Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO₂). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO₂. The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO₂ in the coastal zone are to be expected over the course of this century.

We report a new, autonomous Lab-on-Chip (LOC) microfluidic pH sensor with a 6000 m depth capability, ten times the depth capability of the state of the art autonomous spectrophotometric sensor. The pH is determined spectrophotometrically using purified meta-Cresol Purple indicator dye offering high precision (<0.001 pH unit measurement reproducibility), high frequency (every 8 min) measurements on the total proton scale from the surface to the deep ocean (to 600 bar). The sensor requires low power (3 W during continuous operation or ∼1300 J per measurement) and low reagent volume (∼3 μL per measurement) and generates small waste volume (∼2 mL per measurement) which can be retained during deployments. The performance of the LOC pH sensor was demonstrated on fixed and moving platforms over varying environmental salinity, temperature, and pressure conditions. Measurement accuracy was +0.003 ± 0.022 pH units (n = 47) by comparison with validation seawater sample measurements in coastal waters. The combined standard uncertainty of the sensor in situ pH_T measurements was estimated to be ≤0.009 pH units at pH 8.5, ≤ 0.010 pH units at pH 8.0, and ≤0.014 pH units at pH 7.5. Integrated on autonomous platforms, this novel sensor opens new frontiers for pH observations, especially within the largest and most understudied ecosystem on the planet, the deep ocean.