A metagenomic–based investigation of the planktonic microbial food web in relation with ice melting occurring in the Ross Sea coastal ecosystem

The Ross Sea area in the Southern Ocean is a critical region for the oceanography of the global ocean, being the site of significant deep-water formation, a rapidly changing region, and home to a highly productive ecosystem which, despite being studied intensively in the last decades, is still far from being fully deciphered. The continental shelf of the Ross Sea represents one of the most productive areas in the Southern Ocean, however the coastal food web is here still poorly characterized with several gaps and uncertainties existing, especially with respect to energy and carbon flows through the pelagic food web, the temporal and spatial variations in diversity and ecosystem functioning, and the connections among the key species, including microbes that drive the ecosystem functioning. Changes in the Ross Sea physical and biological features have been observed for decades, and are expected to continue in future. Among these changes, recent rapid warming has been documented at the regional scale in Antarctica, leading to massive loss of ice shelves and retreat of tidewater glaciers, together with reduction of seasonal sea-ice extension (Turner et al., 2005, Sinclair et al., 2014; Rintoul et al., 2018). Besides this, surface water warming and strong salinity changes, especially in coastal Antarctic areas during summer, have been reported (Moline et al., 2004; Meredith and King 2005). In the Ross Sea area, significant yet variable changes in the ice cover have been observed in the last years, with variable magnitudes leading to sea ice retreat and advance dynamics different if compared to other areas of the Antarctica (Stammerjohn et al., 2012). The study of the biological response to ice melting dynamics and changes in the thermohaline conditions have been primarily focused on phytoplankton with cross-food web links from krill to large vertebrates (Smetacek and Nicol 2005; Montes-Hugo et al., 2009; Schofield et al., 2010). However, the responses of the planktonic microbial food webs (from viruses to prokaryotes and unicellular eukaryotes), which are key components driving biogeochemical processes and energy and material flow to higher trophic levels, are still largely unknown (Wilkins et al., 2013; Wilkins et al. 2013). This limits our understanding of the functioning and provisioning of goods and services (biomass production, nutrient cycling) of Antarctic coastal ecosystems. The advances in DNA sequencing and metagenomics make now possible to provide insights on the biodiversity and functions of the microbial food webs, and to explore the complex network of biological and ecological interactions. These tools can provide unprecedented evidences on the role of the microbial “world” in the Antarctic coastal ecosystems, and their response to changing environmental conditions related with ice melting processes.

This project aims at providing new insights on the influence of changes in the thermohaline and trophic conditions, as the consequence of the sea ice melting processes, on biodiversity and functioning of the planktonic microbial food web in Antarctic coastal systems. This project will investigate the microbial responses during intensive field work through cross-cutting multidisciplinary and innovative approaches. In detail, the main objectives of this project are:
1. to investigate the temporal changes of planktonic microbial biodiversity, from viruses to prokaryotes and unicellular eukaryotes, by using high throughput sequencing technologies;
2. to investigate the relationships between microbial biodiversity and key attributes of ecosystem functioning (biomass production, organic matter cycling, nutrient regeneration)
3. to assess the role of viral impact on microbial biodiversity and biogeochemical cycles
4. to identify the main environmental drivers that shape microbial biodiversity and potential metabolic functions (by metagenomic analyses)
5. to shed light on the complex network of interactions within the planktonic food webs, and their response to the ice melting and consequent changes in the environmental conditions
6. to elaborate a new conceptual model of the response of the pelagic microbial food webs in coastal polar ecosystems under future climate change scenarios.