Dynamics of transparent exopolymer particles (TEP) during the VAHINE mesocosm experiment in the New Caledonian lagoon

Abstract

In the marine environment, transparent exopolymeric particles (TEP) produced from abiotic and biotic sources link the particulate and dissolved carbon pools and are essential vectors enhancing vertical carbon flux. We characterized spatial and temporal dynamics of TEP during the VAHINE experiment that investigated the fate of diazotroph-derived nitrogen and carbon in three replicate dissolved inorganic phosphorus (DIP)-fertilized 50 m 3 enclosures in the oligotrophic New Caledonian lagoon. During the 23 days of the experiment, we did not observe any depthdependent changes in TEP concentrations in the three sampled depths (1, 6, 12 m). TEP carbon (TEP-C) content averaged 28.9 ± 9.3 and 27.0 ± 7.2 % of total organic carbon (TOC) in the mesocosms and surrounding lagoon respectively and was strongly and positively coupled with TOC during P2 (i.e., days 15-23). TEP concentrations in the mesocosms declined for the first 9 days after DIP fertilization (P1 = days 5-14) and then gradually increased during the second phase. Temporal changes in TEP concentrations paralleled the growth and mortality rates of the diatomdiazotroph association of Rhizosolenia and Richelia that predominated the diazotroph community during P1. By P2, increasing total primary and heterotrophic bacterial production consumed the supplemented P and reduced availability of DIP. For this period, TEP concentrations were negatively correlated with DIP availability and turnover time of DIP (T DIP ), while positively associated with enhanced alkaline phosphatase activity (APA) that occurs when the microbial populations are P stressed. During P2, increasing bacterial production (BP) was positively correlated with higher TEP concentrations, which were also coupled with the increased growth rates and aggregation of the unicellular cyanobacterial Group C (UCYN-C) diazotrophs that bloomed during this period. We conclude that the composite processes responsible for the formation and breakdown of TEP yielded a relatively stable TEP pool available as both a carbon source and facilitating aggregation and flux throughout the experiment. TEP were probably mostly influenced by abiotic physical processes during P1, while biological activity (BP, diazotrophic growth and aggregation, export production) mainly impacted TEP concentrations during P2 when DIP availability was limited.

Programmed cell death in diazotrophs and the fate of organic matter in the Western Tropical South Pacific Ocean during the OUTPACE cruise

“…4c, p<0.05 (Fig. 4c) (Spungin et al, 2016). 493 TEP concentrations at LDB were positively correlated to TOC, POC, and DOC ( Fig.…”

Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment

“…This indicates that UCYN-C cannot only contribute to direct export but promote indirect export. The high content of TEP measured in traps by Berman-Frank et al (2016) on days 15 and 16 in correlation with the increase in UCYN-C abundances observed by led to the assumption that the presence of TEP in the field would facilitate export flux and especially the sinking of UCYN-C during P2. This phenomenon was taken into account in the model by allowing, each day from day 10, the settling of 10 % of all the model compartments (living and non-living, particulate and dissolved) in addition to the detrital particulate matter.…”

“…Finally, the overestimation of UCYN-C abundance by the model also supports the idea that UCYN-C sinking is underestimated by the model. The aggregation process induced by TEP (Berman-Frank et al, 2016) or by specific molecules such as extracellular polysaccharides (EPSs; Sohm et al, 2011), which is not explicitly represented in the model, might explain the preferential export of UCYN-C in the mesocosms. Hence, though aggregation was probably overestimated in the mesocosms as compared to natural situations, the contribution of this process seems to be significant in C export.…”

“…The aim was to represent the setting of the aggregation process and the subsequent intensification of the sinking process. The aggregation process was indeed supposed to be favored, not only by the reduced eddy fluxes due to the containment of water but also by the release of transparent exopolymer particles (TEPs) as mentioned in Berman-Frank et al (2016). At the bottom of the modeled mesocosms, the sinking material was accumulated to be compared with the particulate matter collected daily in the traps.…”

Biogeochemical and biological impacts of diazotroph blooms in a low-nutrient, low-chlorophyll ecosystem: synthesis from the VAHINE mesocosm experiment (New Caledonia)

“…Bottom panel: simplified evolution of the major standing stocks, rates and plankton abundances measured during P1 (days 5 to 14) and P2 (days 15 to 23). Protocols for each parameter measurements are described in Berthelot et al (2015b), Bonnet et al (2016a, b), Van Wambeke et al (2016), Berman-Frank et al (2016), Leblanc et al (2016), Turk-Kubo et al (2015) and Hunt et al (2016). Squares are represented in green when a significant (p < 0.05) increase was observed between each period (i.e.…”