Culturing and experimental conditions
Stock cultures of the two Atlantic C. watsonii isolates were provided courtesy of Dr. Eric Webb. Both isolates were collected in March 2002, WH0401 from 6º 58.78' N, 49º 19.70' W and WH0402 from 11º 42.12S', 32º 00.64'W. Triplicate cultures were grown using a semi-continuous culturing technique (Garcia et al., 2011) at 28 degrees C in an artificial seawater medium (Chen et al., 1996). Nutrients were added to autoclaved seawater at the concentrations listed in the AQUIL recipe (Morel et al., 1979), except for nitrate, which was omitted. The growth rates of cultures were measured over 2–3 day intervals and were used to determine the dilution rate. Culture cell density was kept low (cells ml–1 = 50–500 × 103 for experiments with WH0401 and 5.0–30 × 103 for WH0402) to prevent light limitation of photosynthesis and deviation from the expected pH values for respective pCO2 culture treatments. Light was supplied with cool-white fluorescent lamps on a 12:12 h light:dark cycle and measured with a LI-250A light meter (LiCor Biosciences, light sensor serial# SPQA 4020). Because of large differences in cell size between WH0401 and WH0402, WH0401 was cultured at higher cell densities to maintain relatively equivalent levels of total culture biomass (0.1–2.5 mM particulate C for cultures of WH0401; 0.1–1.3 mM particulate C for WH0402). Cells were considered fully acclimated to treatment conditions after cultures had remained at steady-state growth for seven generations or more (unless stated otherwise). Fast growing cultures (i.e. high light cultures) were acclimated for more than ten generations while slow growing cultures (i.e. low light and low pCO2 cultures) were acclimated over two months but for fewer generations. Cultures were sampled over the period between 24 and 48 h after the preceding dilution to measure growth rates, gross and net 15N2-fixation rates, CO2-fixation rates, and particulate elemental composition.

Light experiments
In order to quantify differences in growth and in the CO2- and N2-fixation rate capacities of these two isolates of C. watsonii, the investigators measured growth, CO2-fixation and gross and net N2-fixation rates, and particulate carbon and nitrogen composition in response to a range of light intensities.

Growth rate and cell density estimates
Growth rate was determined as an increase in culture cell density over time with the equation N_T=N₀e^µT, where N₀ and N_T are the initial and final culture cell densities, respectively, T is the time in days between culture cell density estimates, and µ is the specific growth rate. Culture cell density was determined using a haemocytometer and an Olympus BX51 microscope. Cell diameter was measured using an ocular micrometer calibrated with the same microscope. Growth rates were fitted to a Monod linear hyperbolic function of light (Monod, 1949) using Sigma Plot 10 software program. The hyperbola was fit to the data without including the origin to yield the highest r² value.

N2 fixation
The acetylene reduction assay described by Capone et al. (1993) was used to estimate the gross N2-fixation rate. Rate measurements were initiated at the beginning of the 12-h dark period, when C. watsonii is known to fix N2 (Mohr et al., 2010a; Saito et al., 2011). Gross N2-fixation rates were calculated in the same way as described in Garcia et al. (2011), using a Bunsen coefficient for ethylene of 0.082 (Breitbarth et al., 2004) and an ethylene production:N2-fixation ratio of 3:1.

Net N2-fixation rates were measured using the 15N2 isotope tracer method (Mulholland & Bernhardt, 2005; Mulholland et al., 2004). Samples were prepared the same way as described in Garcia et al. (2011). Briefly, 169 ml of each experimental replicate was inoculated with 169 µl of 99% doubly labelled 15N2 gas and incubated at 28 degrees C in complete darkness for 12 h during the dark period. The incubation was then terminated by filtering the entire volume onto precombusted (450 degree C, 4 h) GF/F filters for the analysis of particulate 15N, total particulate N, and total particulate C. Filters were dried at 80–90 degrees C, pelleted, and combusted in a quartz column with chromium oxide and silver wool at 1000 degrees C. For this analysis, ammonium sulphate and sucrose were used as standards. At the time the experiments were conducted, the investigators were not aware of the criticisms of the 15N2 uptake method that have been discussed by Mohr et al. (2010b). Thus, for another independent estimate of net N2 fixation, the investigators calculated a particulate N (PN) accumulation rate in cultures over time (deltaPN = PNfinal - PNinitial). Particulate N was measured in subsamples of experimental replicates that were incubated with 15N2 at the end of the dark period and used as the end-period PN measurement (PNfinal). Because only one sample of PN was collected, the investigators back-calculated an estimate of PNinitial based on their measurements of cellular growth rate using the equation: growth rate (d–1) = [ln(PNfinal)–ln(PNinitial)]/(t2–t1), where t1 is the initial time and t2 is the final time. Based on their measurements of growth rates, the investigators assumed that PN per cell was in a daily steady state. The gross N2-fixation rate:PN-accumulation rate ratio (hereafter the gross:PN accumulation ratio) was then calculated and compared to the ratio of gross N2-fixation rate:net 15N2-fixation rate ratio (gross:net), which is a proxy for cellular N retention (Mulholland et al., 2004; Mulholland, 2007).

CO2 fixation
The rate of CO2 fixation was determined as described in Garcia et al. (2011) using the H14CO3- incorporation method. CO2-fixation rates were determined by first calculating the ratio of the radioactivity of 14C incorporated into cells during 24 hours to the total radioactivity of H14CO3–. This ratio was then multiplied by the total CO2 concentration (TCO2). TCO2 concentrations were measured in the CO2-light experiments and were applied to all experiments to calculate CO2-fixation rates for corresponding CO2 treatments. For the light experiments, the investigators used a TCO2 value that was measured in the present-day pCO2 treatments of the CO2-light experiments (2053 µM TCO2).

References:
BREITBARTH, E., MILLS, M.M., FRIEDRICHS, G. & LAROCHE, J. (2004). The Bunsen gas solubility coefficient of ethylene as a function of temperature and salinity and its importance for nitrogen fixation assays. Limnology and Oceanography: Methods, 2: 282–288. DOI: 10.4319/lom.2004.2.282

CHEN, Y.B., ZEHR, J.P. & MELLON, M. (1996). Growth and nitrogen fixation of the diazotrophic filamentous nonheterocystous cyanobacterium Trichodesmium sp. IMS101 in defined media: Evidence for a circadian rhythm. Journal of Phycology, 32: 916-923. DOI: 10.1111/j.0022-3646.1996.00916.x

Garcia, N. S., F.-X. Fu, , C. L. Breene, P. W. Bernhardt, M. R. Mulholland, J. A. Sohm, and D. A. Hutchins. 2011. Interactive effects of irradiance and CO2 on CO2- and N2 fixation in the diazotroph Trichodesmium erythraeum (Cyanobacteria). J. Phycol. 47: 1292-1303. DOI: 10.1111/j.1529-8817.2011.01078.x

MONOD, J. (1949). The growth of bacterial cultures. Annual Review of Microbiology, 3: 371–394.

Morel, F. M. M., J. G. Rueter, D. M. Anderson, and Guillard, R. R. L. 1979. Aquil: Chemically defined phytoplankton culture medium for trace metal studies. J. Phycol. 15:135-141.

MULHOLLAND, M.R. (2007). The fate of nitrogen fixed by diazotrophs in the ocean. Biogeosciences 4: 37–51. DOI: 10.5194/bg-4-37-2007

MULHOLLAND, M.R. & BERNHARDT, P.W. (2005). The effect of growth rate, phosphorus concentration and temperature on N2-fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS101. Limnology and Oceanography, 50: 839–849. DOI: 10.4319/lo.2005.50.3.0839

MULHOLLAND, M.R., BRONK, D.A. & CAPONE, D.G. (2004). N2 fixation and regeneration of NH4+ and dissolved organic N by Trichodesmium IMS101. Aquatic Microbial Ecology, 37: 85–94. DOI: 10.3354/ame037085

C_watsonii_light_expt.csv