PHOTOSYNTHETIC EFFICIENCY OPTIMIZATION STUDIES WITH THE MACROALGA GRACILARIA-TIKVIHAE - IMPLICATIONS FOR CO2 EMISSION CONTROL FROM POWER-PLANTS

The photosynthetic efficiency (PE) with which the macroalga Gracilaria tikvihae converts visible light energy into chemical energy was studied as a function of irradiance, temperature, and salinity in tumble culture systems at the Natural Energy Laboratory of Hawaii. The photosynthesis/irradiance curve exhibited a typical hyperbolic shape, the associated PEs being a maximum at a visible irradiance of about 500 kcal m-2 day-1. The highest PEs were obtained in seawater diluted by 10% with freshwater; the maximum PEs under these conditions exceeded 7% in full sunlight and 12% in the region of optimal irradiance. PEs were almost identical at 21-degrees-C and 25-degrees-C, but declined sharply as the temperature was reduced below 21-degrees-C. Conversion of the algal biomass to methane by anaerobic fermentation resulted in conversion efficiencies as high as 22% at a detention time of 15 days. This efficiency is substantially higher than results reported in an earlier study, the difference apparently reflecting the use of freshwater rather than seawater as the fermentation medium. To the extent that CO2 emissions from electric power plants are reduced by scrubbing the stack gases, growth of algae such as G. tikvihae may be the most logical way to utilize the CO2. If 20% of the CO2 presently emitted by coal-fueled power plants in the US were used to grow algae, an area of land equal to roughly 1% of the area of the United States would be required for the growth of the algae.