# The use of the stable isotope, oxygen-18, as a tracer to measure gross primary production in coastal and oligotrophic waters and in monoclonal cultures of marine phytoplankton

#### Abstract

A new technique was developed to measure the in vitro rates of gross oxygen production in planktonic communities, in which water is enriched with the stable isotope $\sp{18}$O, and photosynthetic evolution of $\sp{18}$O$\sp{16}$O is measured. In order to calculate gross oxygen production, we correct for the $\sp{18}$O fractionation due to respiration.^ The standard technique for measuring production in the oceans, $\sp{14}$C-bicarbonate uptake, has recently been questioned due to discrepancies with other estimates of water column production and suspected intrinsic problems with the $\sp{14}$C technique. One purpose for developing the $\sp{18}$O technique was as a comparison standard for the $\sp{14}$C technique. We compared rates of $\sp{18}$O gross production and $\sp{14}$C production in oligotrophic and coastal sites. Samples were generally incubated under natural lighting with neutral density screening. Some oligotrophic bottles were incubated in situ at the depth of collection. Rates of $\sp{14}$C production were 60-100% of $\sp{18}$O gross production in both coastal and oceanic communities. Assuming a PQ of 1.0 to 1.5, these comparisons suggest that $\sp{14}$C production rates are not seriously underestimating actual in vitro rates of production.^ We also measured rates of light respiration in cultures and natural populations by the $\sp{18}$O technique. The rates of light respiration in algal cultures were generally greater than dark rates by a factor of two to ten. The increased respiration in the light could be due to increased rates of mitochondrial respiration, photorespiration or Mehler respiration. We measured the enhanced $\sp{14}$C production under reduced (O$\sb2$) conditions (the Warburg effect), as an estimate of the rate of photorespiration. In three clones, rates of photorespiration were a significant fraction of light respiration. In five other clones examined, photorespiration was not implicated as a source of light respiration.^ Lastly, in situ rates of production can, theoretically, be calculated from natural $\partial\sp{18}$O of the in situ dissolved oxygen. Unfortunately, the fractionation factors associated with photosynthesis and respiration, as well as the mixing dynamics of the water column, must first be better quantified. ^

Biogeochemistry

#### Recommended Citation

Karen Diane Grande, "The use of the stable isotope, oxygen-18, as a tracer to measure gross primary production in coastal and oligotrophic waters and in monoclonal cultures of marine phytoplankton" (1988). Dissertations and Master's Theses (Campus Access). Paper AAI8903062.
http://digitalcommons.uri.edu/dissertations/AAI8903062

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