Paradox of relatively more phospholipids in phytoplankton in phosphorus limited sea

Abstract

Marine life is threatened by global warming and its indirect consequences, which include, among others, increased stratification leading to phosphorus (P) and nitrogen depletion in the upper water column. Phosphorus plays a key role in all biochemical systems; storage of metabolic energy, formation of genetic material, and subcellular compartmentalization. Our multi‐year study of lipid biogeochemistry in the northern Adriatic (Mediterranean), which is becoming warmer and nutrient‐poorer, particularly regarding P, has shown that under conditions of P scarcity, phospholipids are relatively more abundant and smaller plankton cells dominate. Consistent with the field data, experiments with seven phytoplankton monocultures, comprising microplankton, nanoplankton, and picoplankton, confirmed a relatively higher phospholipid content in the smaller phytoplankton species and, in particular, an increase in those grown under stress conditions in general, including, unexpectedly, P‐limitation. We suggest two reasons for the observed “P paradox” of P‐limited phytoplankton: (1) cell geometry: volume of the plasma membrane relative to the volume of the entire cell is greater in smaller cells and, therefore, the proportion of plasma membrane phospholipids to intracellular lipids is greater in smaller cells, (2) higher proportion of densely packed saturated fatty acids found in stressful conditions, including P oligotrophy, additionally increase the proportion of membrane phospholipids relative to intracellular lipids. Our findings contribute to the understanding of P cycling in the sea. In addition, our data suggest that higher phospholipid export to deep waters is possible by smaller plankton.

Projected climate oligotrophication of the Adriatic marine ecosystems

“…The NM ecoregion, strongly influenced by the dynamics of the northern estuaries via horizontal transport and mesoscale filaments, also shows a significant decrease in annual biological activity: −12.1% in NPP, −33.2% in chlorophyll-a, −33% in phytoplankton biomass year on year. These changes appear aligned with the recent occurrence of extremely low annual discharges (e.g., Gervasio et al, 2023;Montanari et al, 2023;Aragão et al, 2024), and the low levels of nutrients and chlorophyll-a concentrations measured in the water column of the NE ecoregion (e.g., ARPAE, 2021;Gašparović et al, 2023). Although it is still too soon to interpret these data as harbingers of future mean changes, they surely represent an unprecedented event in our records (Bonaldo et al, 2023;Montanari et al, 2023).…”