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 (CO₂) and temperature. We compared Trichodesmium dinitrogen (N₂) and CO₂ fixation rates during steady‐state growth under past, current, and future CO₂ scenarios, and at two relevant temperatures. At projected CO₂ levels of year 2100 (76 Pa, 750 ppm), N₂ fixation rates of Pacific and Atlantic isolates increased 35–100%, and CO₂ fixation rates increased 15‐128% relative to present day CO₂ conditions (39 Pa, 380 ppm). CO₂‐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) CO₂, but N₂ and CO₂ 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 4°C temperature change. Photosynthesis versus irradiance parameters, however, responded to both CO₂ and temperature but in different ways for each isolate. These results suggest that by the end of this century, elevated CO₂ could substantially increase global Trichodesmium N₂ and CO₂ fixation, fundamentally altering the current marine N and C cycles and potentially driving some oceanic regimes towards P limitation. CO₂ limitation of Trichodesmium diazotrophy during past glacial periods could also have contributed to setting minimum atmospheric CO₂ levels through downregulation of the biological pump. The relationship between marine N₂ fixation and atmospheric CO₂ concentration appears to be more complex than previously realized and needs to be considered in the context of the rapidly changing oligotrophic oceans.

Ocean acidification produced by dissolution of anthropogenic carbon dioxide (CO ₂ ) emissions in seawater has profound consequences for marine ecology and biogeochemistry. The oceans have absorbed one-third of CO ₂ emissions over the past two centuries, altering ocean chemistry, reducing seawater pH, and affecting marine animals and phytoplankton in multiple ways. Microbially mediated ocean biogeochemical processes will be pivotal in determining how the earth system responds to global environmental change; however, how they may be altered by ocean acidification is largely unknown. We show here that microbial nitrification rates decreased in every instance when pH was experimentally reduced (by 0.05–0.14) at multiple locations in the Atlantic and Pacific Oceans. Nitrification is a central process in the nitrogen cycle that produces both the greenhouse gas nitrous oxide and oxidized forms of nitrogen used by phytoplankton and other microorganisms in the sea; at the Bermuda Atlantic Time Series and Hawaii Ocean Time-series sites, experimental acidification decreased ammonia oxidation rates by 38% and 36%. Ammonia oxidation rates were also strongly and inversely correlated with pH along a gradient produced in the oligotrophic Sargasso Sea ( r ² = 0.87, P < 0.05). Across all experiments, rates declined by 8–38% in low pH treatments, and the greatest absolute decrease occurred where rates were highest off the California coast. Collectively our results suggest that ocean acidification could reduce nitrification rates by 3–44% within the next few decades, affecting oceanic nitrous oxide production, reducing supplies of oxidized nitrogen in the upper layers of the ocean, and fundamentally altering nitrogen cycling in the sea.

We examined the effects of increased temperature, pCO₂, and irradiance on a calcifying strain of the marine coccolithophore Emiliania huxleyi in semi-continuous laboratory cultures. Emiliania huxleyi CCMP 371 was cultured in four temperature and pCO₂ treatments at both low and high irradiance (50 and 400 µmol photons m⁻² s⁻¹): (i) 20°C and 375 ppm CO₂ (ambient control); (ii) 20°C and 750 ppm CO₂ (high pCO₂); (iii) 24°C and 375 ppm CO₂ (high temperature); and (iv) 24°C and 750 ppm CO₂ (‘greenhouse’). The growth of E. huxleyi was greatly accelerated by elevated temperature at low irradiance. Photosynthesis was significantly promoted by increases in both pCO₂ and temperature at both irradiances. Higher cellular C/P ratios were found in the higher CO₂ treatments at high irradiance, indicating a reduced requirement for P. The PIC/POC (particulate inorganic to organic carbon) ratio remained constant at low light, regardless of CO₂ or temperature conditions. However, both the cellular PIC content and PIC/POC ratio were greatly decreased by elevated irradiance, and were further decreased by increased pCO₂ only at high light, indicating a combined effect of CO₂ and light on calcification. These results suggest that future trends of CO₂ enrichment, sea-surface warming and exposure to higher mean irradiances from intensified stratification will have a large influence on the growth of Emiliania huxleyi, and potentially on the PIC/POC ‘rain ratio’. Our study demonstrates that it is possible to obtain a more complete picture of global change impacts on marine phytoplankton by designing experiments that consider multiple global change variables and their mutual interactions.

International audienceThe North Atlantic spring bloom is one of the largest annual biological events in the ocean, and is characterized by dominance transitions from siliceous (diatoms) to calcareous (coccolithophores) algal groups. To study the effects of future global change on these phytoplankton and the biogeochemical cycles they mediate, a shipboard continuous culture experiment (Ecostat) was conducted in June 2005 during this transition period. Four treatments were examined: (1) 12°C and 390 ppm CO2 (ambient control), (2) 12°C and 690 ppm CO2 (high pCO2), (3) 16°C and 390 ppm CO2 (high temperature), and (4) 16°C and 690 ppm CO2 ('greenhouse'). Nutrient availability in all treatments was designed to reproduce the low silicate conditions typical of this late stage of the bloom. Both elevated pCO2 and temperature resulted in changes in phytoplankton community structure. Increased temperature promoted whole community photosynthesis and particulate organic carbon (POC) production rates per unit chlorophyll a. Despite much higher coccolithophore abundance in the greenhouse treatment, particulate inorganic carbon production (calcification) was significantly decreased by the combination of increased pCO2 and temperature. Our experiments suggest that future trends during the bloom could include greatly reduced export of calcium carbonate relative to POC, thus providing a potential negative feedback to atmospheric CO2 concentration. Other trends with potential climate feedback effects include decreased community biogenic silica to POC ratios at higher temperature. These shipboard experiments suggest the need to examine whether future pCO2 and temperature increases on longer decadal timescales will similarly alter the biological and biogeochemical dynamics of the North Atlantic spring bloom