綠球藻屬

语源

绿球藻的学名Chlorella来自于希腊语字根χλώρος（chloros，绿色）和拉丁语的后缀ella（小）。

发现与研究历史

绿球藻是由荷兰微生物学者马丁努斯·威廉·拜耶林克于1890年间在河流湖泊中发现；而1931年获诺贝尔生理学或医学奖的癌症研究权威的德国生物化学家及细胞生理学家奥托·海因里希·瓦尔堡是第一个以绿藻进行生物学研究的学者。
1961年，美国加州大学的梅尔文·卡尔文以绿球藻研究植物中二氧化碳同化的途径而荣获诺贝尔化学奖。

型态与特征

绿球藻顾名思义有着球形的外型，直径约2到10微米，没有鞭毛，但有细胞壁，由多层纤维素框架组成。绿球藻属物种细胞有叶绿体和和散落在细胞质的线粒体。叶绿体含有绿色光合色素叶绿素-a和-b，其比例与一般高等植物相同。

分类学

绿球藻属物种的单系性仍然存疑：尽管其父系分类单元已不断将关系较远的物种分出去，现时的绿球藻仍然被认为是一个并系群，包含不少因为趋同效应而有着相似型态的物种。

食用

许多人认为绿球藻可以作为食物和能源的潜在来源，因为它的光合效率理论上可以达到8%，超过其他诸如甘蔗之类的高效作物。
早在1960年代，《人民日报》引述中国科学院水生所的研究，发现绿球藻属物种的蛋白质含量高达30%以上，可治疗当时中国国民因为长期营养不良而导致的浮肿病。1960年10月，秘书胡乔木呈书毛泽东称，推广绿球藻，既可治浮肿，又能“保证不饿死人”。1960年10月27日毛将胡乔木的信批转全国，开始大力推广，大规模用人尿来培植绿球藻。
事实上，干燥的绿球藻属物种的蛋白质含量可高达45%，还有20%脂肪酸，20%碳水化合物，5%食用纤维，10%矿物质及维生素。因此现时已有健康食品生产商在大型的人造圆塘大量生产绿球藻以供食用。
当科学家首次大规模的收获养殖的绿球藻时，认为这些绿球藻将会是人类饮食中廉价的蛋白质补充剂的来源。一些倡导者有时专注于藻类的其他假定的健康益处，例如体重控制、癌症预防和免疫系统支持等。据美国癌症协会声称：“可用的科学研究不支持其预防或治疗癌症或任何其他人类疾病的有效性。”
在某些生长条件下， 小球藻能够产生多元不饱和脂肪含量高的油—小绿球藻（Chlorella minutissima）可生产二十碳五烯酸，而且比例还占总脂质的39.9%。

历史

在全球担心在1940年代末到1950年代初期的“婴儿潮”人口爆炸期，由于担心人口不受控的增长会引发粮食危机，绿球藻被视为新的和有前途的主要食物来源，并且是当前世界饥饿危机的可能解决方案。在此期间许多人认为饥饿将是一个压倒性的问题，并认为小球藻通过以相对较低的成本提供大量优质食物来结束这场危机。

许多机构开始研究藻类，包括卡内基研究所、洛克菲勒基金会、美国国立卫生研究院、加州大学伯克利分校、原子能委员会和斯坦福大学。第二次世界大战之后 ，许多欧洲人正在挨饿，而不少马尔萨斯主义者认为战争不是发生饥荒的主因。他们认为：全球粮食生产无法追上人口的增加，才是饥荒的主要成因。根据1946年一份FAO的报告，预期在1960年将要生产出比1939年的粮食产量多25到35%，才能赶上人口上的增长；而若要确所有人的健康，这个比例更要上调至90到100%。

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Because meat was costly and energy-intensive to produce, protein shortages were also an issue. Increasing cultivated area alone would go only so far in providing adequate nutrition to the population. The USDA calculated that, to feed the U.S. population by 1975, it would have to add 200 million acres (800,000 km²) of land, but only 45 million were available. One way to combat national food shortages was to increase the land available for farmers, yet the American frontier and farm land had long since been extinguished in trade for expansion and urban life. Hopes rested solely on new agricultural techniques and technologies. Because of these circumstances, an alternative solution was needed.
To cope with the upcoming postwar population boom in the United States and elsewhere, researchers decided to tap into the unexploited sea resources. Initial testing by the Stanford Research Institute showed Chlorella (when growing in warm, sunny, shallow conditions) could convert 20% of solar energy into a plant that, when dried, contains 50% protein. In addition, Chlorella contains fat and vitamins. The plant's photosynthetic efficiency allows it to yield more protein per unit area than any plant—one scientist predicted 10,000 tons of protein a year could be produced with just 20 workers staffing a 1000-acre (4-km²) Chlorella farm. The pilot research performed at Stanford and elsewhere led to immense press from journalists and newspapers, yet did not lead to large-scale algae production. Chlorella seemed like a viable option because of the technological advances in agriculture at the time and the widespread acclaim it got from experts and scientists who studied it. Algae researchers had even hoped to add a neutralized Chlorella powder to conventional food products, as a way to fortify them with vitamins and minerals.
When the preliminary laboratory results were published, the scientific community at first backed the possibilities of Chlorella. Science News Letter praised the optimistic results in an article entitled "Algae to Feed the Starving". John Burlew, the editor of the Carnegie Institution of Washington book Algal Culture-from Laboratory to Pilot Plant, stated, "the algae culture may fill a very real need," which Science News Letter turned into "future populations of the world will be kept from starving by the production of improved or educated algae related to the green scum on ponds." The cover of the magazine also featured Arthur D. Little's Cambridge laboratory, which was a supposed future food factory. A few years later, the magazine published an article entitled "Tomorrow's Dinner", which stated, "There is no doubt in the mind of scientists that the farms of the future will actually be factories." Science Digest also reported, "common pond scum would soon become the world's most important agricultural crop." However, in the decades since those claims were made, algae have not been cultivated on that large of a scale.

Current status

Since the growing world food problem of the 1940s was solved by better crop efficiency and other advances in traditional agriculture, Chlorella has not seen the kind of public and scientific interest that it had in the 1940s. Chlorella has only a niche market for companies promoting it as a dietary supplement.

Production difficulties

The experimental research was carried out in laboratories, rather than in the field, and
scientists discovered that Chlorella would be much more difficult to produce than previously thought. To be practical, the algae grown would have to be placed either in artificial light or in shade to produce at its maximum photosynthetic efficiency. Also, for the Chlorella to be as productive as the world would require, it would have to be grown in carbonated water, which would have added millions to the production cost. A sophisticated process, and additional cost, was required to harvest the crop, and, for Chlorella to be a viable food source, its cell walls would have to be pulverized. The plant could reach its nutritional potential only in highly modified artificial situations. Another problem was developing sufficiently palatable food products from Chlorella.

Although the production of Chlorella looked promising and involved creative technology, it has not to date been cultivated on the scale some had predicted. It has not been sold on the scale of Spirulina, soybean products, or whole grains. Costs have remained high, and Chlorella has for the most part been sold as a health food, for cosmetics, or as animal feed. After a decade of experimentation, studies showed that following exposure to sunlight, Chlorella captured just 2.5% of the solar energy, not much better than conventional crops. Chlorella, too, was found by scientists in the 1960s to be impossible for humans and other animals to digest in its natural state due to the tough cell walls encapsulating the nutrients, which presented further problems for its use in American food production.

用于二氧化碳还原和氧气生产

1965年，前苏联俄罗斯的CELSS实验场所BIOS-3以大桶在人造光下养殖绿球藻，以去除实验场所内的二氧化碳，并为内里的人提供氧气。实验确定，只要有8平方米的暴露绿球藻面积，就可以为一个在密封环境内的成人去除其排出的二氧化碳，并替代以其所需的氧气。

替代医学

绿球藻在美国和加拿大主要被作为健康补充剂销售，而在日本则作为食品补充剂
绿球藻有一些声称的健康影响，包括：治疗癌症的能力。 然而，根据美国癌症协会的报告，“现有的科学研究不支持其预防或治疗癌症或任何其他人类疾病的有效性”。

健康问题

2002年曾有一项研究发现认为：绿球藻的细胞壁含有脂多糖，是一种在革兰氏阴性菌中发现的内毒素，会影响免疫系统、并可能导致炎症 。然而，较近期的研究认为：在革兰氏阴性菌以外的生物体发现的脂多糖，例如在蓝绿菌中的脂多糖与革兰氏阴性菌中的脂多糖明显不同。

水族箱

当水族箱内有绿球藻，可令水的颜色变绿、降低水的透光度。这是由于在水族箱内其他生物排出的高浓度硝酸盐和磷酸盐，在阳光直接照射下有助绿球藻生长。通过不断补充新水，逐步替换水族箱内的水，有助减低水族箱内硝酸盐和磷酸盐的浓度；为水族箱遮光，亦能帮助缓解问题。