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 C-14 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 C-13 isotope label. Important advantages of the C-13 method include simplicity of sample processing, avoidance of environmental hazards and restrictions of radioactive C-14, and ability to be used as a dual tracer with N-15. 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 light-saturated photosynthesis (P-max) 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 C-13 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.