We examined the combined effects of light and pCO
2 on growth, CO
2-fixation and N
2-fixation rates by strains of the unicellular marine N
2-fixing cyanobacterium
Crocosphaera watsonii with small (WH0401) and large (WH0402) cells that were isolated from the western tropical Atlantic Ocean. In low-pCO
2-acclimated cultures (190 ppm) of WH0401, growth, CO
2-fixation and N
2-fixation rates were significantly lower than those in cultures acclimated to higher (present-day ~385 ppm, or future ~750 ppm) pCO
2 treatments. Growth rates were not significantly different, however, in low-pCO
2-acclimated cultures of WH0402 in comparison with higher pCO
2 treatments. Unlike previous reports for
C. watsonii (strain WH8501), N
2-fixation rates did not increase further in cultures of WH0401 or WH0402 when acclimated to 750 ppm relative to those maintained at present-day pCO
2. Both light and pCO
2 had a significant negative effect on gross : net N
2-fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated light and pCO
2. These data, along with previously reported results, suggest that
C. watsonii may have wide-ranging, strain-specific responses to changing light and pCO
2, emphasizing the need for examining the effects of global change on a range of isolates within this biogeochemically important genus. In general, however, our data suggest that cellular N retention and CO
2-fixation rates of
C. watsonii may be positively affected by elevated light and pCO
2 within the next 100 years, potentially increasing trophic transfer efficiency of C and N and thereby facilitating uptake of atmospheric carbon by the marine biota.
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