Stable isotope (13C) and O2 micro-optode alternatives for measuring photosythesis in seaweeds

Many photosynthetic studies with macroalgae are based on oxygen evolution. This approach is preferred over 14C tracer techniques since oxygen electrodes are generally inexpensive and radioactive wastes are not produced. Nevertheless, oxygen techniques do not directly measure carbon assimilation; thus, primary production estimates rely on the conversion of oxygen units to carbon units via a photosynthetic quotient, which may vary depending on physiological state and nutrient status of the alga. We present in detail 2 methods for measuring photosynthesis in macroalgae, both of which should enhance prospects for photosynthetic research, particularly under field conditions. First, a carbon uptake procedure is described which relies on incorporation of a stable 13C isotope label. Important advantages of the 13C method include simplicity of sample processing, avoidance of environmental hazards and restrictions of radioactive 14C, and ability to be used as a dual tracer with 15N. Second, we employed a fiber-optic micro-optode system for measurement of dissolved oxygen. The fluorescence-based optodes stabilize quickly (<15 s), do not consume oxygen, and are simple to set up and maintain. Oxygen and carbon photosynthesis were assessed concurrently in a light/dark chamber design. Both techniques resolved significant decreases in lightsaturated photosynthesis (Pmax) in the kelp Laminaria hyperborea collected subtidally at 10 m compared to kelp at 0 m (from mean low tide). Oxygen and carbon photosynthetic rates agreed well with published values and were mutually consistent, considering a reasonable photosynthetic quotient. Estimated measurement errors (<2%) associated with the 13C technique were less than intrinsic variation (>20%) in photosynthetic rates between kelp individuals with similar light histories. Both techniques should provide an alternative to traditional photosynthetic protocols and stimulate further primary production research in macrophytes.