Imprint of past and present environmental conditions on microbiology and biogeochemistry of coastal Quaternary sediments

To date, North Sea tidal-flat sediments have been intensively studied down to a depth of 5 m below seafloor (mbsf). However, little is known about the biogeochemistry, microbial abundance, and activity of sulfate reducers as well as methanogens in deeper layers. In this study, two 20 m-long cores were retrieved from the tidal-flat area of Spiekeroog Island, NW Germany. The drill sites were selected with a close distance of 900 m allowing to compare two depositional settings: first, a paleo-channel filled with Holocene sediments and second, a mainly Pleistocene sedimentary succession. Analyzing these cores, we wanted to test to which degree the paleo-environmental imprint is superimposed by present processes. <br><br> In general, the numbers of bacterial 16S rRNA genes are one to two orders of magnitude higher than those of <i>Archaea</i>. The abundances of key genes for sulfate reduction and methanogenesis (<i>dsr</i>A and <i>mcr</i>A) correspond to the sulfate and methane profiles. A co-variance of these key genes at sulfate-methane interfaces and enhanced ex situ AOM rates suggest that anaerobic oxidation of methane may occur in these layers. Microbial and biogeochemical profiles are vertically stretched relative to 5 m-deep cores from shallower sediments in the same study area, but still appear compressed compared to deep sea sediments. Our interdisciplinary analysis shows that the microbial abundances and metabolic rates are elevated in the Holocene compared to Pleistocene sediments. However, this is mainly due to present environmental conditions such as pore water flow and organic matter availability. The paleo-environmental imprint is still visible but superimposed by these processes

Enrichment of intracellular sulphur cycle –associated bacteria in intertidal benthic foraminifera revealed by 16S and aprA gene analysis

Benthic foraminifera are known to play an important role in marine carbon and nitrogen cycles. Here, we report an enrichment of sulphur cycle -associated bacteria inside intertidal benthic foraminifera (Ammonia sp. (T6), Haynesina sp. (S16) and Elphidium sp. (S5)), using a meta barcoding approach targeting the 16S rRNA and aprA -genes. The most abundant intracellular bacterial groups included the genus Sulfurovum and the order Desulfobacterales. The bacterial 16S OTUs are likely to originate from the sediment bacterial communities, as the taxa found inside the foraminifera were also present in the sediment. The fact that 16S rRNA and aprA -gene derived intracellular bacterial OTUs were species-specific and significantly different from the ambient sediment community implies that bacterivory is an unlikely scenario, as benthic foraminifera are known to digest bacteria only randomly. Furthermore, these foraminiferal species are known to prefer other food sources than bacteria. The detection of sulphur-cycle related bacterial genes in this study suggests a putative role for these bacteria in the metabolism of the foraminiferal host. Future investigation into environmental conditions under which transcription of S-cycle genes are activated would enable assessment of their role and the potential foraminiferal/endobiont contribution to the sulphur-cycle.Peer reviewe

Aggregation of Diatoms during a spring bloom in the North Sea as a function of temperature and specific effects of Thalassiosira Rotula

Phytoplankton spring blooms dominated by diatoms and often by Thalassiosira rotula and/or Skeletonema costatum are typical features of coastal seas in the temperate zone. They are usually terminated by nutrient limitation, aggregation and sedimentation events. In the course of climate change increasing temperatures in spring may affect the development of diatom blooms and in particular aggregation, a process in which heterotrophic bacteria play an active role. As heterotrophic microbial processes are favoured more relative to autotrophic processes by increasing temperature, we hypothesize that aggregation will be enhanced with increasing temperature and in particular towards the end of the spring bloom with increasing senescence of the diatoms. To test this hypothesis we examined aggregation in the course of a spring bloom in a mesocosm experiment in the southern North Sea at the in situ temperature of 6°C and at 11°C. Water was transferred to a mesocosm of 720 liter and incubated at the in situ temperature and a 12:12 h light-dark cycle for 3 weeks. Aggregation was assessed in the pre-bloom, bloom and late bloom phase by measuring the formation of aggregates in subsamples incubated for 48 h in rolling tanks at 6°C and 11°C in a 12:12 h light-dark cycle. Aggregation was determined in subsamples withdrawn directly from the mesocosm and in subsamples to which axenic T. rotula cells were added in a concentration of ca. 5000 cells ml-1. Transparent exopolymer particles (TEP), dissolved organic carbon and dissolved carbohydrate as well as the bacterial abundance and community composition by DGGE and FISH were analysed during the bloom and the aggregation experiments. The addition of T. rotula enhanced aggregation relative to the untreated samples. For the latter, aggregation was rather independent of temperature whereas aggregation was enhanced at the higher temperature when T. rotula was added and in particular towards the end of the bloom. The results suggest that increasing temperature enhances aggregation of phytoplankton spring blooms dominated by T. rotula. This enhanced aggregation may have implications for the trophic transfer of diatom blooms in the food web by reducing the available amount of diatom-derived organic matter to zooplankton

Lack of 13C-label incorporation suggests low turnover rates of thaumarchaeal intact polar tetraether lipids in sediments from the Iceland shelf

Thaumarchaeota are amongst the most abundant microorganisms in aquatic environments, however, their metabolism in marine sediments is still debated. Labeling studies in marine sediments have previously been undertaken, but focused on complex organic carbon substrates which Thaumarchaeota have not yet been shown to take up. In this study, we investigated the activity of Thaumarchaeota in sediments by supplying different 13C-labeled substrates which have previously been shown to be incorporated into archaeal cells in water incubations and/or enrichment cultures.We determined the incorporation of 13C-label from bicarbonate, pyruvate, glucose and amino acids into thaumarchaeal intact polar lipid-glycerol dibiphytanyl glycerol tetraethers (IPL-GDGTs) during 4–6 day incubations of marine sediment cores from three sites on the Iceland shelf. Thaumarchaealintact polar lipids, in particular crenarchaeol, were detected at all stations and concentrations remained constant or decreased slightly upon incubation. No 13C incorporation in any IPL-GDGT was observed at stations 2 (clay-rich sediment) and 3 (organic-rich sediment). In bacterial/eukaryoticIPL-derived fatty acids at station 3, contrastingly, a large uptake of 13C label (up to + 80‰ ) was found. 13C was also respired during the experiment as shown by a substantial increase in the 13C content of the dissolved inorganic carbon. In IPL-GDGTs recovered from the sandy sediments at station 1, however, some enrichment in 13C (1–4‰ ) was detected after incubation with icarbonate and pyruvate.The low incorporation rates suggest a low activity of Thaumarchaeota in marine sediments and/or a low turnover rate of thaumarchaeal IPL-GDGTs due to their low degradationrates. Cell numbers and activity of sedimentary Thaumarchaeota based on IPL-GDGT measurements may thus have previously been overestimated