Dataset: Nutrient and cell count data from incubations conducted with methylcyclohexane or methylcyclopentane on samples collected from the Gulf of Mexico during June 2015 on R/V Atlantis cruise AT29-02

Seawater samples were collected aboard R/V Atlantis in June 2015 on cruise AT29-02. Methylcyclohexane (MCH) and methylcyclopentane (MCP) incubations were conducted at stations 1 (27° 30.41ʹ N, 87° 12.41ʹ W), 2 (27° 15.00ʹ N, 89° 05.05ʹ W), 3 (27° 11.60ʹ N, 90° 41.75ʹ W), and 4 (27° 38.40ʹ N, 90° 54.98ʹ W) with seawater collected from 1000 meters (m) depth. Seawater collected from the CTD Niskin bottles was transferred to 250-milliliter (mL) glass serum vials using a small length of Tygon tubing. Vials were filled for at least 3 volumes of water to overflow. Care was taken to ensure no bubbles were present before sealing with a polytetrafluoroethylene (PTFE) coated chlorobutyl rubber stopper and crimp cap seal. All bottles, except for unamended blank controls, immediately received 10 mL of MCH or MCP using a gas-tight syringe (Hamilton) and were maintained in the dark at in-situ temperature (4º Celsius). Before filling, each serum bottle was fixed with a contactless optical oxygen sensor (Pyroscience, Aachen, Germany) on the inner side with silicone glue, and afterward were cleaned from organic contaminants with triple rinses of ethanol, 3% hydrogen peroxide, 10% hydrochloric acid, and MilliQ water, and were sterilized via autoclave. Oxygen concentration was monitored approximately every 8 hours with a fiber optic oxygen meter (Pyroscience, Aachen, Germany). Observed changes in oxygen content were normalized to unamended controls to correct for oxygen loss from background respiration processes and variability due to temperature changes. Bloom onset is operationally defined as three consecutive time points with oxygen loss >0.21 micromoles per hour (µM h-1). Importantly, incubations were conducted with no added headspace and were limited to in-situ availability of oxygen, as well as for key nutrients, such as nitrogen and phosphorous. At the termination of each respiration experiment, samples were harvested and the microbial community was captured on a 0.22-micrometer (μm) polyethersulfone filter. Prior to filtration samples were collected for dissolved inorganic nutrients and bacterioplankton abundance. Samples for prokaryotic cell abundance were fixed with 0.2% paraformaldehyde and quantified using Guava EasyCyte flow cytometer. Dissolved nutrient (nitrate, phosphate, and ammonia) sample collection was conducted following the University of California, Santa Barbara Marine Science Institute Analytical Lab's requirements. Seawater samples from incubations were filtered through a 0.2 μm polyvinylidene into pre-rinsed plastic HDPE 20 mL. Nutrient sample volumes were ∼17 mL water and stored frozen until analysis. Dissolved nutrient concentrations were analyzed by flow injection analysis (FIA) using the QuikChem 8500 Series 2 (Lachat Instruments, Zellweger Analytics Inc.).