Gaube, P. | bioRxiv Science

bioRxiv2022

Characterizing phytoplankton communities in the absence of resource-based competition

AO_SCPLOWBSTRACTC_SCPLOWUnder most natural marine conditions, phytoplankton cells suspended in the water column are too distantly spaced for direct competition for resources to be a routine occurrence. Accordingly, resource-based competitive exclusion should be rare. In contrast, contemporary ecosystem models typically predict an exclusion of larger phytoplankton size classes under low-nutrient conditions, an outcome interpreted as reflecting the competitive advantage of small cells having much higher nutrient affinities than larger cells. Here, we develop mechanistically-focused expressions for steady-state, nutrient-limited phytoplankton growth that are consistent with the discrete, distantly-spaced cells of natural populations. These expressions are then encompassed in an ecosystem model that sustains diversity across all size classes over the full range in nutrient concentrations observed in the ocean. In other words, our model does not exhibit resource-based competitive exclusion between size classes. We show that the basis for species exclusions in earlier models is not a reflection of size-dependent nutrient affinities, but rather a consequence of inappropriate descriptions of non-grazing phytoplankton mortality.

ecology↗

bioRxiv2020

Changes amid constancy: flower and leaf microbiomes along land use gradients and between bioregions

Microbial communities inhabiting above-ground parts of plants affect their hosts development, fitness and function. Although studies on plant-associated microbes are of growing interest, environmental drivers of flower microbiomes in particular are poorly characterized. In this study, we investigated flower and leaf epiphytic bacterial microbiomes of Ranunculus acris and Trifolium pratense using metabarcoding of 16S ribosomal DNA in three German bioregions and along land-use intensity gradients. Our data suggests that the structures of bacterial communities clearly differed between plant species and tissue types. Also, floral bacterial communities of R. acris showed higher variability in comparison to T. pratense. Bacteria usually associated with pollinators were found solely in flower samples, while such usually associated with the rhizosphere were only present in high abundances on leaves. We identified Pseudomonadaceae, Enterobacteriaceae and Sphingomonadaceae as the most abundant taxa on flowers, while Sphingomonadaceae, Methylobacteriaceae and Cytophagaceae dominated bacterial communities on leaves. We found strong bacterial turnover already for short geographic distances, which however did not increase with the long distances between bioregions. High land use intensity caused phylogenetically less diverse and more homogenous bacterial communities. This was associated with a loss of rare bacterial families. Intensification of mowing and fertilization affected almost all plant associated bacterial communities, while grazing had only minor effects on bacterial structures of T. pratense flowers. However, dominant taxa were mostly resilient to mowing, grazing and fertilization. Despite that, we identified indicator taxa for regularly disturbed environments in flower microbiomes.

ecology↗