Examination of gene repertoires and physiological responses to iron and light limitation in Southern Ocean diatoms

Abstract

Diatoms play a fundamental role at the base of the polar marine food web. In the Southern Ocean, low iron concentrations and light levels control diatom abundance and distribution. Diatoms must therefore employ strategies that allow them to cope when iron and/or light availability is growth limiting. Through a combination of physiological and molecular-based approaches, we have investigated the physiological response to variable iron concentrations and light levels along with the expressed gene repertoires of nine newly isolated diatoms from the Western Antarctic Peninsula (WAP) region of the Southern Ocean. The diatoms ranged across five orders of magnitude in biovolume and displayed various degrees of susceptibility to low iron and light availability. Under the performed laboratory culture conditions, the growth rates of most diatoms decreased more due to low light level rather than low iron concentrations. Additionally, most diatoms were not subject to further reductions in growth rates when grown under combined low-light and iron-limiting conditions, indicating they are less likely to be co-limited by an additive effect. By sequencing the transcriptomes of these diatoms, we identified genes that likely facilitate their growth under variable iron and light conditions commonly present in the Southern Ocean. Specifically, we investigated the presence of 20 key genes involved in iron acquisition and homeostasis, iron usage in photosynthesis and nitrogen assimilation, and protection from reactive oxygen species. When comparing gene repertoires of recently sequenced transcriptomes of diatoms isolated from around the globe, the prevalence of certain genes exhibited biogeographical patterns that clearly distinguish Southern Ocean diatoms from those isolated from other regions.