建立一光生物反應系統用於微藻的高密度養殖與二氧化碳的減量

Abstract

微藻為行光合作用之生物體，可利用太陽能將水與二氧化碳（CO2）轉換成生物質能（biomass），由於全球暖化日益嚴重，因此利用微藻減量環境中的二氧化碳並生產生物質是最具有潛力的方法之ㄧ。本研究所採用的五種微藻分別為Chlorella sp.、Nannochloropsis oculata、Skeletonema costatum、Isochrysis aff. galbana及Tetraselmis chui，將此五種微藻於相同的環境下培養，並在f/2培養液與供給空氣的情況下，Chlorella sp.，Nannochloropsis oculata，Skeletonema costatum，Isochrysis aff. galbana及Tetraselmis chui的生長率分別為1.55、1.51、0.5、0.72及0.99 d-1，並且從中篩選出Chlorella sp.及Nannochloropsis oculata進行CO2減量並生產生物質之研究。在本研究中，我們將Chlorella sp.和Nannochloropsis oculata培養於封閉式光生物反應器中，經由半連續式的培養技術達到CO2減量與高密度生產生物質的目標。首先，我們測量細胞密度和CO2通入之濃度對微藻的影響，當微藻培養於10及15% CO2下，生長會被抑制，但可經由高濃度微藻的接種（細胞密度 > 9 × 107 cells/mL）以及藻體事先培養於2% CO2濃度下可改善抑制的情況發生，再將其轉入半連續式光合生物反應系統，通入2、5、10和15%的CO2濃度下持續培養八天，我們測得在不同CO2濃度之間的生長曲線是非常相似的。 利用半連續式光合反應系統（800 mL）通入2、5、10和15%的CO2培養後，CO2減量成果分別為0.261、0.316、0.466和0.573 g/hr，而CO2的減量效率則分別為58、27、20和16%，且即使細胞生長在通入高濃度CO2的培養條件下，生物質產量有下降的趨勢，但仍然看的出在15%CO2條件下生長的Chlorella sp.之生物質的生產量是具有生產潛力的，在本研究中也建立並運作六個反應器並聯式系統，且此系統通入不同濃度的CO2培養之CO2移除效率與單一式光合生物反應系統是相似的。於六個反應器並聯式系統中，其CO2總減量與生物質的總生產量得約為單一式的六倍。再則，在本研究中亦採用半連續式培養系統研究CO2的濃度對於Nannochloropsis oculata生產生物質之影響。在2% CO2培養條件下，生物質之生產量0.480 g/L/day為最高，即使在15% CO2培養條件下，生物質之生產量亦可達0.372 g/L/day。 這些結果指出光合生物反應系統若應用於CO2減量，可以藉由多組並聯的方式來針對大量的廢氣處理。而採用每天置換1/2的半連續式系統並通入2% CO2培養，是對於Chlorella sp.和Nannochloropsis oculata最適的生物質生產條件。

Microalgae as photosynthetic organisms can use solar energy to convert water and carbon dioxide (CO2) into biomass. Facing the increasing concerns about global warming, the reduction of CO2 emission to acceptable levels by utilizing microalgae to consume CO2 and to produce biomass is a potential approach. Five microalgal strains, Chlorella sp., Nannochloropsis oculata, Skeletonema costatum, Isochrysis aff. galbana and Tetraselmis chui, were first used in this study. The growth potential of Chlorella sp., Nannochloropsis oculata, Skeletonema costatum, Isochrysis aff. galbana and Tetraselmis chui cultured in the f/2 medium (designed as normal cultural medium) and given air were 1.55, 1.51, 0.5, 0.72 and 0.99 d-1, respectively. Chlorella sp. and Nannochloropsis oculata were selected for the studies of CO2 reduction and biomass production. In this study, Chlorella sp. and Nannochloropsis oculata, were cultured in a closed system of photobioreactors in the semi-continuous cultivation conditions for the exploration of CO2 reduction and high-density microalgal biomass production. First, we determined the effects of cell density and CO2 concentration in airstreams on the growth of microalgae. The growth inhibition when the microalgal cells were cultured in 10 and 15% CO2 aeration could be overcome via a high density of microalga inoculum (up to 9 × 107 cells/mL) and pre-adapted culture with 2% CO2 aeration. The cultures were then transferred into a semi-continuous photobioreactor system aerated with 2, 5, 10 and 15% CO2. The profiles of growth curve of microalgal cultures aerated with different CO2 concentration were similar. Amount of CO2 reduction and CO2 reduction efficiency of the Chlorella sp. cultures in the semi-continuous cultivation (800 mL) under 2, 5, 10 and 15% CO2 aeration were 0.261, 0.316, 0.466 and 0.573 g/hr and 58, 27, 20 and 16%, respectively. The biomass production of Chlorella sp. cultures could maintain in 15% CO2 aeration although biomass productions showed a decreased trend when the cells exposed to the airstreams with rising CO2 concentration increased. A six-parallel photobioreactor system was also performed in this study, and the CO2 reduction efficiency in the system was similar to the single photobioreactor in different concentrations of CO2 aeration. Performances, including total amount of CO2 reduction and biomass production of the six-parallel photobioreactor system was also determined and the result were around 6 folds compared those in the matched single photobioreactor. And then, the effects of concentration of CO2 aeration on the biomass production and lipid accumulation of Nannochloropsis oculata in a semi-continuous culture were investigated in this study. The maximal biomass productivity in the semi-continuous system was 0.480 g/L/d with 2% CO2 aeration. Even the Nannochloropsis oculata cultured in the semi-continuous system aerated with 15% CO2, the biomass productivity could reach to 0.372 g/L/d. These results indicate that the CO2 reduction by microalgae incorporated photobioreactor could be extended to multiple parallel units of the photobioreactor for a large amount of waste gas treatment. To optimize the condition for long-term biomass yield from Chlorella sp. and Nannochloropsis oculata, these microalgae were suggested growing in the semi-continuous system aerated with 2% CO2 and operating by one-day replacement.