Scaling down the microbial loop: data‐driven modelling of growth interactions in a diatom–bacterium co‐culture

Abstract

An intricate set of interactions characterizes marine ecosystems. One of the most important is represented by the microbial loop, which includes the exchange of dissolved organic matter (DOM) from phototrophic organisms to heterotrophic bacteria. Here, it can be used as the major carbon and energy source. This interaction is one of the foundations of the entire ocean food-web. The carbon fixed by phytoplankton can be redirected to bacteria in two main ways; either (i) bacteria feed on dead phytoplankton cells or (ii) DOM is actively released by phytoplankton (a process resulting in up to 50% of the fixed carbon leaving the cell). Here, we have set up a co-culture of the diatom Phaeodactylum tricornutum and the chemoheterotrophic bacterium Pseudoalteromonas haloplanktis TAC125 and used this system to study the interactions between these two representatives of the microbial loop. We show that the bacterium can thrive on diatom-derived carbon and that this growth can be sustained by both diatom dead cells and diatomreleased compounds. These observations were formalized in a network of putative interactions between P. tricornutum and P. haloplanktis and implemented in a model that reproduces the observed co-culture dynamics, revealing an overall accuracy of our hypotheses in explaining the experimental data.

Shaping the phycosphere: Analysis of the EPS in diatom‐bacterial co‐cultures

“…This was evident from both cell count (Appendix S1: Figure S1b) and chlorophyll a content (Appendix S1: Figure S1c). This stimulating effect, previously reported by Bruckner et al (2011) and Daly et al (2021), is possibly due to the presence of bacterial exudates, which serve as substrates to support P. tricornutum under mixotrophic growth (Villanova & Spetea, 2021). Previous studies revealed that glucose and fructose exerted significant enhancement on the growth of P. tricornutum under mixotrophic conditions (Cerón-García et al, 2013;Villanova et al, 2017).…”

“…The co‐culture of Phaeodactylum tricornutum ‐ Pseudoalteromonas haloplanktis , as well as the bacterial and diatom controls, were set up following the methods described in Daly et al (2021) with some modifications. Briefly, the preculture of Ps.…”

“…Briefly, the preculture of Ps. haloplanktis was grown for 2 d in a marine salt mix, called “Schatz Salts medium modified” (mSS; Daly et al, 2021), supplemented with L‐glutamic acid (11 g · L −1 ) and used as inoculum for the co‐cultivation experiments. For the co‐culture, the bacterial preculture was washed once by centrifugation at 1254 × g for 4 min and then added at a concentration of 10 5 cells · mL −1 to the fresh culture of P. tricornutum (10 4 cells · mL −1 ) in mSS medium with no additional carbon source.…”

Exchange or Eliminate: The Secrets of Algal-Bacterial Relationships

“…In a coculture of P. tricornutum and the bacterium Pseudoalteromonas haloplanktis , the exo-environment described by exo-monosaccharide profiles varied depending on the culture condition [ 149 ]. This bacterium can utilize diatom-released compounds [ 150 ]. Algal-derived dissolved organic carbon and nitrogen and bacterial incorporation of these remineralized compounds were studied with P. tricornutum and several different bacteria.…”

Response of bacterial communities (Marivita, Marinobacter, and Oceanicaulis) in the phycosphere to the growth of Phaeodactylum tricornutum in different inorganic nitrogen sources

“…The decomposition and mineralization of dissolved organic materials by heterotrophic bacteria are essential to their survival and proliferation (Falkowski et al, 2008). Due to the limited available nutrients in the culture medium, the phycosphere bacteria primarily interacted with algae in the indoor culture environment, and the usage of organic substances generated by algae was the primary survival method of the bacteria (Daly et al, 2021;Kim et al, 2022). In this experiment, as P. tricornutum entered the exponential growth phase, a large number of metabolites secreted by algae might have increased the bacterial diversity index.…”