Proportion of marine organic carbon present in self-assembled gels along the subtropical front and its increase in response to reduced pH

Abstract

Dissolved organic matter (DOM), the largest marine reservoir of reduced carbon, which contains a complex mix of polydisperse biopolymers and particularly small molecules in solution. However, we now know that about 10% of these dissolved moieties can spontaneously aggregate forming a huge mass of highly bioreactive microscopic particles by forming self-assembled gels (SAG) with a significant role in global carbon cycling. However, the effect of varying marine environments on DOM self-assembly remains virtually unknown. Here we report variations of the fraction of DOM that self-assemble forming gels (%SAG) measured in seawater samples collected during two different periods — before and during the autumn bloom— along the Subtropical Frontal Zone off New Zealand; an area characterized by strong spatiotemporal physicochemical and biological variability. Results show that %SAG varies in time and space. Measurements of %SAG ranged between 9–28% (14.5±5, mean±SD), and this broad variability could be partly associated to changes in phytoplankton biomass and/or UV radiation. Additional studies on the effect of a pH on the %SAG, reveal that small pH reductions of 0.3units can virtually double the %SAG from 14.5±5 to 27±7% (12.5±4%, mean±SD). The observed increase in the %SAG under reduced pH was observed in all samples, irrespectively of their origin. This outcome suggests that pH might be a critical parameter controlling the formation of gels in marine environments. Decreased pH increases the SW concentration of free ionized Ca ([Ca+2]), the prime DOM crosslinker. Rising [Ca+2] is likely responsible for the observed %SAG increase. The outcome that %SAG can be strongly affected by small changes in seawater properties, suggest that DOM self-assembly is remarkably responsive to subtle variations of environmental conditions. SAG are a leading source of bacterial nutrition and these observations have potentially strong ramifications in the ecology and biogeochemical cycles of the oceans.