DOP Stimulates Heterotrophic Bacterial Production in the Oligotrophic Southeastern Mediterranean Coastal Waters

Abstract

Phytoplankton and heterotrophic bacteria rely on a suite of inorganic and organic macronutrients to satisfy their cellular needs. Here, we explored the effect of dissolved inorganic phosphate (PO 4 ) and several dissolved organic molecules containing phosphorus [ATP, glucose-6-phosphate, 2-aminoethylphosphonic acid, collectively referred to as dissolved organic phosphorus (DOP)], on the activity and biomass of autotrophic and heterotrophic microbial populations in the coastal water of the southeastern Mediterranean Sea (SEMS) during summertime. To this end, surface waters were supplemented with PO 4 , one of the different organic molecules, or PO 4 + ATP, and measured the PO 4 turnover time (Tt), alkaline phosphatase activity (APA), heterotrophic bacterial production (BP), primary production (PP), and the abundance of the different microbial components. Our results show that PO 4 alone does not stimulate any significant change in most of the autotrophic or heterotrophic bacterial variables tested. ATP addition (alone or with PO 4 ) triggers the strongest increase in primary and bacterial productivity or biomass. Heterotrophic bacterial abundance and BP respond faster than phytoplankton (24 h post addition) to the various additions of DOP or PO 4 + ATP, followed by a recovery of primary productivity (48 h post addition). These observations suggest that both autotrophic and heterotrophic microbial communities compete for labile organic molecules containing P, such as ATP, to satisfy their cellular needs. It also suggests that SEMS coastal water heterotrophic bacteria are likely C and P co-limited.

The effects of decomposing invasive jellyfish on biogeochemical fluxes and microbial dynamics in an ultra-oligotrophic sea

“…They hypothesized that the metabolism and propagation of heterotrophic bacteria led to enhanced oxygen consumption. Indeed, our experimental results support this hypothesis, as bacterial abundance was strongly correlated with oxygen levels, whereas the abundance of autotrophic cyanobacteria decreased as they were likely outcompeted by the heterotrophic bacteria (Sisma-Ventura and Rahav, 2019;Thingstad et al, 2005). Thus, jelly-falls can generate hypoxic areas on the seabed and overlying waters (Pitt et al, 2009), and affect the benthic infauna (Chelsky et al, 2016).…”

“…Nutrient fluxes were measured using the whole-core incubation technique previously described by Denis et al (2001). Although restricting this study for testing short-term responses, this method follows the best practices for measuring oxygen and nutrient fluxes and dynamics at the sediment-water interface (Glud, 2008;Hammond et al, 2004;Pratihary et al, 2014;Skoog and Arias-Esquivel, 2009). Pre-filtered coastal water was transferred to a reserve tank and stored under the same conditions as the incubated cores.…”

“…In the EMS, R. nomadica typically peaks in the summer months and collapses at the end of July (Edelist et al, 2020), whereas peak chlorophyll a concentrations in the water column are measured during wintertime (Ignatiades et al, 2009;Rahav et al, 2018a. This may result from the competitive exclusion of phytoplankton by heterotrophic bacteria (Sisma-Ventura and Rahav, 2019). Thus, fertilization of the water column due to nutrient release from R. nomadica decomposition may fail to trigger an algal bloom in the EMS.…”

“…Our study found that the decomposition of R. nomadica induced an increase of 2 orders of magnitude in the heterotrophic bacteria abundance. Autotrophic cyanobacteria, on the other hand, decreased (Synechococcus), or increased to a lower level than the unamended control (Prochlorococcus), likely due to deoxygenation (Bagby and Chisholm, 2015) or outcompetition by heterotrophic bacteria (Sisma-Ventura and Rahav, 2019;Thingstad et al, 2005). The fate of jellyfish DOM consumed by bacteria depends on bacterial growth efficiency -the ratio of bacterial production to substrate assimilation (i.e., the sum of bacterial production and respiration) (Condon et al, 2011).…”

“…Changes in the sediment conditions may result in migration or mortality of infauna (Chelsky et al, 2016), which in turn affect indirectly nutrient cycling (Stief, 2013;Welsh, 2003). The contribution of jellyfish degradation to nutrient cycling was hypothesized to be particularly important in nutrient-depleted, oligotrophic ecosystems (Pitt et al, 2009), such as the ultra-oligotrophic Eastern Mediterranean Sea (EMS), where microbial production is mainly limited by organic carbon (Sisma-Ventura and Rahav, 2019) or nitrogen (Rahav et al, 2018b), or co-limited by nitrogen and phosphorus (Kress et al, 2005).…”