Quantitative proteomics insights into
Chlamydomonas reinhardtii
thermal tolerance enhancement by a mutualistic interaction with
Sinorhizobium meliloti
Authors: Zhao Na , Liu Fei , Dong Wenxiu , Yu Jie , Halverson Larry J. , Xie Bo 
Published: July 2024

Interactions between photosynthetic microalgae and bacteria impact the physiology of both partners, which influence the fitness and ecological trajectories of each partner in an environmental context-dependent manner. Thermal tolerance of
Chlamydomonas reinhardtii
can be enhanced through a mutualistic interaction with vitamin B
12
(cobalamin)-producing
Sinorhizobium meliloti
. Here, we used label-free quantitative proteomics to reveal the metabolic networks altered by the interaction under normal and high temperatures. We created a scenario where the growth of
Sinorhizobium
requires carbon provided by
Chlamydomonas
for growth in co-cultures, and survival of
Chlamydomonas
under high temperatures relies on cobalamin and possibly other metabolites produced by
Sinorhizobium
. Differential abundance analysis identified proteins produced by each partner in co-cultures compared to mono-cultures at each temperature. Proteins involved in cobalamin production by
Sinorhizobium
increased in the presence of
Chlamydomonas
under elevated temperatures, whereas in
Chlamydomonas
, there was an increase in cobalamin-dependent methionine synthase and certain proteins associated with methylation reactions. Co-cultivation and heat stress strongly modulated the central metabolism of both partners as well as various transporters that could facilitate nutrient cross-utilization. Co-cultivation modulated expression of various components of two- or one-component signal transduction systems, transcriptional activators/regulators, or sigma factors, suggesting complex regulatory networks modulate the interaction in a temperature-dependent manner. Notably, heat and general stress-response and antioxidant proteins were upregulated in co-cultures, suggesting that the interaction is inherently stressful to each partner despite the benefits of mutualism. Our results shed insight into the metabolic tradeoffs required for mutualism and how metabolic networks are modulated by elevated temperature.

IMPORTANCE

Photosynthetic microalgae are key primary producers in aquatic ecosystems, playing an important role in the global carbon cycle. Nearly every alga lives in association with a diverse community of microorganisms that influence each other and their metabolic activities or survival. One chemical produced by bacteria that influence algae is vitamin B
12
, an enzyme cofactor used for a variety of metabolic functions. The alga
Chlamydomonas reinhardtii
benefits from vitamin B
12
produced by
Sinorhizobium meliloti
by producing the amino acid methionine under high temperatures which are required for
Chlamydomonas
thermotolerance. Yet, our understanding of this interaction under normal and stressful temperatures is poor. Here, we used quantitative proteomics to identify differentially expressed proteins to reveal metabolic adjustments made by
Chlamydomonas
and
Sinorhizobium
that could facilitate this mutualism. These findings will enhance our understanding of how photosynthetic algae and their associated microbiomes will respond as global temperatures increase.