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Anaerobic ammonia-oxidizing (Anammox) bacteria (AnAOB) rely on nitrite supplied by ammonia-oxidizing bacteria (AOB) and archaea (AOA). Affinities for ammonia and oxygen play a crucial role in AOA/AOB competition and their association with AnAOB. In this work we measured the affinity constants for ammonia and oxygen (half-saturation; k m) of two freshwater AOA enrichments, an AOA soil isolate (N. viennensis), and a freshwater AnAOB enrichment. The AOA enrichments had similar kinetics (μ max ≈ 0.36 d −1 , k m,NH4 ≈ 0.78 µM, and k m,O2 ≈ 2.9 µM), whereas N. viennensis had similar k m values but lower μ max (0.23 d −1). In agreement with the current paradigm, these AOA strains showed a higher affinity for ammonia (lower k m,NH4 ; 0.34-1.27 µM) than published AOB measurements (>20 µM). The slower growing AnAOB (μ max ≈ 0.16 d −1) had much higher k m values (k m,NH4 ≈ 132 µM, k m,NO2 ≈ 48 µM) and were inhibited by oxygen at low levels (halfoxygen inhibition; k i,O2 ≈ 0.092 µM). The higher affinity of AOA for ammonia relative to AnAOB, suggests AOA/AnAOB cooperation is only possible where AOA do not outcompete AnAOB for ammonia. Using a biofilm model, we show that environments of ammonia/oxygen counter diffusion, such as stratified lakes, favors this cooperation.

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designed the experiments, collected the data, and did the primary analysis, interpretation, and reporting of the data. Bruce Rittmann provided further insight for data analysis, interpretation, and reporting. Both wrote the manuscript cooperatively.

The viability of large-scale microalgae cultivation depends on providing optimal growth conditions, for which a key operational parameter is culture density. Using Synechocystis sp. PCC 6803, we conducted a series of fixed-density, steady-state experiments and one batch-growth experiment to investigate the role of culture density on biomass production and light utilization efficiency. In all cases, the fixed-density, steady-state experiments and batch-growth experiment showed good agreement. The highest biomass production rates (260 mg L d ) and efficiency for converting light energy to biomass (0.80 μg (μmol photons) ) occurred together at a culture density near 760 mg L , which approximately corresponded to the lowest culture density where almost all incident light was absorbed. The ratio of OD /OD increased with culture density up to the point of maximum productivity, where it plateaued (at a value of 2.4) for higher culture densities. This change in OD /OD indicates a photoacclimation effect that depended on culture density. Very high culture densities led to a sharp decline in efficiency of biomass production per photons absorbed, likely due to a combination of increased decay relative to growth, metabolic changes due to cell-cell interactions, and photodamage due to mixing between regions with high light intensity and zero light intensity.

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