Reused Cultivation Water Accumulates Dissolved Organic Carbon and Uniquely Influences Different Marine Microalgae.

Sarah E Loftus,Zackary I Johnson

doi:10.3389/fbioe.2019.00101

Abstract

Reusing growth medium (water supplemented with nutrients) for microalgae cultivation is required for economical and environmentally sustainable production of algae bioproducts (fuels, feed, and food). However, reused medium often contains microbes and dissolved organic matter that may affect algae growth. While the accumulation of dissolved organic carbon (DOC) in reused medium has been demonstrated, it is unclear whether DOC concentrations affect algae growth or subsequent rates of algal DOC release. To address these questions, lab-scale experiments were conducted with three marine microalgae strains, Navicula sp. SFP, Staurosira sp. C323, and Chlorella sp. D046, grown in medium reused up to four times. Navicula sp. and Chlorella sp. grew similarly in reused medium as in fresh medium, while Staurosira sp. became completely inhibited in reused medium. Across the three algae, there was no broad trend between initial DOC concentration in reused medium and algae growth response. Navicula sp. released less DOC overall in reused medium than in fresh medium, but DOC release rates did not decrease proportionally with increased DOC concentrations. Net DOC accumulation was much lower than gross DOC released by Navicula sp. and Staurosira sp., indicating the majority of released DOC was degraded. Additionally, biodegradation experiments with reused media showed no further net decrease in DOC, suggesting the accumulated DOC was recalcitrant to the associated bacteria. Overall, these results suggest that taxa-specific factors may be responsible for algae growth response in reused medium, and that DOC release and accumulation are insensitive to prior cultivation rounds. Choosing an algae strain that is uninhibited by accumulated DOC is therefore critical to ensure successful water reuse in the algae industry.

Highlights

Minimizing algae cultivation costs and maximizing algae productivity are priorities for producing algal bioproducts such as fuels, feed, and food
The objectives of this study were to (a) characterize growth responses of microalgae strains in reused medium, (b) measure gross dissolved organic carbon (DOC) excretion and net DOC accumulation over multiple medium reuses, and (c) determine if DOC concentration in reused medium is associated with algae growth response
The range of algal growth responses observed here in reused medium confirms taxa-specificity and suggests that algae screening processes for large-scale cultivation should include reused medium testing. This is the first study to analyze the relationship between growth response and biologically-derived DOC concentration in reused medium across physiologically different algae

Summary

Introduction

Minimizing algae cultivation costs and maximizing algae productivity are priorities for producing algal bioproducts such as fuels, feed, and food. Reusing growth medium (water supplemented with nutrients) after algae harvesting is one crucial strategy to reduce costs (Farooq et al, 2015). Reused medium may affect algae yields because it can contain dissolved compounds, DOC in Reused Microalgae Cultivation Water cell debris, microbes, and any substances used in the harvesting process. Previous studies have tested how reused medium affects algae growth using a variety of algae taxa, harvesting methods, and growth conditions. These studies revealed diverse growth responses ranging from inhibition to stimulation, though most experiments found no substantial effects caused by reused medium (Loftus and Johnson, 2017). One factor that accounted for the wide variety of growth responses was algae taxa

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