I guess you want to do a batch cultivation. Depending on your experimental setup and the algae species you should calculate with 7-14 days as cultivation time to ensure you will get enough datapoints in the exponential growth phase for proper calculation of the specific growth rate.

Yes, I will do batch culture. So I will have a lag phase, a log phase and stationary. To calculate the specific growth rate for each single condition (let's say light=100, T=25 and pH=7.5, which two data points do I have to use for calculation the specific growth rate? at the beginning and the end of the log phase? or the whole cultivation curve?

Only use the data points in the exponential growth phase, preferably the first and the last datapoint in the exponential phase (biggest shift in biomass concentration).

I do not like guessing, so the first point is making clear what your question is, i.e. what do you want to know:  to make clear my point: you might wish to measure growth rates of acclimated cultures, or you might want to know the effect of environmental changes on growth rate.

In the first case, you have to make sure that the acclimation process is complete (this might take a few generations (that is, a succession of several cell reproductions). In the second case, you have to run several growth curves with acclimated cells, get your mean growth rate and use these cultures to start new ones in the changed environment.

Once said this, you may avoid completely the lag phase, by using cultures which are still  growing exponentially or nearly so (you will get the hamng of this after running a few growth curves), and definitely you  will have to run experiments lasting a few days, to make sure of the point where your exponential growth ceases to be so.

here a couple of paper which might help to make clear my points

Domenico:

Thank you very much for your support. Actually based on what you're asking, I would say I want to know the effect of environmental parameters on specific growth rate. Based on what you have uploaded, I think the last article would help me in dealing with this issue

Thanks again

no need to thank. it's why we sign up here. actually, it's why we chose this as a job.

If there's any doubt, you may contact me at [email protected] or [email protected]

The specific growth rate of fed-batch biotechnological processes depends generaly on the temperature and the substrate feed rate. The optimalgrowth rate control as approach is inovative in biotechnology:

Equivalent Forms of Wang-Yerusalimsky Kinetic Model and Optimal Growth Rate Control of Fed-batch Cultivation Processes