# Revealing the Physiological Patterns of Dinoflagellates in North‐Eastern Adriatic Phytoplankton

## TL;DR

This study uses metatranscriptomics to reveal how dinoflagellates and other phytoplankton adapt to seasonal and environmental changes in the northern Adriatic Sea.

## Contribution

The first comprehensive physiological characterization of phytoplankton in the north-eastern Adriatic using metatranscriptomics.

## Key findings

- Dinoflagellates showed the highest metabolic activity across all seasons in the >50 μm size fraction.
- Seasonal shifts in phytoplankton communities were driven by phosphorus, silicon availability, and species interactions.
- Dinoflagellates used both photosynthesis and phagotrophy, with metabolic strategies changing seasonally based on nutrient availability.

## Abstract

The northern Adriatic is a highly dynamic marine ecosystem where multiple environmental stressors, particularly phosphorus limitation, shape phytoplankton communities. Previous studies have established annual phytoplankton succession patterns primarily using light microscopy, while metatranscriptomic analyses have been lacking. This study used a metatranscriptomic approach to investigate the taxonomic and functional dynamics of the northern Adriatic phytoplankton community, focusing on the predominant group of dinoflagellates. Monthly sampling from April 2021 to March 2022 at two coastal stations revealed dinoflagellates as the most metabolically active phylum throughout the year in the size fraction > 50 μm. Peaks in metabolic activity of other studied phyla aligned with the characteristic seasonal species succession observed in previous studies. Community ordination indicated distinct seasonal shifts driven by environmental factors, notably phosphorus and silicon availability, as well as species interactions. Both photosynthesis and phagotrophy emerged as important trophic strategies for dinoflagellates. Nitrogen and phosphorus metabolism showed clear seasonal trends, with dinoflagellates employing various strategies for nutrient acquisition and recycling depending on resource availability. Changes in the activation of different cellular processes highlighted a seasonal shift in metabolic investment, with spring favouring rapid population expansion, while the rest of the year was characterised by prevalent transcription of genes indicative of cellular maintenance and adaptation. This study provides critical new insights into dinoflagellate phytoplankton ecology and emphasises the need for further multi‐method research to fully understand their role in the northern Adriatic ecosystem.

Using metatranscriptomics, we have described the annual taxonomic and functional succession of the phytoplankton community in the north‐eastern Adriatic Sea, taking into account the effects of environmental factors on succession. The results obtained in this way represent the first comprehensive physiological characterisation of the phytoplankton community in the area studied. They are of great importance for answering fundamental questions of the phytoplankton ecology of the north‐eastern Adriatic Sea and at the same time enable the continuation of research on a broader spatial and temporal scale.