Data for: Dissolved organic matter (DOM) offsets the detrimental effects of climate change in the nitrogen fixing cyanobacterium Crocosphaera

Diazotrophs provide a significant reactive nitrogen source in the ocean. Increased warming and stratification may decrease nutrient availability in the future, forcing microbial communities to use dissolved organic matter (DOM). Not depending on reactive nitrogen availability, diazotrophs may be "winners" in a nutrient depleted ocean. However, their ability to exploit DOM may influence this success. We exposed cultures of the widespread Crocosphaera to low (26°C, pH 8.1), moderate (28°C, pH 8.0), and extreme (30°C, pH 7.9) climate change scenarios, under control or DOM-amended conditions. Growth was suboptimal in the low and extreme treatments, and favoured in the moderate treatment. DOM was preferred as a carbon source regardless of the treatment, and promoted N2 fixation in extreme conditions. This was reflected in the increased expression of photosynthesis genes to obtain energy. DOM provides Crocosphaera with a key ecological advantage, possibly dictating diazotroph-derived nitrogen inputs in the future ocean.

Methods
To provide a comprehensive assessment of the behaviour of Crocosphaera under future ocean conditions, we combined temperature and pH levels simulating representative concentration pathways (RCP) 2.6 (26 °C and pH 8.1), 4.5 (28 °C and pH 8.0) and 6.0 (30 °C and pH 7.9) (IPCC 2014), here called "low", "intermediate" and "extreme" treatments (10.6084/m9.figshare.22773503). Cultures of Crocosphaera were acclimated to the different treatments for one month in 500 mL bottles before the start of the experiment in larger volumes (40 L; duplicate low and triplicate intermediate and extreme treatments) with a starting concentration of ~1.5 x 104 cells mL-1 (Fig. 1A). The experiment was run for eight days (here days -2 to 5) under a 12h light:dark cycle with a light intensity of ~250 µmol photons s-1. The temperature and pH levels of each treatment were maintained constant throughout the experiment using an IKS aquastar system. Temperature and salinity were measured daily with a YSI Pro30 probe, while pH was measured spectrophotometrically. Cell abundance and nutrient concentrations were measured daily using standard microscopy and colorimetry methods. On alternate days (day 0, 2, and 4) subvolumes were sampled from each treatment and incubated to measure single-cell N2 and dissolved inorganic carbon (DIC) fixation using dissolved 15N2 and 13C-HCO3, respectively. Similarly, a mix of 13C-labelled organic compounds representative of marine DOM including the amino acids alanine, leucine, and arginine, and the carbohydrates glucose and N-acetylglucosamine was used to measure DOM uptake rates and RNA expression responses. RNA was sequenced using an Illumina NovaSeq 6000 platform generating more than 30 million paired end reads per sample (2 x 50 bp). RNA reads were annotated using Diamond and the databases Kofam UniProt’s Swissprot, trEMBL, and NCBI's RefSeq.