INFLUENCE OF INCREASED CARBON DIOXIDE ON PHYTOPLANKTON TRACE METAL QUOTAS IN CULTURES AND FIELD-COLLECTED NATURAL ASSEMBLAGES

participantA significant consequence of future global climate change scenarios on the world's oceans is a rise in the partial pressure of CO2 (pCO2) and associated acidification. This change in carbonate chemistry is likely to affect both the (bio)availability of trace metal ‘micronutrients' to marine phytoplankton and their trace metal requirements. To assess the influence of pCO2 on phytoplankton, we examined trace metal quotas and cell physiology in both culture and field-based incubation experiments using trace metal clean techniques and air:CO2 mixtures including glacial (190 ppm), current (380 ppm), and year 2100 estimates (750 ppm). From various culture experiments with eukaryotic phytoplankton and natural assemblages in field experiments, from the subtropical Pacific (off New Zealand) and the coastal Northeastern Pacific (off southern California), we observed general declines in Fe and Zn quotas with increasing pCO2 – consistent with current trace metal-CO2 availability hypotheses. Interestingly, when co-limited by the essential vitamin B12, a diatom culture experiment showed a positive correlation in Fe, Zn, and Co quotas with pCO2. This highlights the need for experiments that test not only the influence of pCO2 on trace metal quotas, but also the effect of predicted changes in nutrient availability. The use of culture and field approaches for global change experiments both suggest that rising oceanic pCO2 may lead to shifts in phytoplankton trace metal requirements, which in combination with predicted changes in trace metal availability, could have critical impacts on future phytoplankton productivity, dominant phytoplankton taxa, and major nutrient (C, N, and P) oceanic biogeochemistry

Combined effects of CO2 and light on large and small isolates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii from the western tropical Atlantic Ocean

We examined the combined effects of light and pCO2 on growth, CO2-fixation and N2-fixation rates by strains of the unicellular marine N2-fixing cyanobacterium Crocosphaera watsonii with small (WH0401) and large (WH0402) cells that were isolated from the western tropical Atlantic Ocean. In low-pCO2-acclimated cultures (190 ppm) of WH0401, growth, CO2-fixation and N2-fixation rates were significantly lower than those in cultures acclimated to higher (present-day 385 ppm, or future 750 ppm) pCO2 treatments. Growth rates were not significantly different, however, in low-pCO2-acclimated cultures of WH0402 in comparison with higher pCO2 treatments. Unlike previous reports for C. watsonii (strain WH8501), N2-fixation rates did not increase further in cultures of WH0401 or WH0402 when acclimated to 750 ppm relative to those maintained at present-day pCO2. Both light and pCO2 had a significant negative effect on gross : net N2-fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated light and pCO2. These data, along with previously reported results, suggest that C. watsonii may have wide-ranging, strain-specific responses to changing light and pCO2, 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 CO2-fixation rates of C. watsonii may be positively affected by elevated light and pCO2 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