Effects of light on gravitaxis and velocity in Chlamydomonas reinhardtii

Parallel evolution of gravity sensing

Omnipresent gravity affects all living organisms; it was a vital factor in the past and the current bottleneck for future space exploration. However, little is known about the evolution of gravity sensing and the comparative biology of gravity reception. Here, by tracing the parallel evolution of gravity sensing, we encounter situations when assemblies of homologous modules result in the emergence of non-homologous structures with similar systemic properties. This is a perfect example to study homoplasy at all levels of biological organization. Apart from numerous practical implementations for bioengineering and astrobiology, the diversity of gravity signaling presents unique reference paradigms to understand hierarchical homology transitions to the convergent evolution of integrative systems. Second, by comparing gravisensory systems in major superclades of basal metazoans (ctenophores, sponges, placozoans, cnidarians, and bilaterians), we illuminate parallel evolution and alternative solutions implemented by basal metazoans toward spatial orientation, focusing on gravitational sensitivity and locomotory integrative systems.

RNA-Seq Analysis Reveals Genes Related to Photoreception, Nutrient Uptake, and Toxicity in a Noxious Red-Tide Raphidophyte Chattonella antiqua

Aquaculture industries are under threat from noxious red tides, but harm can be mitigated by precautions such as early harvesting and restricting fish feeding to just before the outbreak of a red tide. Therefore, accurate techniques for forecasting red-tide outbreaks are strongly needed. Omics analyses have the potential to expand our understanding of the eco-physiology of these organisms at the molecular level, and to facilitate identification of molecular markers for forecasting their population dynamics and occurrence of damages to fisheries. Red tides of marine raphidophytes, especially Chattonella species, often extensively harm aquaculture industries in regions with a temperate climate around the world. A red tide of Chattonella tends to develop just after an input of nutrients along the coast. Chattonella displays diurnal vertical migration regulated by a weak blue light, so it photosynthesizes in the surface layer during the daytime and takes up nutrients in the bottom layer during the nighttime. Superoxide produced by Chattonella cells is a strong candidate for the cause of its toxicity to bacteria and fishes. Here we conducted mRNA-seq of Chattonella antiqua to identify genes with functions closely related to the dynamics of the noxious red tide, such as photosynthesis, photoreception, nutrient uptake, and superoxide production. The genes related to photosynthetic pigment biosynthesis and nutrient uptake had high similarity with those of model organisms of plants and algae and other red-tide microalgae. We identified orthologous genes of photoreceptors such as aureochrome (newly five genes), the cryptochrome/photolyase (CRY/PHR) family (6-4PHR, plant CRY or cyclobutane pyrimidine dimer [CPD] Class III, CPD Class II, and CRY-DASH), and phytochrome (four genes), which regulate various physiological processes such as flagellar motion and cell cycle in model organisms. Six orthologous genes of NADPH oxidase, which produces superoxide on the cell membrane, were found and divided into two types: one with 5-6 transmembrane domains and another with 11 transmembrane domains. The present study should open the way for analyzing the eco-physiological features of marine raphidophytes at the molecular level.

Phototaxis Assay for Chlamydomonas reinhardtii

Phototaxis is a behavior in which organisms move toward or away from the light source (positive or negative phototaxis, respectively). It is crucial for phototrophic microorganisms to inhabit under proper light conditions for phototaxis. The unicellular green alga Chlamydomonas reinhardtii rapidly changes its swimming direction upon light illumination, and thus is a nice model organism for phototaxis research. Here we show two methods to assay Chlamydomonas phototaxis; one is a quick, easy and qualitative analysis, so-called the dish assay; and the other is a quantitative single-cell analysis.

Displacement-weighted velocity analysis of gliding assays reveals that Chlamydomonas axonemal dynein preferentially moves conspecific microtubules

In vitro gliding assays, in which microtubules are observed to glide over surfaces coated with motor proteins, are important tools for studying the biophysics of motility. Gliding assays with axonemal dyneins have the unusual feature that the microtubules exhibit large variations in gliding speed despite measures taken to eliminate unsteadiness. Because axonemal dynein gliding assays are usually done using heterologous proteins, i.e., dynein and tubulin from different organisms, we asked whether the source of tubulin could underlie the unsteadiness. By comparing gliding assays with microtubules polymerized from Chlamydomonas axonemal tubulin with those from porcine brain tubulin, we found that the unsteadiness is present despite matching the source of tubulin to the source of dynein. We developed a novel, to our knowledge, displacement-weighted velocity analysis to quantify both the velocity and the unsteadiness of gliding assays systematically and without introducing bias toward low motility. We found that the quantified unsteadiness is independent of tubulin source. In addition, we found that the short Chlamydomonas microtubules translocate significantly faster than their porcine counterparts. By modeling the effect of length on velocity, we propose that the observed effect may be due to a higher rate of binding of Chlamydomonas axonemal dynein to Chlamydomonas microtubules than to porcine microtubules.

Drastic reorganization of the bioconvection pattern of Chlamydomonas: quantitative analysis of the pattern transition response

Motile aquatic microorganisms are known to self-organize into bioconvection patterns. The swimming activity of a population of microorganisms leads to the emergence of macroscopic patterns of density under the influence of gravity. Although long-term development of the bioconvection pattern is important in order to elucidate the possible integration of physiological functions of individuals through bioconvection pattern formation, little quantitative investigation has been carried out. In the present paper, we present the first quantitative description of long-term behavior of bioconvection of Chlamydomonas reinhardtii, particularly focusing on the 'pattern transition response'. The pattern transition response is a sudden breakdown of the steady bioconvection pattern followed by re-formation of the pattern with a decreased wavelength. We found three phases in the pattern formation of the bioconvection of C. reinhardtii: onset, steady-state 1 (before the transition) and steady-state 2 (after the transition). In onset, the wavelength of the bioconvection pattern increases with increasing depth, but not in steady-states 1 or 2. By means of the newly developed two-axis view method, we revealed that the population of C. reinhardtii moves toward the bottom of the experimental chamber just before the pattern transition. This indicates that the pattern transition response could be caused by enhancement of the gyrotaxis of C. reinhardtii as a result of the changes in the balance between the gravitactic and gyrotactic torques. We also found that the bioconvection pattern changes in response to the intensity of red-light illumination, to which C. reinhardtii is phototactically insensitive. These facts suggest that the bioconvection pattern has a potential to drastically reorganize its convection structure in response to the physiological processes under the influence of environmental cues.

Exploring the molecular basis of responses to light in marine diatoms

Light is an essential source of energy for life on Earth and is one of the most important signals that organisms use to obtain information from the surrounding environment, on land and in the oceans. Prominent marine microalgae, such as diatoms, display a suite of sophisticated responses (physiological, biochemical, and behavioural) to optimize their photosynthesis and growth under changing light conditions. However, the molecular mechanisms controlling diatom responses to light are still largely unknown. Recent progress in marine diatom genomics and genetics, combined with well-established (eco) physiological and biophysical approaches, now offers novel opportunities to address these issues. This review provides a description of the molecular components identified in diatom genomes that are involved in light perception and acclimation mechanisms. How the initial functional characterizations of specific light regulators provide the basis to investigate the conservation or diversification of light-mediated processes in diatoms is also discussed. Hypotheses on the role of the identified factors in determining the growth, distribution, and adaptation of diatoms in different marine environments are reported.

Gravitaxis in Chlamydomonas reinhardtii Studied with Novel Mutants

Many free-swimming unicellular organisms show negative gravitaxis, i.e. tend to swim upward, although their specific densities are higher than the medium density. To obtain clues to the mechanism of this behavior, we examined how a mutation in motility or behavior affects the gravitaxis in Chlamydomonas. A phototaxis mutant, ptx3, deficient in membrane excitability showed weakened gravitaxis, whereas another phototaxis mutant, ptx1, deficient in regulation of flagellar dominance displayed normal gravitaxis. Two mutants that swim backwards only, mbo1 and mbo2, did not show any clear gravitaxis. We also isolated two novel mutants deficient in gravitaxis, gtx1 and gtx2. These mutants displayed normal motility and physical characteristics of cell body as assessed by the behavior of anesthetized cells. However, these cells were found to have defects in physiological responses involving membrane excitation. These observations are consistent with the idea that the gravitaxis in Chlamydomonas involves a physiological signal transduction system, which is at least partially independent of the system used for phototaxis.

Photoreceptor current and photoorientation in chlamydomonas mediated by 9-demethylchlamyrhodopsin

Green flagellates possess rhodopsin-like photoreceptors involved in control of their behavior via generation of photocurrents across the plasma membrane. Chlamydomonas mutants blocked in retinal biosynthesis are "blind," but they can be rescued by the addition of exogenous retinoids. Photosignaling by chlamyrhodopsin regenerated with 9-demethylretinal was investigated by recording photocurrents from single cells and cell suspensions, and by measuring phototactic orientation. The addition of a saturating concentration of this analog led to reconstitution of all receptor molecules. However, sensitivity of the photoreceptor current in cells reconstituted with the analog was smaller compared with retinal-reconstituted cells, indicating a decreased signaling efficiency of the analog receptor protein. Suppression of the photoreceptor current in double-flash experiments was smaller and its recovery faster with 9-demethylretinal than with retinal, as it would be expected from a decreased PC amplitude in the analog-reconstituted cells. Cells reconstituted with either retinal or the analog displayed negative phototaxis at low light and switched to positive one upon an increase in stimulus intensity, as opposed to the wild type. The reversal of the phototaxis direction in analog-reconstituted cells was shifted to a higher fluence rate compared with cells reconstituted with retinal, which corresponded to the decreased signaling efficiency of 9-demethylchlamyrhodopsin.