Abstract
BackgroundOchromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments. Species in this genus can be important consumers of bacteria but vary in their ability to perform photosynthesis. We studied the effect of light and bacteria on growth and gene expression of a predominantly phagotrophic Ochromonas species. Axenic cultures of Ochromonas sp. were fed with heat-killed bacteria (HKB) and grown in constant light or darkness. RNA was extracted from cultures in the light or in the dark with HKB present (Light + HKB; Dark + HKB), and in the light after HKB were depleted (Light + depleted HKB).ResultsThere were no significant differences in the growth or bacterial ingestion rates between algae grown in light or dark conditions. The availability of light led to a differential expression of only 8% of genes in the transcriptome. A number of genes associated with photosynthesis, phagotrophy, and tetrapyrrole synthesis was upregulated in the Light + HKB treatment compared to Dark + HKB. Conversely, the comparison between the Light + HKB and Light + depleted HKB treatments revealed that the presence of HKB led to differential expression of 59% of genes, including the majority of genes involved in major carbon and nitrogen metabolic pathways. Genes coding for unidirectional enzymes for the utilization of glucose were upregulated in the presence of HKB, implying increased glycolytic activities during phagotrophy. Algae without HKB upregulated their expression of genes coding for ammonium transporters, implying uptake of inorganic nitrogen from the culture medium when prey were unavailable.ConclusionsTranscriptomic results agreed with previous observations that light had minimal effect on the population growth of Ochromonas sp. However, light led to the upregulation of a number of phototrophy- and phagotrophy-related genes, while the availability of bacterial prey led to prominent changes in major carbon and nitrogen metabolic pathways. Our study demonstrated the potential of transcriptomic approaches to improve our understanding of the trophic physiologies of complex mixotrophs, and revealed responses in Ochromonas sp. not apparent from traditional culture studies.
Highlights
Ochromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments
heat-killed bacteria (HKB) abundances were not quantified after day 1, but daily samples were inspected for the presence of bacteria through day 8
Axenicity tests performed on the last day of the experiment indicated that one of the replicates in the Light + HKB treatment was contaminated with live bacteria on day 8, but it is uncertain when the contamination occurred
Summary
Ochromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments Species in this genus can be important consumers of bacteria but vary in their ability to perform photosynthesis. Mixotrophic species possess chloroplasts, but encompass a spectrum of mixotrophic behaviors ranging from nearly purely phototrophic to predominantly phagotrophic due to their ability to consume prey [1]. The extent of these contrasting trophic processes within a species is dependent on both the genetic composition of the mixotroph as well as resource availability in the environment (e.g. light and prey abundance) [2]. The important ecological role(s) played by these nutritionally flexible organisms has only been recognized within the last few decades [8, 9]
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