Can I use phosphate as a variable to change rates of photosynthesis?

Yes-absolutely!   But, it is not always the easiest to control.

Phosphate is one of the macronutrients.  N:P:K in photosynthetic world are the macronutrients Nitrogen (N), Phosphorus (P), and Potassium (K).  For most algae cultures, a ratio of 5:1:1 to 10:1:1 is appropriate for NPK.  

Knowing that the nitrogen and phosphorus are significantly different in the concentrations needed to satisfy the growth of plants, we at ARS will encourage folks new to growing algae and experiments to use nitrogen, not phosphorus, for their first experiments.   You are much more likely to successful limiting N than P.  

Why:  Higher concentrations of N and it is easier to measure (test strip).  Measuring phosphorus is challenging because very low concentrations can have dramatic effects on photosynthesis.   Your body is composed of NPK, how much phosphate will be added to water by simply dipping in your finger?   

I want to test phosphate.

No problem.   We suggest doing a two-step dose response (video on how to).  Step 1- Test from a concentration of ZERO, and increase the concentration exponentially to a toxic concentration.   In doing so, you can capture the entire range of what is possible.  

Step-2 Refine the range to explore your point of interest.  You could consider exploring the fastest growth, largest cells, minimum concentration for growth, or lethal dose (LD)

How much phosphate should I add?

Water concentrations of phosphate are between 0.001 to 0.5mg/L.  So pick a your dilutions to encapsulate those ranges.  

• Phosphate is an ion PO³⁻ , so when you add a salt of phosphate you need to consider the molar range, not just the mass.
• Consider what other ions are included, it is often expeditious for fertilizer manufacturers to include a nitrogen in the material as well.  
• Some phosphate is insoluble in water- rock phosphate.

sources of phosphate:

• ammonium phosphate: (careful, has ammonia, a nitrogen source)
• calcium phosphate: ( ok to use)
• Trisodium phosphate:  often used as a cleaner be careful if it has additional chemicals.
• rock phosphate:  not readily soluble in water
• phosphoric acid:  our favorite. Its already dissolved and can be pipetted.  

Resources:  

Great article by the US EPA on water monitoring of phosphorus.  

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