Photosynthetic action, absorption, and quantum yield spectra for a natural population of Oscillatoria in the North Atlantic1

Measurements of the photosynthetic action and absorption spectra were made of marine Oscillatoria ( Trichodesmium ) from a summer surface bloom in the North Atlantic. The absorption spectrum was typical of those of other phytoplankton but with small peaks at 627 and 567 nm and a large peak at 493, consistent with the presence of phycocyanin, phycoerythrobilin, and phycourobilin respectively. The photosynthetic action spectrum was determined by measurement of the initial slope of the photosynthesis‐irradiance curve every 25 nm from 400 to 675 nm (25‐nm bandpass); it was dominated by the phycourobilin waveband. Calculations of quantum yield indicated that it was constant from 475 to 650 nm and was near maximal theoretical values. In wavebands corresponding to chlorophyll absorption (400‐450 and 675 nm), quantum yield dropped off sharply. Photosynthesis‐irradiance curves determined on both whole colonies and individual trichomes in “white” light indicated that both the initial slope and the degree of photoinhibition at high light were enhanced when the colonies were disrupted‐the former by a factor of ten, although the maximum rate of photosynthesis was unchanged. These optical and photosynthetic characteristics of marine Oscillatoria are consistent with adaptation to the high light environment where they typically bloom.