Pioreactors at Sea: Assessing Microbial Community Response to Added Dissolved Inorganic Nutrients

Abstract

This study aims to understand interactions within the microbial community of the oligotrophic (nutrient-poor) equatorial Pacific. This experiment attempts to provide a more realistic representation of in-situ conditions, overcoming previous difficulties capturing the dynamic behavior of microbial communities in the field. A novel methodology utilizing a continuous media supply to incubate natural microbial communities from the field accomplishes this objective. Three growth chambers were employed, one as a continuous incubation control (no nutrients added), one as a continuous incubation with added inorganic nutrients (nitrate, phosphate, and silicate), and one as a batch incubation with the same added nutrients. A continuous incubation constantly exchanges a small volume of external media from outside the growth chamber with volume from inside the growth chamber. A batch incubation does not have any media exchanged during its incubation period. All incubations followed a 16:8 light/dark period using LEDs, simulating environmental day-night cycles. Community response was assessed by monitoring the turbidity of the incubation with continuous optical density measurements, with beginning and endpoint subsamples analyzed for total bacterial abundance and DNA content using flow cytometry. There was a distinct day-night turbidity oscillation in all incubations, with the nutrient-enriched continuous incubation being the most pronounced (average normalized optical density 10% greater during simulated daytime). At the conclusion of the experiment, the continuous incubation without added nutrients had about double the bacterial cell concentration and a greater bacterial DNA content, yet similar optical density values compared to its nutrient-added counterpart. This combination of results suggests that the added nutrients selected for larger, slower growing bacteria, such as the picophytoplankton Synechococcus, which would align with previous studies. Additionally, the different turbidity oscillations between incubation methods suggest that nutrient replenishment results in a greater microbial response to diel variability. This study not only advances our understanding of microbial community dynamics in the oligotrophic equatorial Pacific but also introduces a novel experimental method that can be applied across a diversity of marine and aquatic environments.