PhD thesis: New insights on sources and sinks of organic matter in the oligotrophic coastal and open ocean





Esporles, September 28, 2017. Francesca Iuculano has defended her doctoral thesis supervised by the doctors Susana Agustí and Xose Antón Salgado. The event has taken place today, September 28, 2017 at the IMEDEA (UIB-CSIC).



Oligotrophic systems cover approximately 70% of the ocean surface and, due to their low concentrations of nutrient salts and chlorophyll, are essentially known as ocean deserts. However, their huge extension makes them playing an important role in the carbon cycle. While the primary production in the subtropical gyres of the open ocean is dominated by phytoplankton, in the oligotrophic coastal systems of the Mediterranean Sea Posidonia oceanica is the dominant primary benthic producer.


This Ph.D. thesis is focused on investigating two key compartments within the organic matter continuum of the coastal and oceanic oligotrophic ecosystems: the chromophoric dissolved organic matter (CDOM), and the transparent exopolymer particles (TEP). CDOM represents the optically active fraction of the dissolved organic matter (DOM) and it plays a key role in the photochemical and photobiological processes occurring in marine ecosystems.


TEP are organic particles of adhesive nature, formed via abiotic as well as biotic processes and are especially important for the biological carbon pump. Although in the last years the scientific community has increased its knowledge regarding the distribution of CDOM and TEP in the oceans, as well as about their roles in biogeochemical cycles, the current level of comprehension on the sources and sinks that control their dynamics in oligotrophic systems remains elusive. In this context, the identification of new sources of CDOM and TEP, and the deeper exploration of the biogeochemical processes governing them in oligotrophic systems – until now poorly explored – is the leitmotiv of this thesis.


In order to address these questions, we used a combination of field observation and laboratory experimentation.


On one hand, I participated in the Malaspina 2010 Expedition, which crossed 15 tropical and subtropical biogeographic provinces of the global ocean. On the other hand, the marine research station of Cap Ses Salines (Majorca) was used as reference of the oligotrophic coastal system and it was additionally compared with Es Caragol beach, characterized by its extensive P. oceanica meadows. At the global scale this thesis has evaluated, by means of the analysis of the absorption spectra of CDOM in the photic layer of the Atlantic, Indian and Pacific Oceans, the biogeochemical variables that mainly dictate the distribution of the optical properties of CDOM. Chlorophyll and apparent oxygen utilization have been shown to be the most relevant drivers, denoting the catabolic processes within the microbial food web as the main source of colour in the oligotrophic ocean. Moreover, this thesis has allowed the identification of Prochlorococcus as a potentially relevant source of TEP
in the oligotrophic ocean.


At the local scale, the time series results derived from the coastal observations have tested the hypothesis that P. oceanica is an important source of both CDOM and TEP, although the estimations of its contributions to the carbon cycle in the Mediterranean Sea and the biogeochemical processes require additional studies.


The original and novel results presented and discussed in this Ph.D. thesis propel new hypotheses to develop future research lines aimed at deeply comprehend the biotic and abiotic processes that control the dynamics of these components in the oligotrophic systems and its implications in the efficiency of the carbon pumps in future global change scenarios