Work Packages 15 (Wurl, Dittmar, Simon)

"Interaction between microbes and gel particles at the sea surface"

Broader background of the proposed research project

The discovery that the sea-surface microlayer (SML) is a widespread gelatinous and biofilm-like environment (Wurl et al. 2011) has created a new perspective of the air-water interface in terms of biogeochemical cycling in the upper ocean (Cunliffe et al. 2013). The enrichment of gel particles in the SML forms the foundation of the biofilm habitat, but microbial colonization, unattenuated UV radiation, and specific accumulation of surface-active molecules creates a daunting complexity. For example, microbial-free transparent exopolymer particles (TEP), the most abundant gel particles in the ocean, collapses upon UV radiation, but not if colonized (Ortega-Retuerta et al. 2010). It clearly shows the close interaction between the microbial community and chemical environment. For example, TEP from the SML contained significantly more sulfate half-ester groups (Wurl & Holmes 2008), and sulfate content had an influence on the intrinsic viscosity, solubility and hydrolyzability of polymers. TEP can be also produced by bubbling seawater, to mimic breaking waves, via polymerization of specific molecules on ascending air bubbles (Zhou et al. 1998). However, it is unknown if the chemical composition of TEP from the SML differs from those in the underlying water, and how microbes potentially affect their UV-resistance. The interaction between TEP and microbes are likely to be essential for the formation of biofilm-like habitats at the sea surface, and will be investigated within the proposed PhD project.

Outline for the proposed PhD research project

The proposed project investigates the differences in the chemical composition of TEP particles on molecular level collected from the SML and underlying water (hypothesis 1), and if microbes affect their UV-resistance (hypothesis 2). We will investigate the chemical composition of TEP particles collected using ultra-high resolution mass spectrometry in collaboration with Dittmar (WP4), and identify microbial communities colonizing the gel particles with Simon (WP 8). Samples will be collected during an already funded cruise with the research vessel RV Falkor (Schmidt Ocean Insitute, USA) from Darwin, Australia, to Guam in Oct-Nov 2016. The cruise is dedicated to an interdisciplinary program on microlayer research using state-of-the-art technologies. Separated TEP particles (via slow filtration) can be dissolved by chelation of Ca2+ and Mg2+ by adding EDTA (ethylenediamine-tetra-acetic acid) as their divalent bonds are known to stabilize TEP. With this approach, we can prepare extracts from TEP produced on varies conditions (UV-level, colonization, bubbling) for the mass spectrometric analysis. Bacterial colonization is estimated from double staining with Alcian Blue (stains TEP) and acridine orange (stains cells on TEP). Bacterial communities are also subjected to molecular DNA/RNA fingerprints. Data sets will be explored using advanced statistical and data exploratory tools, including principal component and cluster analysis.