Eelgrass (Zostera marina) is a marine foundation species essential for coastal ecosystem services around the northern hemisphere. Like all macroscopic organisms, it possesses a microbiome (here defined as an associated prokaryotic community) which may play critical roles in modulating the interaction of eelgrass with its environment. For example, its leaf surface microbiome could inhibit or attract eukaryotic epibionts which may overgrow the eelgrass leading to reduced primary productivity and subsequent eelgrass meadow decline. We used amplicon sequencing of the 16S and 18S rRNA genes of prokaryotes and eukaryotes to assess the leaf surface microbiome (prokaryotes) as well as eukaryotic epibionts in- and outside lagoons on the German Baltic Sea coast. Prokaryote microbiomes varied substantially both between sites inside lagoons and between open coastal and lagoon sites. Water depth, leaf area and biofilm chlorophyll a concentration explained a large amount of variation in both prokaryotic and eukaryotic community composition. The prokaryotic microbiome and eukaryotic epibiont communities were highly correlated, and network analysis revealed disproportionate co-occurrence between a limited number of eukaryotic taxa and several bacterial taxa. This suggests that eelgrass leaf surfaces are home to a mosaic of microbiomes of several epibiotic eukaryotes, in addition to the microbiome of the eelgrass itself. Our findings thereby underline that eukaryotic diversity should be taken into account in order to explain prokaryotic microbiome assembly and dynamics in aquatic environments.
bleasurements of microblal biomass and actlvity were carried out at 6 representative locatlons along a gradient of eutrophlcatlon In the Nordrugensche Bodden (southern Baltic Sea Germany) h4easurements in the water column revealed that turbid~ty, seston content and concentratlons of chlorophyll a and inorganic nutnents (ammonla nitnte, nitrate phosphate) increased from the outer to the inner parts of the Bodden whereas salin~ty decreased Investigations of sedlments conf~rmed this gradlent of eutrophlcatlon Whereas in the outer parts of the Bodden sandy sediments prevailed sandy mud and muddy s e d~m e n t s dominated towards the inner parts Generally, organic carbon and nltrogen, concentratlons of chlorophyll a and phospholip~ds (indicator of microbial biomass) oxygen consumption and hydrolytlc enzyme actlv~tles Increased wlth Increasing level of eutrophlcatlon At the relatively unpolluted locatlon in the outer parts of the Bodden proteolytic enzymes dominated at the sedlment surface In subsurface horlzons carbohydrate-decomposing enzymes galned more importance At the heavily polluted locatlons In the inner parts of the Bodden proteolytlc enzymes were even more Important Wlth increasing sediment depth enzyme actlvlties were greatly reduced h o w ever, and shlfts In the spectrum of hydrolytlc enzymes were less pronounced Among the blologlcal and chemlcal parameters, charactenstlc patterns of lnterrelatlonships became obvious whlch led to the conclus~on that microblal blomass and enzyme actlvlties in sediments of the outer and central parts of the Bodden are limited by organlc carbon The organic-nch sediments of the lnner parts of the Bodden however did not support furthe] increases In microbial biomass and decompos~tlon act~vltles Enzyme actlvltles are discussed In relatlon to the composition and degradablhty of substrates The enzymatic decomposltlon potential was measured by means of fluorogenlc model substrates Methodological investigations showed that the methylumbelllferyl substrates used reacted specifically enough to ]ustlfy the11 use In ecological studies Sedlments can be stored refngerated for over 2 mo without changes in the spectrum of hydrolytic enzymes
This study investigated the microbial community structure and functioning in relation to properties of dominant sediment types (sand, mud) of hypertrophic inner coastal waters. A transect in the Großer Jasmunder Bodden (inner part of the Nordrügensche Bodden, Southern Baltic Sea), with 6 stations from the shore to the central part was sampled in June 1999. In light-exposed nearshore sandy sediments, microphytobenthos was the driving source for benthic metabolism. Positive relationships between photoautotrophic biomass and available dissolved organic carbon (measured with a microbial biosensor) suggested that the pool of available dissolved organic nutrients was supplied by photosynthesis products of the benthic microalgae. Organic matter produced by these algae was rapidly recycled and led to low standing stocks of organic matter. In contrast, muddy organic-rich accumulation sites in greater water depths with limited or no light were heterotrophically dominated. These sediments retained large amounts of deposited organic matter, especially organic carbon and phosphorus. Intensive microbial degradation of deposited organic matter resulted in enhanced enzymatic activity, and in enhanced concentrations of ammonium and phosphate in porewaters. Our findings revealed that sedimentary bulk parameters such as concentration and degradability of particulate organic carbon as well as microbial biomass and activity depend on mud content in pure sandy sediments (with a mud content <10%), while they remain unchanged in sediments with higher mud content. Carbon-normalized enzymatic activity rates revealed that substrate turnover rates in muddy sediments of hypertrophic waters were relatively low compared to sediments with comparable total organic carbon concentrations in eutrophic waters. These findings imply that the availability of degradable organic matter (accounting only for <1% of total organic matter) and probably inorganic nutrients might be of much greater importance in regulating microbial biomass and activity than total organic carbon.
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