# Variable responses to ocean acidification among mixotrophic protists with different lifestyles

## TL;DR

This study explores how different strains of a mixotrophic protist respond to ocean acidification, revealing varied growth and metabolic adaptations.

## Contribution

The study provides new insights into the variable responses of mixotrophic protists to ocean acidification.

## Key findings

- Two Ochromonas strains showed increased growth rates under high-CO2 conditions.
- Strains exhibited diverse shifts in phototrophic and heterotrophic metabolism under ocean acidification.

## Abstract

Marine phytoplankton are facing increasing dissolved CO2 concentrations and ocean acidification caused by anthropogenic CO2 emissions. Mixotrophic organisms are capable of both photosynthesis and phagotrophy of prey and are found across almost all phytoplankton taxa and diverse environments. Yet, we know very little about how mixotrophs respond to ocean acidification. Therefore, we studied responses to simulated ocean acidification in three strains of the mixotrophic chrysophyte Ochromonas (CCMP1391, CCMP2951, and CCMP1393). After acclimatization of the strains to treatment with high-CO2 (1000 ppm, pH 7.9) and low-CO2 concentrations (350 ppm, pH 8.3), strains CCMP1393 and CCMP2951 both exhibited higher growth rates in response to the high-CO2 treatment. In terms of the balance between phototrophic and heterotrophic metabolism, diverse responses were observed. In response to the high-CO2 treatment, strain CCMP1393 showed increased photosynthetic carbon fixation rates, while CCMP1391 exhibited higher grazing rates, and CCMP2951 did not show significant alteration of either rate. Hence, all three Ochromonas strains responded to ocean acidification, but in different ways. The variability in their responses highlights the need for better understanding of the functional diversity among mixotrophs in order to enhance predictive understanding of their contributions to global carbon cycling in the future.