10.3 Algae Growth and Reaction Conditions
There are two primary ways that algae reproduce. Some algae are unicellular and demonstrate the simplest possible life cycles (see Figure 10.6a). Note that there is a generative phase and a vegetative phase. During the generative phase, cysts are freed. The cysts open to form gametes and then form the zygote. From there, the vegetative phase occurs so the plant grows and new cysts can form. Most algae have two recognizable phases, sporophyte and gametophyte. Figure 10.6b shows a schematic of the two phases. The main difference is a male and female type is required to form the zygote. I will not be expecting you to know the details in depth, but want you to recognize there are differences.
Algae have a particular path of growth, beginning with a lag phase, and continuing on to an exponential phase, a linear phase, and stationary phase, and decline of death phase. Figure 10.7 shows a schematic of algal growth rate in a batch culture.
There are several factors that influence the growth rate. The temperature will vary with algae species. The optimal temperature range for phytoplankton cultures is 20-30°C. If temperatures are higher than 35°C, it can be lethal for a number of algal species, especially green microalgae. Temperatures that are lower than 16°C will slow down the growth of algae.
Light also has an effect on the growth of algae: it must not be too strong or weak. In most algal growth cultivation, algae only need about 1/10 of direct sunlight. In most water systems, light only penetrates the top 7-10 cm of water. This is due to bulk algal biomass, which blocks light from reaching into deeper water.
Mixing is another factor that influences the growth of algae. Agitation or circulation is needed to mix algal cultures. An agitator is used for deep photo reactor systems. Paddle wheels are used for open pond systems. And pump circulation is used for a photo-tube system.
Of course, algae need nutrients and the proper pH to grow effectively. Autotrophic growth requires carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur, iron, and trace elements. The compositional formula of C O1.48 H1.83 N0.11 P0.01 can be used to calculate the minimum nutrient requirement. Under nutrient limiting conditions, growth is reduced significantly and lipid accumulation is triggered. Algae prefer a pH from neutral to alkaline.
There are particular steps for algal biodiesel production. Figure 10.8 shows the processing steps for algae production in biodiesel production. The first step is the cultivation of algae, which includes site selection, algal culture selection and process optimization. Process optimization includes design of the bioreactor and necessary components for algal cell growth (nutrients, light, and mass transfer). Once the algae grow to the necessary level, the algae are harvested. The biomass must first be processed in order to dewater, thicken, and dry the algae in order to extract the oil that will then be processed into biodiesel. The biomass process differs depending on the method of oil extraction and biodiesel production. You primarily learned about transesterification to make biodiesel in Lesson 9, but there are other processes being researched and developed.