Metabolic co-dependence drives the evolutionarily ancient Hydra-Chlorella symbiosis

Elife. 2018 May 31;7:e35122. doi: 10.7554/eLife.35122.


Many multicellular organisms rely on symbiotic associations for support of metabolic activity, protection, or energy. Understanding the mechanisms involved in controlling such interactions remains a major challenge. In an unbiased approach we identified key players that control the symbiosis between Hydra viridissima and its photosynthetic symbiont Chlorella sp. A99. We discovered significant up-regulation of Hydra genes encoding a phosphate transporter and glutamine synthetase suggesting regulated nutrition supply between host and symbionts. Interestingly, supplementing the medium with glutamine temporarily supports in vitro growth of the otherwise obligate symbiotic Chlorella, indicating loss of autonomy and dependence on the host. Genome sequencing of Chlorella sp. A99 revealed a large number of amino acid transporters and a degenerated nitrate assimilation pathway, presumably as consequence of the adaptation to the host environment. Our observations portray ancient symbiotic interactions as a codependent partnership in which exchange of nutrients appears to be the primary driving force.

Keywords: Chlorella; Hydra; evolutionary biology; genome; nitrogen metabolism; symbiosis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Biological Evolution*
  • Chlorella / drug effects
  • Chlorella / genetics
  • Chlorella / metabolism*
  • Conserved Sequence
  • Darkness
  • Epithelial Cells / drug effects
  • Epithelial Cells / metabolism
  • Gene Expression Regulation
  • Genome
  • Hydra / drug effects
  • Hydra / genetics
  • Hydra / growth & development
  • Hydra / metabolism*
  • Molecular Sequence Annotation
  • Nitrates / metabolism
  • Nitrogen / metabolism
  • Photosynthesis / genetics
  • RNA, Ribosomal, 18S / genetics
  • RNA, Ribosomal, 18S / metabolism
  • Species Specificity
  • Sugars / pharmacology
  • Symbiosis* / drug effects
  • Symbiosis* / genetics


  • Nitrates
  • RNA, Ribosomal, 18S
  • Sugars
  • Nitrogen

Grant support

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.