An ecological basis for extracellular carbonic anhydrase in marine unicellular algae

Extracellular carbonic anhydrase (CA,) is expressed by many, but not all, autotrophic species of aquatic unicellular protists. We measured CA, activities in unicellular marine algae characteristic of either high nutrient spring, fall, and winter blooms or low nutrient summer populations to provide ecological/evolutionary information about the enzyme. Highest activities occurred in spring bloom and opportunistic diatoms exposed to long photoperiods (16 h) when pH was highest and CO(2) was lowest. Lower activities were recorded for a fall-bloom diatom exposed to the long photoperiod, and lowest values were found under all culture conditions for one diatom and a number of flagellated species typical of summer low nutrient environments. Other potential sources of variance in measurements of CA, activity were examined. Maximum activities of CA, were recorded for the diatom, Skeletonema costatum (Greville) Cleve, during late exponential phase of growth and within 8 h of the beginning of the photoperiod. We concluded that ecological factors are important in determining CA, activities in marine unicellular protists. Potential functions of CA, in the metabolism of marine unicellular algae are discussed.