Fig. 2

Microbial colonization of silicate sand grain surfaces and associated zones of O₂ production and consumption that result in highly heterogeneous O₂ concentrations at the surface of sand grains. (A) Development of heterogenous O₂ concentrations on the surface of single sand grain over time. A.1 grey scale image of a sand grain. A.2 Auto-fluorescence (exc.: 469/35 nm, em.: > 590 nm) as an indicator for oxygen-producing photosynthetic microorganisms. A.3 Heterogeneities in O₂ concentrations developing over time at the sand grain surface. (B) Grey-scale image of silicate sand grains incubated within the microfluidic chip, (C) auto-fluorescence image (exc.: 469/35 nm, em.: > 590 nm) showing the location of photosynthetic microorganisms on the sand grain surfaces, (D) O₂ consumption and production on the sand grain surface in the microfluidic chip (blue color indicates O₂ consumption rate and red color indicates O₂ production rate. (B–D) The same field of view. (E) Normalized density distribution of the O₂ rate for microenvironments, depicted in grey. The O₂ rates were subsequently classified based on whether photosynthetic microorganisms were present in the pixels where the rate was measured, with net production highlighted in red, and other microenvironments showing net consumption, indicated in blue. Arrows indicate the median O₂ rates of 19 µmol L⁻¹ h⁻¹ and -26 µmol L⁻¹ h⁻¹ for the photosynthetic microorganisms and other microorganisms, respectively. Note that the O₂ consumption could have been associated with heterotrophic remineralization of organic matter as well as chemolithotrophic processes such as nitrification. Scalebars denote 200 μm.