An ecophysiological study of Calanus finmarchicus at high latitudes using environmental metabolomics.

Abstract

The marine copepod, Calanus finmarchicus, dominates the mesozooplankton communities throughout much of the North Atlantic and Arctic Oceans and plays important roles in food webs and the global carbon cycle. Their highly plastic life cycle centres around rapidly accumulating lipids and somatic reserves during the growing season, which then fuel diapause, sexual maturation and may also support spawning. The majority of lipids for diapause is stored in a specialised organ as wax esters, a long-term energy storage lipid which aids buoyancy regulation. However, C. finmarchicus also store lipid within cells as triacylgylcerols (TAGs). Comparatively little is known about TAGs metabolism but TAGs are thought to reflect recent feeding as the short-term energy store. These lipid classes and the associated polyunsaturated fatty acids (PUFAs) are vital for C. finmarchicus’ life history strategy. PUFAs are obtained by grazing on the microalgae and effectively cannot be synthesised de novo. However, climate change is altering the microplankton community, reducing the abundance of PUFA rich diatoms, and potentially limiting C. finmarchicus’ access to PUFAs.
This thesis used metabolomics, the study of low molecular weight products of metabolism, to study the ecophysiology of C. finmarchicus during three major life history stages; diapause, ontogenetic development, and reproduction. Chapter 2 investigated diapause by comparing active shallow (0-200 m) with deep (>200 m) animals from the North Atlantic in winter and late summer. During winter, deep animals demonstrated significant TAG reconfiguration, with longer more unsaturated TAGs increasing in peak intensity. This potentially reflects a sophisticated buoyancy mechanism, keeping animals at depth whilst they consume their somatic ballast. Chapter 3 explored the metabolomes of stage CV copepodites preparing to diapause and spawning females across the Fram Strait in August 2019, comparing their physiology to the food environment. TAG composition reflected ontogeny rather than the ambient food conditions and the metabolomes of both developmental stages indicated subsistence off internal reserves during low food conditions.
Chapter 4 investigated reproduction by comparing the physiology of C. finmarchicus females at high and low egg production sites (HEPS and LEPS, respectively) across the Fram strait, during May 2018 and August 2019. When compared with food environment metrics, metabolomic analysis indicated C. finmarchicus was income breeding in May 2018, when food was plentiful, yet capital breeding in August 2019, when food was scarce. During both cruises, TAGs increased in unsaturation and length at HEPS, indicating their importance for reproduction regardless of breeding mode.
This thesis has shown the power of metabolomics, particularly when applied in combination with traditional physiological metrics. It has demonstrated a need to expand our current definition of TAGs beyond the “short-term energy store” and highlighted the importance of PUFAs in diapause, ontogenetic development and reproduction, signalling C. finmarchicus’ current life history strategy is vulnerable to climate change.

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Identifiers

ORCID for Florence Atherden: orcid.org/0000-0001-6779-2274

ORCID for Catherine Lucas: orcid.org/0000-0002-5929-7481

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