Hunting for Extremophiles: A Systematic Screening of Freshwater Microalgae for Tolerance to High-pH and High-Alkalinity Cultivation

TL;DR

This study screens freshwater microalgae for their ability to grow in high-pH and high-alkalinity conditions, identifying strains that could be useful for sustainable industrial applications.

Contribution

A high-throughput screening of freshwater microalgae reveals new alkaline-tolerant strains and insights into their growth under extreme conditions.

Findings

Moderate alkalinity significantly increases algae growth, including potentially harmful strains.

Higher alkalinity inhibits most strains, but a few green algae and cyanobacteria tolerate extreme conditions.

Algae from normal ecosystems can be extremophilic, suggesting potential for local bioprospecting.

Abstract

Microalgae hold the potential to supply sustainable food,
fuel,
plastics, and chemicals at commercial scales. Cultivating microalgae
at extreme pH (>10) and high alkalinity provides multiple benefits,
including (1) reducing the risk of contamination by undesired organisms
and (2) enabling direct air capture of CO2, which expands
the land area suitable for algae farming compared to using CO2 point sources alone. However, we currently have a limited
understanding of which algal taxa can grow under these conditions.
Therefore, we conducted a high-throughput screening of 49 freshwater
microalgae strains, comprising 40 species, for their ability to grow
in moderate (pH 8.5, 25 mM alkalinity), high (pH 10, 75 mM alkalinity),
and extreme (pH 10, 150 mM alkalinity) cultivation environments. Our
results show that moderate alkalinity tends to significantly increase
algae growth (including potentially harmful strains). However, higher
levels inhibited all but a small subset of green algae and cyanobacteria.
Effects of salinity and alkalinity differed, indicating that they
are broadly decoupled. Our results identify new industrially relevant
alkaline-tolerant strains, show that algae isolated from “normal”
ecosystems can be extremophilic, and suggest that future bioprospecting
efforts for alkaline-tolerant algae adapted to local climatic conditions
could yield additional productivity gains for the algae industry.

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