A simple, high resolution colormetric planar optode imaging approach is presented. The approach is simple and inexpensive yet versatile, and can be used to study the two-dimensional distribution and dynamics of a range of analytes. The imaging approach utilizes the inbuilt color filter of standard commercial digital single lens reflex cameras to simultaneously record different colors (red, green, and blue) of luminophore emission light using only one excitation light source. Using the ratio between the intensity of the different colors recorded in a single image analyte concentrations can be calculated. The robustness of the approach is documented by obtaining high resolution data of O 2 and pH distributions in marine sediments using easy synthesizable sensors. The sensors rely on the platinum(II)octaethylporphyrin (PtOEP) and lipophilic 8-Hydroxy-1,3,6-pyrenetrisulfonic acid trisodium (HPTS) salt derivate for O 2 and pH measurements, respectively. The brightness of both indicators is dramatically enhanced by making use of energy transfer from a donor molecule (Macrolex yellow coumarin). Furthermore, the emission from the donor serves as an internal reference for the O 2 sensor. The approach relies on semitransparent sensors, facilitating visual inspection of the sediment behind the sensors during measurements. Software for data acquisition and calibration will be available from the authors, whereas all hardware is available from a range of commercial sources. The total cost of the complete measuring system is approximately $3000 US.

The genetic composition of marine microbial communities varies at the microscale between particleassociated (PA; 41.6 μm) and free-living (FL; 0.2-1.6 μm) niches. It remains unclear, however, how metabolic activities differ between PA and FL fractions. We combined rate measurements with metatranscriptomics to quantify PA and FL microbial activity in the oxygen minimum zone (OMZ) of the Eastern Tropical North Pacific, focusing on dissimilatory processes of the nitrogen (N) cycle. Bacterial gene counts were 8-to 15-fold higher in the FL compared with the PA fraction. However, rates of all measured N cycle processes, excluding ammonia oxidation, declined significantly following particle (41.6 μm) removal. Without particles, rates of nitrate reduction to nitrite (1.5-9.4 nM N d − 1 ) fell to zero and N 2 production by denitrification (0.5-1.7 nM N d − 1 ) and anammox (0.3-1.9 nM N d − 1 ) declined by 53-85%. The proportional representation of major microbial taxa and N cycle gene transcripts in metatranscriptomes followed fraction-specific trends. Transcripts encoding nitrate reductase were uniform among PA and FL fractions, whereas anammox-associated transcripts were proportionately enriched up to 15-fold in the FL fraction. In contrast, transcripts encoding enzymes for N 2 O and N 2 production by denitrification were enriched up to 28-fold in PA samples. These patterns suggest that the majority of N cycle activity, excluding N 2 O and N 2 production by denitrification, is confined to a FL majority that is critically dependent on access to particles, likely as a source of organic carbon and inorganic N. Variable particle distributions may drive heterogeneity in N cycle activity and gene expression in OMZs.