Projected expansion of Trichodesmium’s geographical distribution and increase of growth potential in response to climate change

Estimates of marine N₂ fixation range from 52 to 73 Tg N yr‾¹, of which we calculate up to 84% is from Trichodesmium based on previous measurements of nifH gene abundance and our new model of Trichodesmium growth. Here we assess the likely effects of four major climate change‐related abiotic factors on the spatiotemporal distribution and growth potential of Trichodesmium for the last glacial maximum (LGM), the present (2006‐2015) and the end of this century (2100) by mapping our model of Trichodesmium growth onto inferred global surface ocean fields of pCO₂, temperature, light and Fe. We conclude that growth rate was severely limited by low pCO₂ at the LGM, that current pCO₂ levels do not significantly limit Trichodesmium growth and thus, the potential for enhanced growth from future increases of CO₂ is small. We also found that the area of the ocean where sea surface temperatures (SST) are within Trichodesmium’s thermal niche increased by 32% from the LGM to present, but further increases in SST due to continued global warming will reduce this area by 9%. However, the range reduction at the equator is likely to be offset by enhanced growth associated with expansion of regions with optimal or near optimal Fe and light availability. Between now and 2100, the ocean area of optimal SST and irradiance is projected to increase by 7%, and the ocean area of optimal SST, irradiance and iron is projected to increase by 173%. Given the major contribution of this keystone species to annual N₂ fixation and thus pelagic ecology, biogeochemistry and CO₂ sequestration, the projected increase in the geographical range for optimal growth could provide a negative feedback to increasing atmospheric CO₂ concentrations

An integrated response of Trichodesmium erythraeum IMS101 growth and photo-physiology to Iron, CO₂, and light intensity

We have assessed how varying CO 2 (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m -2 s -1 ) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe') concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rP m ). Under iron-limiting concentrations, high-light increased growth rates and rPm; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO 2 increased growth rates across all iron concentrations, enabled growth to occur at lower Fe' concentrations, increased rPm and lowered the iron half saturation constants for growth (K m ). We attribute these CO 2 responses to the operation of the CCM and the ATP spent/saved for CO 2 uptake and transport at low and high CO 2 , respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO 2 , light intensity and iron-limitation. These results are important given predictions of increased dissolved CO 2 and water column stratification (i.e., higher light exposures) over the coming decades

Inorganic carbon and pH dependency of Trichodesmium's photosynthetic rates.

We established the relationship between photosynthetic carbon fixation rates and pH, CO2 and HCO3- concentrations in the diazotroph Trichodesmium erythraeum IMS101. Inorganic 14C-assimilation was measured in TRIS-buffered ASW medium where the absolute and relative concentrations of CO2, pH and HCO3- were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (< 0 to ~ 5 mM) at constant pH, so ratios of CO2 and HCO3- remained relatively constant. Second, we varied pH (~ 8.54 to 7.52) at constant TIC, so CO2 increased whilst HCO3- declined. We found that 14C-assimilation could be described by the same function of CO2 for both approaches but showed different dependencies on HCO3- when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of Trichodesmium's CCM showed carboxylation rates are modulated by HCO3- and pH. The decrease in Ci assimilation at low CO2, when TIC was varied, is due to HCO3- uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO3- and CO2 leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions

CO2 modulation of the rates of photosynthesis and light-dependent O2 consumption in Trichodesmium

As atmospheric CO₂ concentrations increase, so too does the dissolved CO₂ and HCO₃‾ concentrations in the world’s oceans. There are still many uncertainties regarding the biological response of key groups of organisms to these changing conditions, which is crucial for predicting future species distributions, primary productivity rates, and biogeochemical cycling. In this study, we established the relationship between gross photosynthetic O₂ evolution and light-dependent O₂ consumption in Trichodesmium erythraeum IMS101 acclimated to three targeted pCO₂ concentrations (180 µmol mol‾¹=low-CO₂, 380 µmol mol‾¹=mid-CO₂, and 720 µmol mol‾¹=high-CO₂). We found that biomass- (carbon) specific, light-saturated maximum net O₂ evolution rates (PnC,max) and acclimated growth rates increased from low- to mid-CO₂, but did not differ significantly between mid- and high-CO₂. Dark respiration rates were five times higher than required to maintain cellular metabolism, suggesting that respiration provides a substantial proportion of the ATP and reductant for N₂ fixation. Oxygen uptake increased linearly with gross O₂ evolution across light intensities ranging from darkness to 1100 µmol photons m¯² s¯¹. The slope of this relationship decreased with increasing CO₂, which we attribute to the increased energetic cost of operating the carbon-concentrating mechanism at lower CO₂ concentrations. Our results indicate that net photosynthesis and growth of T. erythraeum IMS101 would have been severely CO₂ limited at the last glacial maximum, but that the direct effect of future increases of CO₂ may only cause marginal increases in growth

A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101

Trichodesmium is a globally important marine diazotroph that accounts for approximately 60-80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt

Modelling human choices: MADeM and decision‑making

Research supported by FAPESP 2015/50122-0 and DFG-GRTK 1740/2. RP and AR are also part of the Research, Innovation and Dissemination Center for Neuromathematics FAPESP grant (2013/07699-0). RP is supported by a FAPESP scholarship (2013/25667-8). ACR is partially supported by a CNPq fellowship (grant 306251/2014-0)

The median growth rate, and mean (± S.E.) <i>p</i>CO₂ and maximum dark-adapted photochemical efficiency (<i>F</i>_<i>v</i><i>/F</i>_<i>m</i>) of <i>Trichodesmium erythraeum</i> IMS101 when acclimated across a range of temperatures, light intensities, and at three target <i>p</i>CO₂ concentrations (Low = 180 ppm, Mid = 380 ppm and High = 720 ppm) (174 treatments).

<p>For the temperature response: LL = 40 μmol photons m^-2 s^-1; HL = 400 μmol photons m^-2 s^-1. Note the dashed line represents the initial <i>p</i>CO₂ concentration for each treatment culture once diluted (T = 0), while the data points are the final CO₂ concentration post culturing.</p

The mean (± S.E.) growth conditions of <i>Trichodesmium erythraeum</i> IMS101 cultures for the temperature and light response.

<p>The mean (± S.E.) growth conditions of <i>Trichodesmium erythraeum</i> IMS101 cultures for the temperature and light response.</p

The light-dependent growth curve parameters (± S.E.) for <i>Trichodesmium erythraeum</i> IMS101 generated by fitting a three-parameter P-I function to each of the three CO₂ (Low = 180 ppm, Mid = 380 ppm and High = 720 ppm) treatments at optimal temperature (26°C).

<p>The light-dependent growth curve parameters (± S.E.) for <i>Trichodesmium erythraeum</i> IMS101 generated by fitting a three-parameter P-I function to each of the three CO₂ (Low = 180 ppm, Mid = 380 ppm and High = 720 ppm) treatments at optimal temperature (26°C).</p

The temperature dependent growth curve parameters (± S.E.) for <i>Trichodesmium erythraeum</i> IMS101 generated by fitting a five-parameter function to each of the two light (LL = 40 μmol photons m^-2 s^-1; HL = 400 μmol photons m^-2 s^-1) treatments at the three CO₂ (Low = 180 ppm, Mid = 380 ppm and High = 720 ppm) treatments.

<p>The temperature dependent growth curve parameters (± S.E.) for <i>Trichodesmium erythraeum</i> IMS101 generated by fitting a five-parameter function to each of the two light (LL = 40 μmol photons m^-2 s^-1; HL = 400 μmol photons m^-2 s^-1) treatments at the three CO₂ (Low = 180 ppm, Mid = 380 ppm and High = 720 ppm) treatments.</p