r -1 (= 1000 m g P l i te r -1)NaNO3 30-0 g l i te r -1 (= 50 00 m gN l i te r -1)Na2SiO3 20 .0 g l i ter -~ (= 1.5 g Si l i ter -l)M ed ium renewa l ra te s : a lgal s tock m a in tenan ce , 0 .1 -1 -5% week 1 o f s tock so lu t ion ;a lga l cu l tu re ma in tenance : 0 .1 -0 .2% week 1 o f s tock so lu t ion .

w i t h a p h o t o p e r i o d o f 1 4 : 1 0 ( L /D ) . C e ll d e n s i t y w a s m e a s u r e d b ys p e c t r o p h o t o m e t r y a t 6 7 8 n m ( S t e in , 1 9 7 3 ) in a 1 - cm c u v e t te fo ro p t i c a l d e n s i t y ( O D ) a b s o r b a n c e . C e ll c o u n t s o f 6 0 0 - 1 0 0 0 c ells w e r em a d e u n d e r a m i c r o sc o p e w i t h a M a la ss ez h e m a t o c y t o m e t e r ( B o rd e a u x-C h i m i c , B o r d e a u x , F r a n c e ) b e f o r e e a c h f l o c c u l a ti o n .Flocculat ion procedureT h e c o m p l e t e f l o c c u l a t io n p r o c e d u r e is s h o w n in F ig . 1 . C h i t o s a ne f f i c i e n c y w a s e v a l u a t e d a n d o p e r a t i n g c o n d i t i o n s , p r i m a r i l y f l o c c u l a n tc o n c e n t r a t i o n a n d p H s e n s i t iv i t y , w e r e d e t e r m i n e d in l - li te r f l o c c u l a t i o nt e s t s ( ' j a r t e s t s ' ) .

Fig. 1.

A l g a l c u l t u r ehom ogenizat ion (1 m in)

A d d i t i o n o f f l o c c u l a t i n g a g e n t (c h it o sa n )~ mixing

p H a d j u s t m e n t ( H C I 0 .1 N o r N a O H 0 -1 N)st irr ing(1 m in at lO Orp m and 4 rain at 40 rpm)

f l o c c u l a t i o nFloccu la t ion p rocedure by ch i to san (Modi f i ed a f t e r Lavo ie and de l aNofie, 1983).


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Flocculation of marine microalgae with chitosan 26 1T h e p H w a s a d j u s t e d b y d r o p w i s e a d d i t io n o f 0 .1 N H C1 o r N a O H .

F o l l o w i n g f l o c c u l a t i o n , t h e f in a l O D a n d p H o f t h e s u p e r n a t a n t w e r ed e t e r m i n e d . T h e f l o c c u l a t i o n e f f i c i e n c y w a s e s t a b l i s h e d b y t h e d i ff e r-e n c e f r o m t h e o r i g i n a l O D .C h i t o s a n

C h i t o s a n , o b t a i n e d b y d e - a c e t y l a t i o n o f a ~ - N - a c e t y l- o - g lu c o s a m i n e ,m a i n l y e x t r a c t e d f r o m c r u s t a c e a n e x o s k e l e t o n (M u z z a r e ll i, 1 9 7 7 ;N i g a m e t a l . , 1 9 8 0 ) w a s p r o v i d e d b y K y p r o C o . ( S e a t t le , U S A ) .C h i t o s a n f l a k e s w e r e d i s s o l v e d i n a 1 % a c e t i c a c i d s o l u t i o n t o o b t a i n af in a l c h i to s a n s o l u t i o n o f 0 . 5 % ( w / w ) a t p H 3 -5 .

R E S U L T SA . I n f lu e n c e o f c h i to s a n c o n c e n t r a t i o n o n f lo c c u l a t io n e f f ic i e n c yS e v e ra l c h i t o s a n c o n c e n t r a t i o n s w e r e t e s t e d t o d e t e r m i n e t h e b e s t c o n d i -t i o n f o r a m a x i m a l f l o c c u l a t i o n e f f i c i e n c y . A l ga l c u l t u r e f e a t u r e s b e f o r ea n d a f t e r f l o c c u l a t i o n a r e s h o w n i n T a b l e s 3 a n d 4 .

TABLE 3Physico-chemical F eature s of Algal Cultures Befo re Floc culat ionSpec ies AIgal eoncentrat ion OD pH

( 106 cells, m1-1] (6 78 nm )Chlorella sp. 80.4 0.8 9.50S k e l e t o n e m a 3-0 0.215 9.85

cos ta tumThalassiossira 5.7 0 .34 9 .70nordensko ld i iThalassionema sp. 3.0 0-21 9.7 0Dunaliella 2.2 0.305 9.25ter t iolecta


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262 J. Morales, J. de la Nolle, G. PicardT A B L E 4Flocculation Efficiency for Different Algal Species as a Function

of Chitosan ConcentrationChitosan

concentrat ion(rag lite r -I)pH a f t e r add i t ion F ina l OD Floccu la tion

o f f ioccu lan t (6 78 nm ) e f f i c iency {%)Spec ies

304050607080

7.7 0-30 62.57-1 0-00 100- 0-00 100- 0 . 0 0 1 0 0- 0 . 0 0 1 0 0- 0 - 0 0 1 0 0

20 8.4 0.30 1-630 7.9 0.27 11.540 7.6 0.23 24.645 7.1 0-00 10050 6.9 0.00 10060 6.5 0.00 10010 9-4 0.18 47.120 8.8 0.03 91.230 8-4 0.03 91-240 7.6 0-00 100

Chorella sp.

Dunaliellatertiolecta

Thalassiosiranordenskoldi i

30 9.3 0.1940 9.0 O. 1350 8.9 0-1560 8.6 0.1070 8.2 0.0380 7.4 0.00

11-6 }39.530.0 S k e l e t o n e m a5 3 . 5 cos ta tum86-0100

40 9-0 0.13 38.1 )50 8.9 0-10 52.460 8.7 0.10 52.4 T h a l a s s i o -70 8.4 0-09 57.4 nema sp.80 8.0 0.08 61.990 7.1 0-01 95.2


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Flocc ula t ion o f mar ine microalgae wi th chi tosan 263The chitosan concentration needed to obtain 100% flocculation

effic iency was d ifferent for each algal culture (Table 4). Several factorsrelated to the flocculation process could explain such differences: celldensity, algal size, ageing of cultures, phase of growth, etc.

Chitosan concentra tions were always above 40 ra g liter -I andfrequen tly reached 80 and 90 mg liter -a. These values are intermed iatebetween those of Nigam e t a l . (1980), Venkataraman e t a l . (1980) andthose of Lavoie and de la Notie (1983) obtained with fresh watermicroalgae. However, they are of the same range as those reported byLavoie and de la NoiSe (1983) for the marine diatom P h a e o d a c t y l u mt r i c o r n u t u m .

In these experiments, pH was not adjusted prior to flocculation, butafter adding chitosan solution, it was readjusted to 8.0 by dropwiseaddit ion of 0.1 N HC1 or NaOH.

The addition of acidic chitosan solution reduced pH values but onlywhen pH dropped to near 7.1, before readjustment to 8.0, was a maxi-mal flocculation efficiency obtained. Due to seawater buffer capacity,the quantities of chitosan needed to drop the pH of algal solutionsbelow 7.5 are higher than those mentioned for freshwater media(Lavoie and de la Nofie, 1983).B. I n f l u e n c e o f f i n a l p H v a l u e s o n f l o c c u l a t i o n e f f i c ie n c yThe importance of pH on flocculation processes was demonstratedby several authors (Ives, 1959; Tenney and Stumm, 1965; Tenneye t a l . , 1969; McGarry and Tongkasame, 1971; Tilton e t a l . , 1972;Venkataraman e t a l . , 1980; Lavoie and de la NoiJe, 1983; Lavoie e t a l . ,1984), but this parameter has never been studied in salt-water media.

Since pH values obtained with the same chitosan concentration weredifferent for each algal culture, we chose the smallest quantity ofchitosan needed to reach an OD reduction higher than 95% (Table 4).Immediately after adding the flocculant, pH was brought to differentfinal values ranging from 7.4 to 8-2 and higher .

We have compared flocculation efficiency for all these final pHvalues on the basis of optical density fluctuations of the supernatant.Figure 2 shows a strong influence of final pH on flocculation efficiency.For pH values between 7.8 and 8.0 we noticed a slight decrease inflocculation efficiency. For pH below 7-8, we observed a marked


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264 J. Morales, J. de la Nolle, G. Picard

I 0 0

8 0 -c ,-

~ 6 0 -

= 4 00~D

~ 20-i,

a / m I

?/a

A7 ~ _ _ ~_ m7 1 6 8 1 o { 4 9 ' 2 9 1 6

F i n o l p HFig. 2. Effe ct of f inal pH of f locculat ion efficiency: D,Dunal ie l la ter t io lec ta (at 45mg l i ter - i chi tosan); o, Thalassiossira nordenskoldi i (a t 4 0 m g liter -1 chitosan); z~,S k e l e t o n e m a c o s t a t u m (at 80 mg liter -1 chitosan); o , Chlorel la sp. (at 60 mg liter -I

chitosan).

r e d u c t i o n o f f l o c c u l a t i o n e f f i c ie n c y . T h i s f a c t is r e l a te d t o c h i t o s a nc h a r a c te r i st ic s a s w e l l a s t o t h e p h y s i c o c h e m i c a l r e a c t i o n s b e t w e e n t h ef l o c c u l a n t a n d t h e a l g a l c e l l s .

F l o c c u l a t i o n e f f i c i e n c y w a s m a x i m a l o n l y f o r p H v a l u e s h i gh e r t h a n8 . 0 . W e d id n o t o b s e r v e a n y r e d u c t i o n i n f lo c c u l a t i o n e f f i c i e n c y i n t h ep H r a ng e 8 . 0 to 9 . 0 f o r a n y o f t h e m i c r o a lg a e s p e c i e s s t u d i e d . T h isc h i t o s a n a c t i v i t y r a n g e i s l a rg e r t h a n t h e 0 . 5 p H r an g e r e p o r t e d b yL a v o i e a n d d e la N o iS e ( 1 9 8 3 ) a n d L a v o i e e t a l . ( 1 9 8 4 ) f o r S c e n e d e s m u sc u l t u r e s i n C a n a d a . V e n k a t a r a m a n e t a l . ( 1 9 8 0 ) m e n t i o n e d a m a x i m a ls e d i m e n t a t i o n r a t e o f S c e n e d e s m u s c u l t u r e i n I n d i a f o r p H v a l u e sb e t w e e n 7 - 5 a n d 8 . 5 .C . I n f l u e n c e o f p H p r e a d ju s t m e n t o n f i o c c u l a t i o n e f f i c i e n c yI n t h e e x p e r i m e n t s d e s c r i b e d i n s e c t i o n A , m a x i m a l f l o c c u l a t i o ne f f i c ie n c y w a s o b s e r v e d a t p H v a lu e s b e l o w 7 . 5 , w h i c h w e r e o b t a i n e db y a d d i n g t h e a c i d ic s o l u t i o n o f c h i to s a n . I n t h e p r e s e n t e x p e r i m e n t s , ap H p r e a d j u s t m e n t w a s d o n e p r io r t o c h i t o s a n a d d i t i o n in o r d e r to f in d


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Flocculation of marine microalgae with chitosan 265the minimal flocculant concen tratio n leading to a sedime ntation greaterthan 95%.

These experiments were carried out with Chlorella sp. and Skele-tonema costatum as representatives of two of the main groups ofmarine microalgae. Algae culture features are shown in Table 3.

The preadju stment of pH was done by dropwise addition of 0-1 NHCI. Three minutes after chitosan addition, pH was readjusted to afinal value of 8.0 by adding 0.1N NaOH. This preadjustment of pHallowed a 50% reduction of flocculant concentration for Chlorellaculture, while a maximal sedimentation of 100% was obtained onSkeletonema culture with a c oncent rat ion of 2 mg liter-~; this is aflocculant conce ntration 40 times lower than the original concen tration(Table 5).

Several important observations can be reported at this point:- For maximal algal flocculation, the agitation time as well as the

stirring speed mu st be increased by 50%.- Microalgal-chitosan flocs are smaller as compared to those observed

in section A experiments.

T A B L E 5Flocculation Efficiency for Chlorella sp. and Skeletonema costatum with pH

AdjustmentChitosan pH pH after Final DO Flocculation

concentration preadjustment chitosan (678 nm) efficiency(mg liter -1) {drops f addition (%)HCl I N)

Species

10 7-9 (9) 7-1 0.14 82.520 8-1 (8) 6.9 0-03 96.230 8.4 (8) 6.9 0-02 97-540 8-3 (7) 6.6 0.01 98.310 7.0 (10) 6.6 0.00 1002 5-7 (15) 5.3 0-00 100a

Chlorella sp.Chlorella sp.Chlorella sp.Chlorella sp.Skeletonema

eostatumSkeletonemacostatuma This includes both sedimented and floated flocs (see text).


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266 J. Morales, J. de la Noiie, G. Picard

- W h e n p H v a lu e s a re b r o u g h t t o a s l o w as 5 . 3 f o r S k e l e t o n e m ac u l t u r e , s m a l l b u b b l e s a p p e a r o n a lg a l f lo c s l if t in g m o s t o f t h e a lg a ls u s p e n s i o n t o t h e s u r f a c e ; t h i s c o u l d b e d u e t o c h e m i c a l r e a c t i o n sb e t w e e n t h e a c i d i c s o l u t i o n a n d t h e s il ic a f ru s t u l e o f t h e d i a t o m s .

D I S C U S S I O NS i nc e t h e 1 9 5 0s , f l o c c u l a t io n a n d c o a g u l a t io n p h e n o m e n a h a ve b e e n t h es u b j e c t o f i n te n s iv e r e s ea r c h b y d i f f e r e n t a u t h o r s w h o h a v e d e v e l o p e dt w o b r o a d t h e o r i e s t o e x p l a i n t h e m e c h a n i s m s o f c o l lo i d p r e c i p it a t io n .T h e c h e m i c a l t h e o r y , w h i c h i s t h e o l d e r o n e , a s s u m e d t h a t d e s t a b i l i z a -t i o n ( a n d f in a l p r e c i p i t a t i o n ) o f c o l l o id s is d u e t o c h e m i c a l i n t e r a c t i o n ss u c h a s c o m p l e x f o r m a t i o n a n d p r o t o n t r an s f e r (S m e l li e a n d L a M e r ,1 9 5 8 ). L a t e r o n , a p h y s ic a l t h e o r y e m p h a s i z e d t h e im p o r t a n c e o fp h y s i c a l f a c t o r s s u c h a s z e t a p o t e n t i a l r e d u c t i o n o r i o n p a i r f o r m a t i o ni n t h e d e s t a b i l i z a t io n o f c o ll o i d s ( S t u m m a n d M o r g a n , 1 9 6 2 ) .

A t p r e s e n t, t h e i n f lu e n c e o f b o t h p h y s ic a l a n d c h e m i c a l f a c t o rs o nt h e c o a g u l a t i o n a n d f l o c c u l a t i o n p r o c e s s e s is r e c o g n i z e d . C l a r k e t a l .( 1 9 7 7 ) d e s c r i b e d c o a g u l a t i o n a s a r e d u c t i o n o f n e t e l e c tr ic a l r e p u ls iv ef o r c e s a t p a r t ic l e s u r f a c es b y e l e c t r o l y t e s i n s o l u t i o n , w h i l e f l o c c u l a t i o ns h o u l d b e a n a g g r e g a t i o n b y c h e m i c a l b r i d g in g b e t w e e n p a r ti c le s .M i c r o a lg a e a s w e l l a s b a c t e r i a o r p r o t o z o a c a n b e c o n s i d e r e d h y d r o -p h il ic b io c o l lo i d s ( T e n n e y a n d S t u m m , 1 9 6 5) .

T h e s t a b i li t y o f h y d r o p h i l i c c o l l o id s (l a c k o f t e n d e n c y t o a g g lo -m e r a t e ) d e p e n d s u p o n a m a r k e d a f f in i ty f o r w a t e r r a t h e r t h a n u p o nt h e s li g ht m u t u a l l y r e p u l s iv e c h a r g e s (u s u a l ly n e g a t i v e ) t h a t t h e y p o s se s s( C l a r k e t a l . , 1 97 7 ). T e n n e y a n d S t u m m ( 1 9 6 5 ) p o i n t e d o u t t h a t t h es u r fa c e c h a r ge d e n s i t y o f b a c t e r i a is s t r o n g l y p H - d e p e n d e n t . T h i sc r i t e r i o n is a l s o v a l id f o r m i c r o a l g a e ( I v e s, 1 9 5 9 ) . C e l l w a l ls o f m i c r o -a lg ae a re m a i n l y c o m p o s e d o f g l u c o - a n d m u c o - p o l y s a c c h a r i d e s ( J e n se n ,1 9 8 3 ) ; t h e s e s u b s t a n c e s h a v e l o w is o e l e c tr i c p o i n t s ( T e n n e y a n d S t u m m ,1 9 6 5 ) a n d t h e y a r e n e g a ti v e ly c h a r g e d d u e t o i o n i z e d c a r b o x y l ic g r o u p s( S t u m m a n d M o r ga n , 1 9 6 2) .

A m e c h a n i s m e x p l a i n in g t h e f l o c c u l a ti o n p r o c es s o f b i o c o l lo i d s w a sp r o p o s e d b y T e n n e y a n d S t u m m ( 1 9 6 5 ) . I ts m a i n f e a tu r e s ar e t h ef o ll o w i ng : n a t u r a l p o l y m e r s s u c h a s c o m p l e x p o l y sa c c h a r id e s a n d p o l y -a m i n o a c id s ar e e x c r e t e d o r e x p o s e d a t t h e s u rf a ce , p r e d o m i n a n t l yd u r i n g t h e d e c l in i n g g r o w t h p h a se a n d e n d o g e n o u s r e s p ir a t io n


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Flocc ula t ion o f rnar ine microalgae wi th chi tosan 267( A v n i m e l e c h e t a l . , 1 9 8 2 ) ; t h e s e p o l y m e r i c m o l e c u l e s a re o f s u f f ic i e n tl e n g t h t o f o r m b r i d g e s b e t w e e n m i c r o b i a l p a r ti c le s . C h i t o s a n , a s w e l la s c a t io n i c p o l y e l e c t r o l y t e s , b e i n g p o s i t iv e l y c h a rg e d , s h o u l d f o r m c o m -p l e x b r i d g e s b e t w e e n t h e n e g a t i v e ly c h a r g e d s u b s t a n c e s o f t h e a lg a es u rf ac e ( T e n n e y a n d S t u m m , 1 9 6 5; A v n i m e l e c h e t a l . , 1 9 8 2 ) . T e n n e ya n d S t u m m ( 1 9 6 5 ) r e p o r t e d t h a t in t h e i n t e r a c ti o n o f p o l y e l e c tr o -l y r e s w i t h c e l l s , e l e c t r o s t a t i c f o r c e s a r e n e c e s s a r y t o ' b i n d ' p o s i t i v e l yc h a r g e d s e g m e n t s o f t h e p o l y e l e c t r o l y t e t o t h e m i c r o b i a l su r fa c e .

O u r e x p e r i m e n t s e m p h a s i z e t h e i m p o r t a n c e o f d i m i n i s h i n g p H v a lu e sb e l o w 7 . 0 p r i o r t o r e a d j u s t m e n t t o 8 . 0 f o r f i n a l p r e c i p i t a t i o n . T h i s p Hr e d u c t i o n i s l i k el y t o h a v e t w o m a i n c o n s e q u e n c e s : 1 , i n c r e a s e d c h i t o s a na c t iv i ty b y v i sc o s it y r e d u c t i o n ; 2 , i n d u c e d r e d u c t i o n o f m e a n s u r fa c ec h a r g e o f m i c r o a l g a e a n d a l t e r a t i o n o f i t s s t a b i l i t y . O n c e t h e b r i d g i n gis m a d e , a f i n a l p H a d j u s t m e n t t o 8 . 0 i n c r e a s e s f i r s tl y c h i t o s a n v i s c o s i tya n d f i n al ly c a u s e s m i c r o a l g a e - c h i t o s a n f l o c p r e c i p i t a t i o n .

W e h a ve o b s e r v e d t h e n e c e s s it y t o e x t e n d a g i t a ti o n w h e n w e w o r k e da t v e r y l o w c h i t o s a n c o n c e n t r a t i o n ( i. e. 2 m g l it e r -1 ) a n d p H p r e a d j u s t -m e n t . T h e f ir st o b s e r v a ti o n is i n li ne w i t h w h a t T e n n e y a n d S t u m m( 1 9 6 5 ) m e n t i o n e d , n a m e l y ' . . . w i t h a g iv en m i c r o o r g a n is m - p o l y e l e c t r o -l yr e s u s p e n s i o n , th e e x t e n t o f p o l y e l e c t r o l y t e a d s o r p t i o n a p p a r e n t l yi nc r ea s es w i t h p r o l o n g e d a g i t a t i o n . . . '. M o r e r e se a r ch i s n e e d e d t oe s t a b li s h t h e o p t i m a l a g i t a t i o n t i m e s f o r a g i ve n c h i t o s a n c o n c e n t r a t i o na n d t h e o p t i m a l m i n i m a l p H v a lu e f o r a m a x i m a l c h it o s a n - m i c r o a lg a lb r id g i ng w i t h a m i n i m a l f l o c c u l a n t c o n c e n t r a t i o n .

F i n a l ly , i t a p p e a r s t h a t t h e g e n e r a l f l o c c u l a t i o n p r o c e s s d e s c r i b e d f o rf r e s h w a t e r m i c r o o r g a n i s m s a ls o h o l d s f o r m a r i n e a lg a l s p e c ie s .

A C K N O W L E D G E M E N T ST h i s w o r k w a s s u p p o r t e d b y g r a n t s f r o m t h e N a t u r a l S c ie n c es a n dE n g i n e e r in g R e s e a r c h C o u n c i l o f C a n a d a a n d t h e F o n d s F C A C o ft h e m i n i s t 6 r e d e l ' E d u c a t i o n d u Q u 6 b e c . W e t h a n k M r A l a i n L a v o i e f o rh e l p f u l d i s c u ss io n s , D r N . E i d h e n f o r i m p r o v i n g t h e m a n u s c r i p t a n d t h eK y p r o C o . ( S e a t t l e , U S A ) f o r p r o v i d in g u s w i t h t h e c h i to s a n .

R E F E R E N C E SAvnimelech, Y., Troeger, B. W. & R eed, L . W. (1982). M utua l flocculation o f algaeand clay: Evidence and implications. Sc i ence , 216, 63-5.


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2 6 8 J. M orales, J. de la Noiie, G. PicardBa r d a c h , J . E . , Ry t h e r , J . H . & Mc La r n e y , W . O . ( 1 9 7 2 ) . Aquaculture. The Farming

and H usbandry o f Fresh-Water and Marine Organisms, W f l e y - I n t e r s c i e n c e , Ne wY o r k .

B e c k e r , E . W . & V e n k a t a r a m a n , L . V . ( 1 9 8 2 ) . B i o t e c h n o l o g y a n d e x p l o i t a t i o n o fa lg ae - T h e I n d i a n a p p r o a c h . G e r m a n A g e n c y f o r T e c h n i c a l C o o p e r a t i o n ( G T Z ) .

C l a r k , J . W ., V i e s s ma n , W . & Ha m m e r , M. ( 1 9 7 7 ) . Water Supply and Pollut ionControl, 3 r d e d n , H a r p e r I n t e r n a ti o n a l E d i t io n s , N e w Y o r k .

C o o k , H . L . & Mu r p h y , M. A . ( 1 9 6 9 ) . Th e c u l t u r e o f l ar va l p e n a i e d s h r imp . Trans.Am. Fish . Soc. , 9 8 ( 4 ) , 7 5 1 - 4 .

D e P a u w , N . , V e r l e t , H . & d e L e e n h e e r , J r , L . ( 1 9 8 0 ) . H e a t e d a n d u n h e a t e d o u t d o o rc u l t u r e s o f ma r i n e a l ga e w i t h a n i ma l m a n u r e . I n : Algae Biomass, e d s . G . Sh e l e fa n d C . J . So e d e r , E l s e v i e r / No r t h Ho l l a n d B i o me d i c a l P r e s s , Ams t e r d a m, p p .3 1 5 - 4 1 .

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chi tosa n as a f lo cculan t for the cul tures of the green a lgae: Scenedesm us acutus .Arch. Hydrobiol . , 88 (3 ) , 378-87 .Persoone , G. & Claus , C . (1980) . M ass cul ture o f a lgae: A bot t lene ck in the nurserycul tur ing of mol luscs . In : Algae Biomass, eds G. S hele f & C. J . Soede r, Elsevier/No r th Ho l l and B iomed ica l P res s, Am s te rdam, pp . 2 65-85 .

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Morales 1985 Aquacultural-Engineering

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