No abstract

Recent findings indicate that N 2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N 2 fixation remains unknown. Here we explore N 2 fixation in the deep chlorophyll maximum and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N 2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43 nmol N L À1 d À1 ) and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics and associated with the presence of more oxygenated ventilated water masses. Finally, and despite that the aphotic N 2 fixation contributes largely to total water column diazotrophic activity (>50%), its contribution to overall nitrogen inputs to the basin is negligible (<0.5%).

Dissolved organic matter (DOM) plays a key role in global biogeochemical cycles and experiences changes in molecular composition as it undergoes processing. In the semi-closed basins of the oligotrophic Mediterranean Sea, these gradual molecular modifications can be observed in close proximity. In order to extend the spatial resolution of information on DOM molecular composition available from ultrahigh resolution mass spectrometry in this area, we relate this data to optical (fluorescence and absorption spectroscopy) measurements. Covariance between molecular formulae signal intensities and carbon-specific fluorescence intensities was examined by means of Spearman’s rank correlations. Fifty two per cent of the assigned molecular formulae were associated with at least one optical parameter, accounting for 70% of the total mass spectrum signal intensity. Furthermore, we obtained significant multiple linear regressions between optical and intensity-weighted molecular indices. The resulting regression equations were used to estimate molecular parameters such as the double bond equivalent, degradation state and occurrence of unsaturated aliphatic compounds from optical measurements. The statistical linkages between DOM molecular and optical properties illustrate that the simple, rapid and cost-efficient optical spectroscopic measurements provide valuable proxy information on the molecular composition of open ocean marine DOM.

The molecular composition of marine dissolved organic matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid‐phase extraction (SPE‐DOM) and molecularly characterized using Fourier‐transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE‐DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in molecular weight (+20 Da), oxygenation (+5%), degradation index (Ideg +22%), and the proportional decrease of unsaturated aliphatic compounds (+34%) along the Levantine Intermediate Water. This reworked SPE‐DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg −25%). In the deep eastern and western overturning cells, the molecular composition of the deep waters varied according to their area and/or time of formation. SPE‐DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre‐EMT waters formed in the Adriatic Sea (molecular weight and the proportion of unsaturated aliphatic compounds were increased by 5 Da and 9%, respectively). Furthermore, pre‐EMT waters contain more reworked SPE‐DOM (Ideg +7%) than post‐EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a laboratory basin where degradation processes and diagenetic transformations of DOM can be observed on close spatial and temporal scales.