FITNESS TRADE-OFFS OF SELENASTRUM-CAPRICORNUTUM STRAINS SELECTED FOR RAPID GROWTH ON COPPER-SPIKED SOLIDIFIED NUTRIENT MEDIUM

Stock (ancestral) cultures of Selenastrum capricornutum grew optimally in low Cu liquid nutrient medium (LNM) containing 0.004 mu g L(-1) Cu2+. Growth of stock cultures was optimized on low Cu solidified nutrient medium (SNM) containing 20 mu g L(-1) Cu2+. Cell strains (clones) were isolated from unusually large colonies on high Cu SNM (1,000 mu g L(-1) Cu2+). Most isolated strains performed the same as ancestral populations in high Cu LNM (50 mu g L(-1) Cu2+), but three strains grew more slowly. These ''variant strains'' were Cu tolerant on SNM (equal growth with low and high Cu) and produced larger colonies than did the ancestral population on both low and high Cu SNM. In contrast, colonies from ancestral cell samples grew more slowly in high versus low Cu SNM. No significant growth differences were observed between ancestral populations and variant strains in low Cu LNM, suggesting low fixed costs of adaptation to SNM. However, slower growth of variant strains in high Cu LNM constituted a fitness trade-off associated with adaptation. Rapid adaptation to pollution may often result in important biological trade-offs even when costs of adaptive mechanisms are low.