West, Maria.

Cyanobacteria are classified as alkalophiles despite their preferential uptake of the acidic form of dissolved inorganic carbon. Long term impacts of external pH on the expression of photosynthetic and structural pigments in Schizothrix calcicola were investigated as potential contributing factors to this phenomenon. More robust cell walls in S. calcicola at pH <7 are suggested by significantly greater expression of myxoxanthophylls. Direct and indirect physiological costs of altering cell walls may contribute to S. calcicola's depressed growth at acidic pH. Comparison of chlorophylls expression suggests that alkaline rather than neutral external pH is only beneficial for S. calcicola growth in absence of nutrient limitation. While the cyanobacterial biomarker ratio of chlorophylls to echinenone was stable across the pertinent pH range of 6-8, other pigment ratios in S. calcicola were affected by pH with an approximately two week lag between the change of pH and the corresponding change of pigment expression.