Photosynthesis, Respiration, and Growth of Five Benthic Diatom Strains as a Function of Intermixing Processes of Coastal Peatlands with the Baltic Sea

Benthic diatom community response to the sudden rewetting of a coastal peatland

The coastline of the shallow southern Baltic Sea is a highly dynamic system of spits, lagoonal systems and subtidal flats. On the land side, coastal peatlands are common. Intact peatlands provide important ecosystem services, however, many peatlands have been severely degraded due to human activities. Many coastal peatlands are also separated from the Baltic Sea by artificial dunes or dykes in order to facilitate drainage and coastal protection measures. Due to their potential to act as carbon sinks, as well as buffer zones against sea level rise and flooding events, there is an increasing interest in the restoration and rewetting of coastal peatlands. Microphytobenthic communities are important primary producers in coastal systems and peatlands and diatoms are often the dominant microphytobenthic group in temperate regions. In November 2019, the removal of the dyke that separated the Polder Drammendorf (a drained, oligohaline peatland), from the adjacent Baltic Sea lagoon Kubitzer Bodden represented a unique chance to study the sudden rewetting of a degraded coastal peatland with brackish water and its effect on the microphytobenthos. The aims of this study were to investigate the poorly known diatom community composition of coastal peatlands, to determine the effects of flooding events on diatom composition and biomass in coastal peatlands as well as the effects of nutrient and substance release from the peatlands on the diatom communities of adjacent coastal waters. We investigated and documented the impact of this sudden rewetting on the benthic diatom communities of the peatland and the adjacent lagoon over the course of one year, applying the latest taxonomy and measuring key environmental factors. The results deepen the understanding of diatom community composition, taxonomy and ecology and explore the influence of land-sea exchange processes on the benthic diatoms of the Baltic Sea coast.

Regime shift in microalgal dynamics: Impact of water level changes on planktonic and benthic algal biomass

Whole-lake microalgal biomass surveys were carried out in Lake Balaton to investigate the seasonal, spatial, and temporal changes of benthic algae, as well as to identify the drivers of the phytobenthos. Phytobenthos was controlled mainly by light: the highest benthic algal biomass was in the shallow littoral region characterized by large grain size (sand) with good light availability but lower nutrient content in the sediment. During the investigated period, phytoplankton biomass showed a significant decrease in almost the entire lake. At the same time, the biomass of benthic algae increased significantly in the eastern areas, increasing the contribution of total lake microalgae biomass (from 20 % to 27 %). Benthic algal biomass increase can be explained by the better light supply, owing to the artificially maintained high water level which greatly mitigates water mixing. The decrease in planktonic algal biomass could be attributed to increased zooplankton grazing, which is otherwise negatively affected by mixing. As a result of the high water level, the trophic structure of the lake has been rearranged in recent decades with a shift from the planktonic life form to the benthic one while the nutrient supply has largely remained unchanged.

The metabolites of light: Untargeted metabolomic approaches bring new clues to understand light-driven acclimation of intertidal mudflat biofilm

The microphytobenthos (MPB), a microbial community of primary producers, play a key role in coastal ecosystem functioning, particularly in intertidal mudflats. These mudflats experience challenging variations of irradiance, forcing the micro-organisms to develop photoprotective mechanisms to survive and thrive in this dynamic environment. Two major adaptations to light are well described in literature: the excess of light energy dissipation through non-photochemical quenching (NPQ), and the vertical migration in the sediment. These mechanisms trigger considerable scientific interest, but the biological processes and metabolic mechanisms involved in light-driven vertical migration remain largely unknown. To our knowledge, this study investigates for the first time metabolomic responses of a migrational mudflat biofilm exposed for 30 min to a light gradient of photosynthetically active radiation (PAR) from 50 to 1000 μmol photons m-2 s-1. The untargeted metabolomic analysis allowed to identify metabolites involved in two types of responses to light irradiance levels. On the one hand, the production of SFAs and MUFAs, primarily derived from bacteria, indicates a healthy photosynthetic state of MPB under low light (LL; 50 and 100 PAR) and medium light (ML; 250 PAR) conditions. Conversely, when exposed to high light (HL; 500, 750 and 1000 PAR), the MPB experienced light-induced stress, triggering the production of alka(e)nes and fatty alcohols. The physiological and ecological roles of these compounds are poorly described in literature. This study sheds new light on the topic, as it suggests that these compounds may play a crucial and previously unexplored role in light-induced stress acclimation of migrational MPB biofilms. Since alka(e)nes are produced from FAs decarboxylation, these results thus emphasize for the first time the importance of FAs pathways in microphytobenthic biofilms acclimation to light.