一般在商業性螺旋藻養殖中較侷限於室外溝渠式養殖(Tredici et al., 1993),僅有少數公司使用封閉式生物反應器培養(Specktorova et al., 1997)。主要原因是因為螺旋藻為一種熱帶性的藻種,喜強光及高溫,所以在室外式養殖頗受環境限制,而僅限於熱帶及亞熱帶地區。然而,封閉式生物反應器,雖然能一年四季提供非常固定的產量,但因建造價格昂貴而無法大規模生產,所以使用的普遍性較低。
利於室外池塘式的養殖有最主要的優點兩個:成本較低及可使用太陽能,也因此螺旋藻目前大部分以室外養殖方式居多。
物理因素對於螺旋藻生長濃度及產量的影響
如前所述,螺旋藻因需要較長時間的自然光照射及高溫來達到理想生長,因此一般的養殖是侷限於熱帶及亞熱帶地區。在部分溫帶地區,雖然夏天的高溫可達到螺旋藻生長的理想溫度,但到晚上會有明顯的溫差,甚至凌晨的溫度卻只有10~15℃(Richmond, 1987)。如此一來,每天都會有幾個小時為生長緩慢或生長停滯時期。然而另有其他報告亦有指出,Spirulina subsalsa為一種生長在河口的種類,可以在一些較高緯度的地區進行商業化養殖(Hargraves and Viquez., 1982; Shimada et al.,1989)。
螺旋藻最適當的成長溫度為35~37℃。在室外最低的成長溫度為18℃,當低於12℃時會停止生長(Richmond, 1986)。但某些種類的螺旋藻的生長溫度範圍是大於一般螺旋藻的生長溫度範圍(24~42℃)(Vonshake and Tomaselli,2000)。Jiménez(2003)指出Spirulina platensis可以在低溫12℃時達到2 g 乾重/每平方公尺/每天的生產量;15℃有6 g 乾重/每平方公尺/每天的生產量;18℃為8 g 乾重/每平方公尺/每天的生產量。令人驚訝的是,在9℃下,螺旋藻也有少許生長。
光度對植物而言是相當重要的。綠色植物能吸收的光波的範圍為可見光(400~700nm)。針對於不同的色素有特定的吸收光波(譜),葉綠素a、b吸收藍光和紅光,類胡螺蔔素則以吸收藍光為主,藻藍素以紅光為主要吸收的光譜(吸光值以565nm偵測),藻紅素以藍光為主;且螺旋藻在波長較短的光波下(藍光),易產生較高的生產量。 Qiand et al.(1998)指出螺旋藻的生長密度及光度是呈線性關係,在8000μmol m-2 s-1(為夏天正中午的2~3倍),螺旋藻有最大的產率為16.8 g 乾重/每小時,但似乎尚未呈現最大的產率。在生物反應器中,不同直徑管子會有不同的生產量:當管徑減少27倍時(200mm變成7.5mm),生產量增加50倍。細胞密度與光度有密切相關:當細胞密度太高時,光度相對降低,會使生產率下降。但光度太強時,會導致葉綠素的褪色(Singh et al., 1995)。
至於培養過程中的pH值以10.2~10.4為最理想,當低於9.5或高於10.5生長率會下降(Jiménez et al.,2003)。溶氧量最好維持在20~22ppm,當大於25ppm時,反而生物濃度會降低(Jiménez et al., 2003)。至於導電度(22~28mS cm-1)對於生長濃度及產量無明顯影響。
綜合以上所言,夏季光度為最主要的影響;而冬天最主要受到溫度的影響(Vonshak, 1997),然而過量的溶氧及太低或太高的pH值對於生長率反而會使產率降低。
結論
螺旋藻的發展已有三十幾年的歷史,因其營養成分豐富,人類醫學上功效顯著,常被利用作為健康食品。其成份中以其含鈣膠質的功效為最多,醫療指數為200~6000(Ayehunie et al.,1998)。因此,如何提高螺旋藻的膠質分泌,來提升其經濟價值,成為相當重要的課題。藻膠對於藻類而言是一種保護的物質,在生長狀況較差時,會影響膠質分泌 (Shepherd et al., 1999)。因此可運用這個特性,改變一些生長因子(如:溫度、光度及營養鹽)來刺激藻膠的大量分泌。
而且養殖螺旋藻在世界會掀起一股風潮,除了有較豐富的營養所帶來較高的商業契機之外,在養殖的過程所需要的成本非常低。扣除掉進口螺旋藻藻種的成本之外,除了需要有土地及收成時消耗較多物力及人力之外,剩下就是加工的成本。一般螺旋藻培養液的pH值可以高達11,所以較少有其他生物會在當中存活而影響其生產率,也因此收成相當方便。微藻養殖有一特別之處就是能夠利用太陽能轉換成生物能,不像魚蝦貝類的養殖需要消耗大量的飼料成本。相對的,投資報酬率一定相當可觀。
台灣的螺旋藻產品大多是仰賴進口,本身並未有像外國如此大規模的養殖。其實螺旋藻的發展最主要還是要與政府的配合有關,像中國大陸在近幾年中,政府投入大規模的人力、物力與資金。如今,中國生產螺旋藻的產量號稱已佔全世界的二分之一。台灣就地理位置而言(特別指高屏地區),發展螺旋藻的市場是相當被看好,加上國人的消費能力水準相當的高,而且在近幾年來我國掀起一股健康食品的熱潮,只要看好這一股市場趨勢,前景是不可限量的。而且我國微藻養殖技術相當進步,只要相關產業之間的配合,一定能創造台灣的養殖綠色奇蹟。
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