Phytoplankton release hundreds of millions of tonnes of hydrocarbons per year into the marine environment, especially medium-chain alkanes/alkenes from the membranes of lysed cells. Cyanobacteria alone release between 118 to 649 million tonnes per year of C₁₅, C₁₇ and C₁₉ n-alkanes into the sea, 100 to 500 times more than all petroleum sources [1,2]. These hydrocarbons fuel communities of hydrocarbon degraders that contribute to the global carbon cycle which in turn influences climate, food webs and potentially the natural attenuation of oil spills.

Microbial enrichments, which inform on biogeochemical processes and precede cultivation, are frequently performed with unnaturally high concentrations of carbon/energy sources and nutrients, which results in repeat detection/cultivation of alkane degraders, like Alcanivorax spp. However, concentration-dependent niche partitioning is common in microbes, e.g. methanotrophs that specialise in consuming atmospheric concentrations of methane[4].

The student will test which microbes are preferentially enriched with n-pentadecane (previously measured as the main hydrocarbon from phytoplankton in the North Sea), at concentrations several orders of magnitude lower than previously tested. By employing dynamic partitioning of a single alkane, to give concentrations that approach those found in seawater, they will identify microbes (whether Bacteria or Archaea, or whether low-concentration-specialists or concentration-agnostics) that are likely to be key consumers in the short-term hydrocarbon cycle[1,2].

Seawater will be sampled in triplicate; half will be filtered (0.2 µm) for measurement of in-situ nutrient concentrations (filtrate), hydrocarbon concentrations and microbial community analysis (cells). Filtered (abiotic controls) and unfiltered (live) seawater (5 ml) will be inoculated into artificial seawater (45 ml) and distributed into serum bottles for sacrificial laboratory incubations (16°C, dark, 8 days, 4 timepoints) that will be continuously supplied with n-pentadecane via passive dosing using alkane-impregnated PDMS silicone O-rings[5]. [Pre-experiments by the ARIES student will identify optimal alkane loading, etc.] The alkane concentration will be varied (by adding multiples of O-rings (e.g. 0 (no-hydrocarbon control), 1 and 10), mimicking the steady low-concentration release of alkane as phytoplankton continuously die.

In addition to the in-situ and time-0 samples, at four timepoints alkane concentrations in live treatments and abiotic controls will be measured using GC-MS, and cells will be preserved from live treatments at equivalent timepoints. Filtrates will be analysed for nitrate, nitrite, ammonia and available phosphorous. Based on these data, nucleic acids will be extracted from two live timepoint samples after filtration via Sterivex filters using DNeasy PowerWater Sterivex kits. Metabarcoding of the 16S and 18S rRNA genes will resolve bacterial, archaeal and eukaryote community composition[3]. Prebooking these 24 samples with Novogene will ensure turnaround within 2 weeks. During this period, the student will be trained in microbial community analysis using an equivalent data set (R fundamentals, DADA-2, SIMPER), and write up / present the nutrient and hydrocarbon data. In the last two weeks, the student will use these tools to analyse changes in the microbial community, with the primary goal of identifying taxa that are differentially abundant in the alkane treatments, and using databases to identify their global distribution.

1_Love_CR_et_al._2021_Nat_Microbiol_6:489–498

2_McGenity_TJ_et_al._2021_Nat_Microbiol_6:419-420

3_Parada_AE_et_al._2016_Environ_Microbiol_18:1403-1414

4_Schmider_T_et_al._2024_Nat_Commun_15:4151

5_Smith_KEC_et_al._2012_Environ_Sci_Technol_46:4852-4860