Photosynthetic unicellular organisms, known as microalgae, are key contributors to carbon fixation on Earth. Their biotic interactions with other microbes shape aquatic microbial communities and influence the global photosynthetic capacity. So far, limited information is available on molecular factors that govern these interactions. We show that the bacterium Pseudomonas protegens strongly inhibits the growth and alters the morphology of the biflagellated green alga Chlamydomonas reinhardtii. This antagonistic effect is decreased in a bacterial mutant lacking orfamides, demonstrating that these secreted cyclic lipopeptides play an important role in the algal–bacterial interaction. Using an aequorin Ca2+-reporter assay, we show that orfamide A triggers an increase in cytosolic Ca2+ in C. reinhardtii and causes deflagellation of algal cells. These effects of orfamide A, which are specific to the algal class of Chlorophyceae and appear to target a Ca2+ channel in the plasma membrane, represent a novel biological activity for cyclic lipopeptides.

ORCID ID: 0000-0003-3414-9850 (M.M.)Microalgae contribute significantly to carbon fixation on Earth. Global warming influences their physiology and growth rates. To understand algal short-term acclimation and adaptation to changes in ambient temperature, it is essential to identify and characterize the molecular components that sense small temperature changes as well as the downstream signaling networks and physiological responses. Here, we used the green biflagellate alga Chlamydomonas reinhardtii as a model system in which to study responses to temperature. We report that an RNA recognition motif (RRM)-containing RNA-binding protein, Musashi, occurs in 25 putative splice variants. These variants bear one, two, and three RRM domains or even lack RRM domains. The most abundant Musashi variant, 12, with a molecular mass of 60 kD, interacts with two clock-relevant members of RNA metabolism, the subunit C3 of the RNA-binding protein CHLAMY1 and the 5ʹ-3ʹ exoribonuclease XRN1. These proteins are able to integrate temperature information by up-or down-regulation of their protein levels in cells grown at low (18°C) or high (28°C) temperature. We further show that the 60-kD Musashi variants with three RRM domains can bind to (UG) 7 repeat-containing RNAs and are up-regulated in cells grown at a higher temperature during early night. Intriguingly, the 60-kD Musashi variant 12, as well as C3 and XRN1, confer thermal acclimation to C. reinhardtii, as shown with mutant lines. Our data suggest that these three proteins of the RNA metabolism machinery are key members of the thermal signaling network in C. reinhardtii.

The antagonistic bacterium Pseudomonas protegens secretes the cyclic lipopeptide orfamide A, which triggers a Ca2+ signal, causing the deflagellation of the green microalga Chlamydomonas reinhardtii. By investigating targeted synthetic orfamide A variants and inhibitors, we found that at least two Ca2+-signalling pathways and TRP channels are involved in this response.

The freshwater microalga Chlamydomonas reinhardtii, which lives in wet soil, has served for decades as a model for numerous biological processes, and many tools have been introduced for this organism. Here, we have established a stable nuclear transformation for its marine counterpart, Chlamydomonas sp. SAG25.89, by fusing specific cis‐acting elements from its Actin gene with the gene providing hygromycin resistance and using an elaborated electroporation protocol. Like C. reinhardtii, Chlamydomonas sp. has a high GC content, allowing reporter genes and selection markers to be applicable in both organisms. Chlamydomonas sp. grows purely photoautotrophically and requires ammonia as a nitrogen source because its nuclear genome lacks some of the genes required for nitrogen metabolism. Interestingly, it can grow well under both low and very high salinities (up to 50 g · L‐1) rendering it as a model for osmotolerance. We further show that Chlamydomonas sp. grows well from 15 to 28°C, but halts its growth at 32°C. The genome of Chlamydomonas sp. contains some gene homologs the expression of which is regulated according to the ambient temperatures and/or confer thermal acclimation in C. reinhardtii. Thus, knowledge of temperature acclimation can now be compared to the marine species. Furthermore, Chlamydomonas sp. can serve as a model for studying marine microbial interactions and for comparing mechanisms in freshwater and marine environments. Chlamydomonas sp. was previously shown to be immobilized rapidly by a cyclic lipopeptide secreted from the antagonistic bacterium Pseudomonas protegens PF‐5, which deflagellates C. reinhardtii.

Summary The antagonistic bacterium Pseudomonas protegens secretes the cyclic lipopeptide (CLiP) orfamide A, which triggers a Ca2+ signal causing rapid deflagellation of the microalga Chlamydomonas reinhardtii. We performed chemical synthesis of orfamide A derivatives and used an aequorin reporter line to measure their Ca2+ responses. Immobilization of algae was studied using a modulator and mutants of transient receptor potential (TRP)‐type channels. By investigating targeted synthetic orfamide A derivatives, we found that N‐terminal amino acids of the linear part and the terminal fatty acid region are important for the specificity of the Ca2+‐signal causing deflagellation. Molecular editing indicates that at least two distinct Ca2+‐signaling pathways are triggered. One is involved in deflagellation (Thr3 change, fatty acid tail shortened by 4C), whereas the other still causes an increase in cytosolic Ca2+ in the algal cells, but does not cause substantial deflagellation (Leu1 change, fatty acid hydroxylation, fatty acid changes by 2C). Using mutants, we define four TRP‐type channels that are involved in orfamide A signaling; only one (ADF1) responds additionally to low pH. These results suggest that the linear part of the CLiP plays one major role in Ca2+ signaling, and that orfamide A uses a network of algal TRP‐type channels for deflagellation.