Fig 4.

Photoassimilation of ¹³C Bicarbonate varies with temperature and top phototrophic genera. A. Rates of oxygenic and anoxygenic photosynthesis plotted by temperature. Rates of anoxygenic photosynthesis (squares) were calculated by subtracting the Dark assimilation values from Light + DCMU group. Rates of oxygenic photosynthesis were calculated by subtracting the Dark and Light + DCMU values from the Light values (triangles). Adjusted rates shown here were log plus one transformed to meet normality requirements for statistical tests. Error bars are present for assimilation rates but are often within the data point symbol. B. Box and whisker plots of the relative abundance of OTUs of Chloroflexus and Roseiflexus (phototrophic Chloroflexi) and Leptococcus (phototrophic Cyanobacteria) plotted by temperature and are shaded by highest taxonomic assignment in mothur. Dots represent outliers.

Microcosm set up and statistical analysis in 4A:

To assess the activity of anoxygenic photoautotrophs and oxygenic photoautotrophs, we performed inorganic ¹³C assimilation microcosms in Light, Light + inhibitor and Dark treatments in seven Rabbit Creek sites and one Bison Pool site in June 2018. Only one Bison Pool site was included in this experiment because of sampling limitations but is included here to test productivity at the upper limit of photosynthesis (72°C). To parse the photoassimilation of anoxygenic versus oxygenic photosynthesis, we amended a subset of microcosms with DCMU (3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea), an inhibitor of oxygenic photosynthesis. A Dark control group was set up and wrapped in foil to halt photoassimilation of inorganic carbon. Microcosm experiments were conducted in triplicate, incubated at in situ spring temperature for 2 h and placed on dry ice following the incubation period. Carbon stable isotope signals of biomass were determined via a Costech Instruments elemental analyser (EA) periphery connected to a Thermo Scientific Delta V Advantage IR‐MS at the UC Davis Stable Isotope Facility. Microcosm samples were thawed, and biomass was rinsed with 1 M HCl to remove any extra ¹³C‐labelled DIC, triple rinsed with 18.2 MΩ cm⁻¹ deionized water, and then dried (60°C for 3 days). Natural abundance samples were not treated with acid. Samples were ground/homogenized with a cleaned mortar and pestle (ground with ethanol silica slurry, triple rinsed with 18.2 MΩ cm⁻¹ deionized water, and dried), weighed, and placed into tin boats, sealed, and submitted to the UC Davis Stable Isotope Facility for analyses. Rates of ¹³C‐labelled DIC uptake (carbon assimilation rates) reflect the difference in uptake between the biomass in the assays that received NaH¹³CO₃ and the natural abundance biomass samples. Using the organic carbon content, the uptake rate was calculated from the total micrograms of C taken up divided by the grams of organic C per gram of sediment, and that was divided by the number of hours of incubation (typically ∼2 h). Raw rates are provided in the Supporting Information Table S3.

To test the hypothesis that rates within sites are not significantly different, an analysis of variance was conducted followed by the post hoc Tukey's Honest Significant Difference test under the null hypothesis that light and DCMU + light ¹³C assimilation rates were not significantly different within sites. Similarly, to test the hypothesis that photosynthetic rates differ from site to site down the temperature gradient, an analysis of variance was conducted followed by the post hoc Tukey's Honest Significant Difference test under the null hypothesis that light and dark rates are not significantly different between sites (see Supporting Information Tables S3 and S4 for a list of pairwise comparisons and respective P‐values). None of the anoxygenic photosynthesis rates were significantly different between sites; however, three of the pairwise comparisons of oxygenic photosynthesis rates between sites were significantly different and are shown via black bars bars (* indicates a P < 0.05).