Unifying chemical and biological perspectives of carbon accumulation in the environment

Large-scale study on groundwater dissolved organic matter reveals a strong heterogeneity and a complex microbial footprint

Dissolved organic matter (DOM) in terrestrial groundwater is generally low in concentration compared to inland surface waters. However, the overall amount of groundwater DOM is huge, as there is 100 times more fresh groundwater than fresh surface water. To date, research on groundwater DOM has merely focused on specific threats to humans such as e.g. DOM and heavy metal complexations and DOM from hydrocarbon contamination. A comprehensive, large-scale study of groundwater is still missing. Here, we examine DOM properties in a large-scale approach with regards to surface characteristics such as land use and altitude, aquifer characteristics as well as microbial features. We analyzed 1600 water samples from 100 groundwater bodies all over Austria with regards to their DOM quantity, quality and bacterial abundance (BA). DOM quality was evaluated with self-organizing maps on fluorescence excitation-emission-matrices (EEMs) combined with Ward clustering and subsequent parallel factor analysis to describe DOM properties of each cluster. We evaluated how these clusters differed among each other, based on DOC and nitrate concentrations, BA and selected environmental characteristics. Our results show that fluorescence components in groundwater resemble components found in other groundwater studies, in studies from forest streams, the dark ocean, agricultural catchments and wastewater treatment plants. The latter fluorescence components were associated with a cluster that is characterized by agricultural and urban land use, as well as by high nitrate concentrations. Clusters with an increased abundance of high-molecular weight and humic components, commonly associated with vascular plant and soil origin, correlated with a higher bacterial abundance. This observation provides evidence that elevated numbers of suspended bacteria mainly originate from the surface. Our study shows that DOM fluorescence can be a fast monitoring tool to identify aquifers under anthropogenic stress and delineate sensitive recharge areas with high surface-groundwater interaction.

Single-cell measurements and modelling reveal substantial organic carbon acquisition by Prochlorococcus

Marine phytoplankton are responsible for about half of the photosynthesis on Earth. Many are mixotrophs, combining photosynthesis with heterotrophic assimilation of organic carbon, but the relative contribution of these two lifestyles is unclear. Here single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea obtain only ~20% of carbon required for growth by photosynthesis. This is supported by laboratory-calibrated calculations based on photo-physiology parameters and compared with in situ growth rates. Agent-based simulations show that mixotrophic cells could grow tens of metres deeper than obligate photo-autotrophs, deepening the nutricline by ~20 m. Time series from the North Atlantic and North Pacific indicate that, during thermal stratification, on average 8-10% of the Prochlorococcus cells live without enough light to sustain obligate photo-autotrophic populations. Together, these results suggest that mixotrophy underpins the ecological success of a large fraction of the global Prochlorococcus population and its collective genetic diversity.