Growth and chain formations of diatoms (Pseudo-nitzschia) under different turbulent conditions: a laboratory analysis

Diatoms have high productivity and are highly influenced by turbulent conditions. We consider here diatoms of the species Pseudo-nitzschia,  which are chain forming. The objective of this work was to show how the turbulent environment affects the growth and the chain forming of these species. For this, cultures of the species Pseudo-nitzschia multiseries and Pseudo-nitzschia fraudulenta were performed in the laboratory and submitted to stationary turbulent conditions, using the Agiturb system developed in the LOG at Wimereux (Le Quiniou et al. 2022). 

In the Agiturb system, the turbulent flow is produced using four contra-rotating agitators that are placed under a cubic tank, generating a statistically stationary, spatially inhomogeneous flow with compression and stretching. The injection of the energy in the flow is produced by 4 stirring bars activated by 4 magnetic stirrers situated at symmetric positions. The cubic tank is almost half-full with 15 liters of sea water. For each experiment, the magnitude of the rotation rate of each agitator was identical, with two agitators rotating clockwise and two anti-clockwise, the same directions being along the diagonal. Different values of the rotation rate were chosen to reach different turbulence levels, characterized by the microscale Reynolds number Rλ going from 130 to 360. These Reynolds numbers correspond to typical values found in the ocean, from the epicontinental zone, to coastal, surf zones and even storm conditions. 

In the experiments, all the other parameters that affect the diatoms’ proliferation were kept the same. Formation and growth of the chains were assessed through microscopy.  P. fraudulenta displayed higher growth than P. multiseries in all turbulence levels except from the control condition (Rλ=0) where the growth was approximately the same. The level of turbulence that was more beneficial for the growth of P. multiseries was the agitated (Rλ= 240) whereas for P. fraudulenta it was for a smaller Reynolds number (Rλ = 160). The chain length were also considered in relation with turbulence level, by considering the probability density of single chains, small chains (2 or 3 cells) and long chains (more than 4 cells). The result was that the predominant form of the cells for both species was the single cells. However, P. multiseries presented higher variations in chain forming throughout the whole experiment than P. fraudulenta. Within this approach, the optimal turbulence level, for growth as well as chain formation, can be assessed for each phytoplankton species.