Photosynthesis in the symbiotic planktonic foraminifer Orbulina universa, and its potential contribution to oceanic primary productivity

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Abstract
The morphology and photosynthetic-irradiance characteristics of the planktonic foraminifer, Orbulina universa, and its symbiotic algae are discussed. Symbiont concentrations ranged between 130 and 3,300 cells per foraminifer in measured specimens, and was highly correlated with the combined maximum length of the trochospiral "juvenile" chambers. Estimates of symbiont concentrations in large spherical "adult" specimens suggests densities of 23,000 algae per foraminifer. This provides a new and easily measured indicator of symbiont density. Freshly collected foraminifera had photosynthetic characteristics which resembled sun-adapted algae: a saturation intensity (Ik) of 386 .mu.E m-2 s-1, a high light-saturated photosynthetic rate of 1.72 .times. 10-6 .mu.Mol C symbiotic alga-1 h-1, and no susceptibility to photoinhibition at intensities up to 800 .mu.E m-2 s-1. Afternoon light-saturated photosynthetic rates were ten times higher than evening and early morning rates. From these data, and published measurements of primary productivity in oligotrophic environments, it is estimated that a single, large Orbulina universa would be 20,000 times more productive than an equivalent volume of seawater. Foraminiferal densities of only 5/m3, therefore, would contribute about 1.0% of the total inorganic carbon fixed by all primary producers in 1 m3 of seawater. In patches of high foraminiferal density, this value may be much higher.