Award: OCE-1536121

Microscopic plants in the surface ocean called phytoplankton use photosynthesis to convert carbon dioxide (CO2) into organic compounds like carbohydrates, fats and proteins. Measuring the rate of photosynthesis by phytoplankton has traditionally relied on enclosing a volume of seawater containing phytoplankton and measuring the rate of conversion of CO2 to organic compounds between dawn and dusk. This measurement method requires a lot of effort which has restricted its application in the ocean. Furthermore, the process of enclosing the phytoplankton in a bottle alters the phytoplankton’s natural environment and thus the photosynthesis rates measured in a bottle likely do not represent the rates occurring in place (referred to as in situ) in the open ocean. In this study we applied a new method to measure photosynthesis rates by phytoplankton. The method was based on measuring small changes in the concentration of dissolved oxygen (O2) and phytoplankton (particles) that occur over a 24 hour day-night (diel) cycle. During the day photosynthesis produces O2 and phytoplankton particles and at night via a process called respiration phytoplankton consume O2 and reduce the size number of phytoplankton particles. (Respiration is the process all animals use to produce the energy required to live.) Notably, this new method does not require bottles and thus measures the in situ rates of photosynthesis and respiration. We measured the diel cycle in O2 and phytoplankton particles at five sites in the North Pacific Ocean across regions with very different photosynthesis rates. The results of our method indicated that the photosynthesis rate varied by a factor of three in the North Pacific with the highest rates occurring in the regions where there were highest concentrations of nutrients (phosphate and nitrate) required by phytoplankton to produce carbohydrates, proteins and fats by photosynthesis. We found that the nighttime rate of respiration almost exactly equaled the daytime rate of photosynthesis at all five sites. We found that the phytoplankton had very short lifetimes of 2 to 4 days based on the measured photosynthesis and respiration rates. We determined that our new method to measure photosynthesis and respiration rates by phytoplankton could be applied using a remotely controlled device called a ‘float’ which measures the diel changes in O2 and particles using sensors attached to the float. The possibility of measuring photosynthesis rates of phytoplankton in the ocean by floats deployed at many locations has the potential to greatly expand our knowledge of how rates of photosynthesis rates vary across the ocean and whether phytoplankton photosynthesis rates could change in response to the global warming that is occurring across the ocean. Last Modified: 12/14/2020 Submitted by: Paul D Quay