Globally Important Haptophyte Algae Use Exogenous Pyrimidine Compounds More Efficiently than Thiamin

Area of Science:

• Marine microbiology
• Biogeochemistry
• Phytoplankton ecology

Background:

• Vitamin B1 (thiamin) is essential for marine algae, but its scarcity in surface oceans raises questions about phytoplankton nutrition.
• Some algae can utilize thiamin precursors, like HMP, for growth, suggesting alternative pathways for vitamin acquisition.

Purpose of the Study:

• To investigate thiamin biosynthesis and salvage pathways in diverse marine haptophytes and other phytoplankton.
• To determine the growth responses of phytoplankton to thiamin and its precursors under varying supply levels.
• To quantify cellular thiamin quotas in oceanic phytoplankton.

Main Methods:

• Comparative genomic analysis of transcriptomes and genomes from 30 phytoplankton species.
• Axenic culture experiments with thiamin and its precursors (HMP, AmMP).
• Thiochrome assay for quantifying cellular thiamin quotas in Emiliania huxleyi.

Main Results:

• HMP synthase is absent in studied haptophytes, but other thiamin biosynthesis genes are present.
• Phytoplankton species grew equally well on HMP or thiamin, and some showed enhanced growth on HMP and AmMP under environmentally relevant concentrations.
• Cellular thiamin quotas in E. huxleyi were lower under simulated environmental conditions than previously reported.
• Identification of a salvage protein subfamily (TENA_E) in multiple phytoplankton groups.

Conclusions:

• Marine phytoplankton can utilize thiamin precursors (HMP, AmMP) for growth, indicating the importance of salvage pathways.
• Environmental availability of thiamin precursors, rather than thiamin itself, may be a key factor structuring phytoplankton communities.
• Lowered cellular thiamin quotas and precursor utilization highlight the need to refine biogeochemical models for oceanic microbial ecosystems.