Experimental evidence of how extreme air temperatures influence microphytobenthos up and down migration

Microphytobenthos (MPB) are microalgae that form biofilms on sediment surfaces and play a key role in coastal ecosystems by supporting food webs, regulating carbon (CO₂) fluxes, and stabilizing mudflats.

Some species are known to migrate vertically within the sediment as a protective strategy. During daytime low tides, MPB migrates to the surface to perform photosynthesis (S), whereas during other periods, MPB moves deeper (“buried” state, B) for nutrients and protection from grazers. The transitional state of the biofilm between B and S depends on the migration speed, which is estimated to range between 0.11 and 0.45 µm.s^-1 [1]. When in B, the biofilm cannot be detected through optical remote sensing methods, and has a reduced photosynthetic rate.

It is known that extreme air temperature events will become more frequent in the coming years due to climate change. The aim of this study is to demonstrate, under controlled conditions, that an extreme air temperature event affects the up and down migration of the biofilm, and therefore the services it provides and its detectability.

Sediment containing biofilm was collected from the Loire estuary in France during two different seasons (in fall and spring), homogenized, and placed in two experimental intertidal chambers for one week, with tide, light, and temperature controlled. A one-day acclimation period simulating field conditions was applied in both chambers, after which two scenarios were implemented. One chamber served as a control, with air temperature following a sinusoidal pattern between the mean daily minimum and mean daily maximum temperatures for the 2000–2024 period, whereas a sudden extreme air temperature event was applied in the other chamber. The experiment was repeated three times for each season, using extreme air temperature events corresponding to (1) the maximum air temperature observed from hourly data, at the site, for the season, for the 2000–2024 period (29.2°C for October and 37.5°C for June), (2) the maximum observed air temperature plus a delta corresponding to an RCP4.5 scenario at long-term horizon (29.2+2.25°C for October and 37.5+1.96°C for June), and (3) the maximum observed air temperature plus a delta corresponding to an RCP8.5 scenario at long-term horizon (29.2+3.82°C for October and 37.5+3.46°C for June). Biofilm concentration in state S was measured every 30 seconds, using a non-destructive hyperspectral reflectance method. The normalized difference vegetation index (NDVI) was used as a proxy for biomass.

An increase in NDVI was assumed to indicate upward migration, while a decrease in NDVI indicated downward migration. The data were interpolated allowing comparison between the control and the treatment. For each day, the mean signed difference (MSD) between control and treatment was calculated. A positive MSD indicated stimulation of the biofilm by the treatment, while a negative MSD indicated inhibition. The initial hypothesis was that the treatment would stimulate the biofilm at the beginning of the event, followed by a progressive inhibition over the week. Results are discussed to confirm, or not, the hypothesis.

[1] Serôdio et al. (2023). Light niche construction: Motility of sediment-inhabiting diatoms determines the experienced light environment.