Diazotrophic marine cyanobacteria in the genus Trichodesmium contribute a large fraction of the new nitrogen entering the oligotrophic oceans, but little is known about how they respond to shifts in global change variables such as carbon dioxide (CO 2 ) and temperature. We compared Trichodesmium dinitrogen (N 2 ) and CO 2 fixation rates during steady-state growth under past, current, and future CO 2 scenarios, and at two relevant temperatures. At projected CO 2 levels of year 2100 (76 Pa, 750 ppm), N 2 fixation rates of Pacific and Atlantic isolates increased 35-100%, and CO 2 fixation rates increased 15-128% relative to present day CO 2 conditions (39 Pa, 380 ppm). CO 2 -mediated rate increases were of similar relative magnitude in both phosphorus (P)-replete and P-limited cultures, suggesting that this effect may be independent of resource limitation. Neither isolate could grow at 15 Pa (150 ppm) CO 2 , but N 2 and CO 2 fixation rates, growth rates, and nitrogen : phosophorus (N : P) ratios all increased significantly between 39 Pa and 152 Pa (1500 ppm). In contrast, these parameters were affected only minimally or not at all by a 4uC temperature change. Photosynthesis versus irradiance parameters, however, responded to both CO 2 and temperature but in different ways for each isolate. These results suggest that by the end of this century, elevated CO 2 could substantially increase global Trichodesmium N 2 and CO 2 fixation, fundamentally altering the current marine N and C cycles and potentially driving some oceanic regimes towards P limitation. CO 2 limitation of Trichodesmium diazotrophy during past glacial periods could also have contributed to setting minimum atmospheric CO 2 levels through downregulation of the biological pump. The relationship between marine N 2 fixation and atmospheric CO 2 concentration appears to be more complex than previously realized and needs to be considered in the context of the rapidly changing oligotrophic oceans.

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Although Fe limitation is well documented in open ocean high-nutrient, low-chlorophyll

We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (ϳ0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH) 4 : NO drawdown ratios were low in both experiments (ϳ0.2-0.7), but net particulate silica to nitrogen production Ϫ 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to

Abstract. Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4 • C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments.