Seasonality of Coastal Picophytoplankton Growth, Nutrient Limitation, and Biomass Contribution

Abstract

Picophytoplankton in the Baltic Sea includes the simplest unicellular cyanoprokaryotes (Synechococcus/Cyanobium) and photosynthetic picoeukaryotes (PPE). Picophytoplankton are thought to be a key component of the phytoplankton community, but their seasonal dynamics and relationships with nutrients and temperature are largely unknown. We monitored pico- and larger phytoplankton at a coastal site in Kalmar Sound (K-Station) weekly during 2018. Among the cyanoprokaryotes, phycoerythrin-rich picocyanobacteria (PE-rich) dominated in spring and summer while phycocyanin-rich picocyanobacteria (PC-rich) dominated during autumn. PE-rich and PC-rich abundances peaked during summer (1.1 × 105 and 2.0 × 105 cells mL–1) while PPE reached highest abundances in spring (1.1 × 105 cells mL–1). PPE was the main contributor to the total phytoplankton biomass (up to 73%). To assess nutrient limitation, bioassays with combinations of nitrogen (NO3 or NH4) and phosphorus additions were performed. PE-rich and PC-rich growth was mainly limited by nitrogen, with a preference for NH4 at >15°C. The three groups had distinct seasonal dynamics and different temperature ranges: 10°C and 17–19°C for PE-rich, 13–16°C for PC-rich and 11–15°C for PPE. We conclude that picophytoplankton contribute significantly to the carbon cycle in the coastal Baltic Sea and underscore the importance of investigating populations to assess the consequences of the combination of high temperature and NH4 in a future climate.

Seasonal dynamics in picocyanobacterial abundance and clade composition at coastal and offshore stations in the Baltic Sea

“…Peak abundances at the coastal K-station and offshore LMO during 2019 and 2020 were in the same range, with the exception of the summer of 2018, when K-station peak abundances reached 4.7 × 10 5 cells mL −1 . These numbers are comparable to other observations in the Baltic Proper during summer (10 5 cells mL −1 ), and confirm previous observations that picocyanobacterial abundances are as high at coastal and offshore locations 55 , 57 , 68 . It is also important to note that the picocyanobacterial abundances at < 10 °C reported in this study were notably higher than previous reports in the Gulf of Finland and in other temperate ecosystems 8 , 9 .…”

“…The increase in contribution of S5.2 took place in June-July, when temperature was > 18 °C and NO 3 concentration was low. This result suggests that S5.2 affiliated picocyanobacterial strains are adapted to high temperatures and may use NH 4 as a primary nitrogen source 33 , 55 , 77 . On the other hand, clade A/B dominated during the colder months, indicating that picocyanobacteria strains in this clade have a preference for low temperatures and high NO 3 concentration.…”

“…Both PE-SYN and PC-SYN showed moderate negative correlation with NO 3 concentration. Some studies have suggested preference of picocyanobacteria for NH 4 over NO 3 at high temperature conditions (> 15 °C) 55 , 77 . Thus, newly fixed nitrogen in the form of NH 4 from N 2 -fixers or regeneration during blooms may be an important driver supporting picocyanobacterial growth during summer 31 – 33 , in line with the positive correlation between N 2 -fixers and PE-SYN.…”

“…K-station samples until 12th May 2020 and LMO samples until 10th July 2019 were analyzed using a Cyflow® Cube8 flow cytometer (Partec®, Germany) at 10 µL s −1 while after that date a BD FACSverse (BD Biosciences) was used instead. Picophytoplankton were counted as three populations: photosynthetic picoeukaryotes (PPE), phycoerythrin rich (PE-SYN) SYN cells and phycocyanin rich (PC-SYN) SYN based on the gating described in Alegria Zufia et al 55 . The observations for PPE abundance variation are presented in Supplementary Figure S1 .…”

“…In the Baltic Proper, picocyanobacterial blooms take place during the summer at temperatures up to 20–24 °C and nitrogen limited conditions 54 – 56 . Peak abundances during this period reach up to 10 5 cells mL −1 55 , 57 corresponding to 21% contribution to the total phytoplankton community in terms of Chl a 58 and 56% in terms of carbon biomass 52 . The diversity of the picocyanobacterial community in the Baltic Sea has been characterized by amplification and sequencing of the V4-V5 16S rRNA gene hypervariable region (e.g.…”