Award: PLR-1341479

The Southern Ocean (SO) plays a key role in Earth?s ocean and atmospheric systems, and phytoplankton that reside in these waters contribute to rich, marine food webs that support fish, birds, penguins, seals and whales. However, low iron and light availability can limit the growth rates of phytoplankton in the SO resulting in patchy primary productivity and unutilized macronutrients in surface waters. Within the last half-century, regions of the SO such as the Western Antarctic Peninsula (WAP), have experienced dramatic changes in ocean and air temperatures, impacting phytoplankton and other biological communities. The overall objective of this project was to examine the molecular basis for the physiological responses of SO diatom isolates to variations in iron and light availability and to study the ecological role of iron and light limitation in natural phytoplankton assemblages in the WAP region. Using physiological and molecular-based approaches, we investigated the growth response and expressed gene repertoires of nine isolates from the WAP region to varying iron concentrations and light levels. Under the performed laboratory culture conditions, the growth rates of most diatoms exhibited a further decline in growth rates due to reduced light levels rather than the lower iron concentrations. Using transcriptomics, 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 distinguish SO diatoms from those isolated in other regions. To examine how iron and light limitation effects the molecular physiology of the ecologically important polar diatom Fragilariopsis kerguelensis, we employed the use of comparative transcriptomics. We investigated both independent and interactive effects of iron and light limitation on the gene expression changes in this diatom. In particular, we examined pathways that are predicted to be significantly affected by iron and/or light limitation, such as photosynthesis, nitrogen assimilation, carbon metabolism, and iron homoeostasis (Fig. 1). F. kerguelensis did not appear to suffer from an additive effect of iron and light co-limitation in relation to growth rates and photophysiology, confirming our previous findings that many SO diatoms do not exhibit additive effects due to a combination of low iron and light. The largest transcriptomic response (i.e., total number of differentially expressed genes and metabolic pathways) was detected in the low light and the combined low iron and low light treatments compared to the control, indicating an increased cellular response to low light availability. Understanding how F. kerguelensis has adapted to its present environment and how it may be affected by future ocean change is imperative due to the significant influence this diatom has on biogeochemical cycling in the SO. To examine how iron limitation affects elemental stoichiometry and associated molecular physiology, we measured the elemental composition and performed comparative transcriptomics in four SO diatoms. Two centrics and pennates diatoms isolated from WAP waters were used in this study to determine if there are any lineage specific differences. Our analysis will be applied to answer the broad question of whether changes in stoichiometric ratios of Si:N and Si:C are a direct function of iron availability, or another physiological parameter such as changes in biovolume, and if there are any associated genes and metabolic pathways that can explain the physiological response to low iron. To examine the ecological importance of iron and light limitation within natural phytoplankton assemblages we participated on two Palmer-LTER cruises in 2016 and 2018 in the WAP region. On these cruises, we conducted both iron/light manipulation experiments and collected samples along the transects to examine expressed genes in natural diatom assemblages and develop molecular indicators to assess iron and light status of diatoms in the field. RNA and DNA were extracted from both the incubation experiments and from the transect samples. Our sequencing efforts are focused on evaluating a cost effective technique, termed Tag-seq, which sequences short 50bp tags at the 3? end of the transcript. We will use genes found to be responsive to iron and light in our diatom isolate studies that could serve as molecular indicators to assess the iron and light status of natural diatom communities. This research provides a further understanding of the molecular bases behind the response of SO diatoms to iron and light limitation. Unlike temperate diatoms, many SO diatoms were not found to be additively affected by these two abiotic factors, suggesting a dinstint physiology within polar diatoms . Similarly, based on their gene repertoires and transcriptome expression profiles we have characterized unique strategies for coping with iron and/or light limitation. We propose the use of molecular indicators as an incubation-independent method to examine the iron and light status of diatoms within natural assemblages. Last Modified: 11/15/2018 Submitted by: Adrian Marchetti