The Photoprotective Behavior of a Motile Benthic Diatom as Elucidated from the Interplay Between Cell Motility and Physiological Responses to a Light Microgradient Using a Novel Experimental Setup.

It has long been hypothesized that benthic motile pennate diatoms use phototaxis to optimize photosynthesis and minimize photoinhibitory damage by adjusting their position within vertical light gradients in coastal benthic sediments. However, experimental evidence to test this hypothesis remains inconclusive, mainly due to methodological difficulties in studying cell behavior and photosynthesis over realistic spatial microscale gradients of irradiance and cell position. In this study, a novel experimental approach was developed and used to test the hypothesis of photosynthesis optimization through motility, based on the combination of single-cell in vivo chlorophyll fluorometry and microfluidic chips. The approach allows the concurrent study of behavior and photosynthetic activity of individual cells of the epipelic diatom species Craspedostauros britannicus exposed to a light microgradient of realistic dimensions, simulating the irradiance and distance scales of light microgradients in benthic sediments. Following exposure to light, (i) cells explored their light environment before initiating light-directed motility; (ii) cells used motility to lower their light dose, when exposed to the highest light intensities; and (iii) motility was combined with reversible non-photochemical quenching, to allow cells to avoid photoinhibition. The results of this proof-of-concept study not only strongly support the photoprotective nature of photobehavior in the studied species but also revealed considerable variability in how individual cells reacted to a light microgradient. The experimental setup can be readily applied to study motility and photosynthetic light responses of other diatom species or natural assemblages, as well as other photoautotrophic motile microorganisms, broadening the toolset for experimental microbial ecology research.

Contrasted impact of two macrofaunal species (Hediste diversicolor and Scrobicularia plana) on microphytobenthos spatial distribution and photosynthetic activity at microscale.

Microphytobenthos is most often the primary source of carbon for coastal soft-sediment communities, especially in intertidal and shallow subtidal environments. The influence of benthic macrofaunal organisms on microphytobenthic biomass, spatial distribution and photosynthetic capacities is not only resulting from their feeding intensity but also indirectly from their bioturbation activity, which regulates nutrient fluxes and sediment mixing. This study compares the impact of two species (Hediste diversicolor and Scrobicularia plana) that dominate macrofaunal communities in estuarine intertidal mudflats on microphytobenthic biomass and photosynthetic activity. Imaging-PAM fluorescence was used to non-invasively map the development of microphytobenthic biomass and to assess its spatial extent. Our results showed that, due to intense deposit feeding, Scrobicularia plana quickly limited microphytobenthos growth and photosynthetic activity, even at low density (<250 ind m-2). In contrast, the negative impact of Hediste diversicolor on microphytobenthos development due to direct consumption was very low. Thereby, the stimulation of nutrient fluxes at the sediment-water interface resulting from bioirrigation seems to enhance microphytobenthos growth and photosynthesis.

Evidence for better microphytobenthos dynamics in mixed sand/mud zones than in pure sand or mud intertidal flats (Seine estuary, Normandy, France).

Understanding the dynamics of microphytobenthos biomass and photosynthetic performances in intertidal ecosystems will help advance our understanding of how trophic networks function in order to optimize ecological management and restoration projects. The main objective of this study was to investigate microphytobenthic biomass and photosynthetic performances as a function of the sedimentary and environmental variabilities in the range of intertidal habitats in the downstream Seine estuary (Normandy, France). Our results highlight higher biomass associated with more stratified biofilms and better photosynthetic performances in areas characterized by a sand/mud mixture (40-60% of mud) compared to pure sand or pure mud environments. This type of sediment probably offers an efficient trade-off between the favorable characteristics of the two types of sediments (sand and mud) with respect to light penetration and nutrient accessibility. Moreover, the large quantities of exopolysaccharides produced in sand/mud mixtures emphasizes the functional role played by microphytobenthos in promoting sediment stability against erosion. This allows us to show that despite the strong increase in sand content of the downstream Seine estuary, intertidal flats are still productive since microphytobenthic biomass, photosynthetic performances and exopolysaccharides secretion are highest in sand-mud mixtures. This study also underlines the impact of ecosystem modifications due to human disturbance and climate change on the dynamics of key primary producers in estuaries.

Electron requirements for carbon incorporation along a diel light cycle in three marine diatom species.

Diatoms account for about 40% of primary production in highly productive ecosystems. The development of a new generation of fluorometers has made it possible to improve estimation of the electron transport rate from photosystem II, which, when coupled with the carbon incorporation rate enables estimation of the electrons required for carbon fixation. The aim of this study was to investigate the daily dynamics of these electron requirements as a function of the diel light cycle in three relevant diatom species and to apprehend if the method of estimating the electron transport rate can lead to different pictures of the dynamics. The results confirmed the species-dependent capacity for photoacclimation under increasing light levels. Despite daily variations in the photosynthetic parameters, the results of this study underline the low daily variability of the electron requirements estimated using functional absorption of the photosystem II compared to an estimation based on a specific absorption cross section of chlorophyll a. The stability of the electron requirements throughout the day would suggest it is potentially possible to estimate high-frequency primary production by using autonomous variable fluorescence measurements from ships-of-opportunity or moorings, without taking potential daily variation in this parameter into consideration, but this result has to be confirmed on natural phytoplankton assemblages. The results obtained in this study confirm the low electron requirements of diatoms to perform photosynthesis, and suggest a potential additional source of energy for carbon fixation, as recently described in the literature for this class.

Dynamics of phytoplankton productivity and exopolysaccharides (EPS and TEP) pools in the Seine Estuary (France, Normandy) over tidal cycles and over two contrasting seasons.

Exopolysaccharides (EPS) play an important role in the carbon flux and may be directly linked to phytoplankton and microphytobenthos production, most notably in estuarine systems. However the temporal and spatial dynamics of estuarine EPS are still not well understood, nor how primary productivity triggers this variability at these different scales. The aim of this study was to investigate the primary productivity of phytoplankton and EPS dynamics in the Seine estuary over a tidal cycle in three different haline zones over two contrasted seasons. The other objectives was to investigate the origin of pools of soluble carbohydrates (S-EPS) and transparent exopolymeric particles (TEP) in phytoplankton, microphytobenthos or other compartments. High frequency measurements of productivity were made in winter and summer 2015. Physical and chemical parameters, biomass and EPS were measured at hourly intervals in sub-surface waters and just above the water sediment-interface. Our results confirmed that high frequency measurements improve the accuracy of primary productivity estimations and associated carbon fluxes in estuaries. The photosynthetic parameters were shown to be strongly controlled by salinity and by the concentrations of suspended particle matter at the smallest temporal and at spatial scales. At these scales, our results showed an inverse relationship between EPS concentrations and biomass and productivity, and a positive relationship with sediment resuspension. Additionally, the distribution of EPS appears to be linked to hydrodynamics with the tide at daily scale and with the winter at seasonal scale. At spatial scale, the maximum turbidity zone played an important role in the distribution of TEP. Our results suggest that, in the Seine estuary, between 9% and 33% of the S-EPS pool in the water column can be attributed to phytoplankton excretion, while only 0.4%-1.6% (up to 6.14% in exceptional conditions) originates from the microphytobenthos compartments. Most EPS was attributed to remobilization of detrital carbon pools in the maximum turbidity zone and in the sediment or allochthonous origin.