This invention relates to the synthesis of algae biopolymers and more particularly to a method and apparatus for the enhancement of alga biopolymer production by nitrogen limitation.
The cultivation of microalgae to recover biopolymers as well as other products in the algal biomass is well known in the art. Such algae biopolymers are useful in various applications such as thickening agents for mobility control in waterflood oil recovery, as food additives, as flocculants useful in waste water treatment, soil conditioning, and as drilling mud extenders. Algae biopolymers may be synthesized by microalgae from the divisions Chlorophyta, Cyanophyta, and Rhodophyta. Genera within these divisions include: Chlorophyta--Chlorella, Ulva, Chlamydomonas, Scenedesmus, and Stichococcus; Cyanophyta--Anabaena; and Rhodophyta--Porphyridium. Particular algae species which may be employed in the synthesis of such biopolymers include: Chlorella stigmataphora, Chlorella vulgaris, Chlorella pyrenoidosa, Chlamydomonas mexicana, Ulva lactuca, Scenedesmus obliquus, Scenedesmus braziliensis, Stichococcus bacillaris, Anabaena flos-aquae, Porphyridium aerugineum, and Porphyridium cruentum.
Cultivation of the algae requires a nutrient medium containing nitrogen and other mineral nutrients and micronutrients, a source of assimilable carbon, illumination with light energy, and favorable conditions of temperature, pH, and salinity. Normally carbon dioxide is employed and this is required in the case of the obligate photoautotrophs which are capable of growth only by photosynthetically incorporating carbon dioxide. However, in the case of algae capable of photoheterotrophic growth, assimilable carbon may be provided by a pre-formed organic carbon source such as glucose, mannose, fructose, either alone or in combination with carbon dioxide.
Various processes for the cultivation of algae biomass and production of biopolymers are carried out in two stages, a first stage in which algae growth is initiated and a second stage in which biopolymer is carried to completion. Thus, U.S. Pat. No. 4,079,544 to Savins discloses a biopolymer synthesis procedure employing a culture medium having designated amounts of sodium nitrate and sodium glycerophosphate. In the first stage, the algae culture is subjected continuously to artificial illumination under conditions in which certain radiant energy parameters are controlled. In the second stage, artificial illumination is terminated and the culture is subjected to diurnal cycles of solar radiation and darkness provided by natural outdoor illumination. U.S. Pat. No. 4,078,332 to Savins also discloses a two-stage process for the synthesis of algae biopolymer employing diurnal cycles of solar radiation and darkness in the second stage as well as an apparatus which may be employed in implementing the process. In this procedure, the culture in the first stage is contacted with a mixture of carbon dioxide and air and irradiated with artificial illumination of an intensity and for a time to begin the synthesis of the algae biopolymer. The algae culture is also contacted with a mixture of carbon dioxide and air during the second stage in which biopolymer production is carried to conclusion.
U.S. Pat. No. 4,087,936 to Savins and Anderson discloses a two-stage process for the production of algae biopolymer employing a seed reactor and a main reactor. In the seed reactor, the algae culture is subjected to artificial illumination to initiate algae growth. At least a portion of the algae culture is then transferred from the seed reactor to the main reactor where it is subjected to artificial illumination to effect growth of the algae and synthesis of the algae biopolymer. In both stages a carbon dioxide-air mixture is employed and the linear gradient of the cumulative absorbed radiant energy is manipulated over an initial period of less than two days along with the cumulative absorbed moles of radiant energy quanta from the illumination and the ratio of these two parameters.
Yet another two-stage synthesis procedure in which a nitrogen deficiency is created to enhance biopolymer production is disclosed in U.S. Pat. No. 3,958,364 to Schenck et al. In this procedure a first culture tank is employed as a vegetative growth chamber and a second culture tank is employed as a polymer production chamber. In the first tank, the alga is cultivated in a nitrogen-containing nutrient medium sustaining exponential cell growth until a desired cell density is reached. Thereafter, approximately one-half of the culture volume is transferred from the first tank to the second tank. The first tank is then filled with fresh nutrient medium to its original volume and the second tank is filled with water. In the second tank, a nitrogen deficiency is created which enhances biopolymer production at the expense of cell growth. When the culture in the second tank reaches a desired optical density, it is then removed and the biopolymer recovered.