High amino acid osmotrophic incorporation by marine eukaryotic phytoplankton revealed by click chemistry

The osmotrophic uptake of dissolved organic compounds in the ocean is considered to be dominated by heterotrophic prokaryotes, whereas the role of planktonic eukaryotes is still unclear. We explored the capacity of natural eukaryotic plankton communities to incorporate the synthetic amino acid L-homopropargylglycine (HPG, analogue of methionine) using biorthogonal noncanonical amino acid tagging (BONCAT), and we compared it with prokaryotic HPG use throughout a 9-day survey in the NW Mediterranean. BONCAT allows to fluorescently identify translationally active cells, but it has never been applied to natural eukaryotic communities. We found a large diversity of photosynthetic and heterotrophic eukaryotes incorporating HPG into proteins, with dinoflagellates and diatoms showing the highest percentages of BONCAT-labelled cells (49 ± 25% and 52 ± 15%, respectively). Among them, pennate diatoms exhibited higher HPG incorporation in the afternoon than in the morning, whereas small (≤5 μm) photosynthetic eukaryotes and heterotrophic nanoeukaryotes showed the opposite pattern. Centric diatoms (e.g. Chaetoceros, Thalassiosira, and Lauderia spp.) dominated the eukaryotic HPG incorporation due to their high abundances and large sizes, accounting for up to 86% of the eukaryotic BONCAT signal and strongly correlating with bulk 3H-leucine uptake rates. When including prokaryotes, eukaryotes were estimated to account for 19-31% of the bulk BONCAT signal. Our results evidence a large complexity in the osmotrophic uptake of HPG, which varies over time within and across eukaryotic groups and highlights the potential of BONCAT to quantify osmotrophy and protein synthesis in complex eukaryotic communities.

Abiotic plastic leaching contributes to ocean acidification

Ocean acidification and plastic pollution are considered as potential planetary boundary threats for which crossing certain thresholds could be very harmful for the world's societies and ecosystems well-being. Surface oceans have acidified around 0.1 units since the Industrial Revolution, and the amount of plastic reaching the ocean in 2018 was quantified to 13 million metric tonnes. Currently, both ocean threats are worsening with time. Plastic leaching is known to alter the biogeochemistry of the ocean through the release of dissolved organic matter. However, its impact in the inorganic chemistry of the seawater is less studied. Here we show, from laboratory experiments, that abiotic plastic degradation induces a decrease in seawater pH, particularly if the plastic is already aged, as that found in the ocean. The pH decrease is enhanced by solar radiation, and it is probably induced from a combination of the release of organic acids and the production of CO2. It is also related to the amount of leached dissolved organic carbon, with higher acidification as leaching increases. In coastal areas, where plastic debris accumulates in large quantities, plastic leaching could lead to a seawater pH decrease up to 0.5 units. This is comparable to the projected decrease induced in surface oceans by the end of the twenty-first century for the most pessimistic anthropogenic emissions scenarios.