From Net to Gross: Measuring True Gross Photosynthesis

H218O method

We’re excited to see the publication of a new application for our MIMS.  Recently, we wrote about work done at the University of Hawaii (see “Oh-squared! Ocean Oxygen) and measurements of O2/Ar for net oxygen exchange and ocean productivity.  The new study, again led by Dr. Sara Ferrón at UH, validates the use of MIMS for gross oxygen production measurements in the ocean using the H218O method.  This isotope method measures the ‘water splitting’ reaction of photosynthesis, which is the reaction that provides all of the reducing energy for photosynthesis.  Over the last 30 years, oceanographers have used this method to measure the ‘true’ gross photosynthetic rate of phytoplankton, which is the rate unaffected by respiration and oxygen consumption by the algae or non-algae organisms.


Despite their importance for understanding ocean productivity, true gross photosynthesis measurements have been relatively limited because the method required shipboard experiments followed by precise and slow analyses back in the home laboratory using isotope ratio mass spectrometry (IRMS).  When applied to the oceans, the method is particularly challenging because of the low phytoplankton biomass, low rates of photosynthesis, and small changes in the isotope enrichment of the O2 product.  Nominally, parts per thousand (per mil) differences in the mass 34/32 ratio are required to resolve gross photosynthesis and this precision has heretofore been obtained in IRMS laboratories.

The Ferrón et al. study demonstrated adequate precision using MIMS of the 34/32 ratio for detecting gross production in oligotrophic ocean waters.  Moreover, they validated the MIMS measurements using direct (parallel incubation) comparisons with the IRMS method.  They point out a number of practical advantages of MIMS over IRMS for these measurements, including the option of conducting isotope measurements at sea, lower cost instrumentation, and significantly faster and easier sample processing.  This study was conducted in the most challenging waters with low phytoplankton biomass.  For coastal and inland waters the H218O method should be possible with higher signal to noise and shorter incubations given the higher photosynthesis rates.  We look forward to other laboratories implementing this important technique in a broader range of water bodies.

Posted in MIMS Applications, Oxygen.