The role of phosphorus, magnesium and potassium availability in soil fungal exploration of mineral nutrient sources in Norway spruce forests.

We investigated fungal growth and community composition in buried meshbags, amended with apatite, biotite or hornblende, in Norway spruce (Picea abies) forests of varying nutrient status. Norway spruce needles and soil collected from forests overlying serpentinite had low levels of potassium and phosphorus, those from granite had low levels of magnesium, whereas those from amphibolite had comparably high levels of these nutrients. We assayed the fungal colonization of meshbags by measuring ergosterol content and fungal community with 454 sequencing of the internal transcribed spacer region. In addition, we measured fine root density. Fungal biomass was increased by apatite amendment across all plots and particularly on the K- and P-deficient serpentinite plots, whereas hornblende and biotite had no effect on fungal biomass on any plots. Fungal community (total fungal and ectomycorrhizal) composition was affected strongly by sampling location and soil depth, whereas mineral amendments had no effect on community composition. Fine root biomass was significantly correlated with fungal biomass. Ectomycorrhizal communities may respond to increased host-tree phosphorus demand by increased colonization of phosphorus-containing minerals, but this does not appear to translate to a shift in ectomycorrhizal community composition. This growth response to nutrient demand does not appear to exist for potassium or magnesium limitation.

Long-term effect of apatite on ectomycorrhizal growth and community structure.

Ectomycorrhizal (ECM) fungi are efficient at taking up phosphorus (P) from mineral sources, such as apatite, which are not easily available to the host trees. Since ECM fungal species differ in P uptake rates, it can be expected that the composition of the ECM fungal community will change upon exposure to apatite, provided that the P transfer is rewarded by more carbon being transferred to the fungal symbiont. Control and apatite-amended mesh bags were buried in pairs in the humus layer of a P-poor Norway spruce forest. The ECM fungal community that colonized these bags was analyzed by DNA extraction, PCR amplification of the internal transcribed spacer (ITS) region, cloning, and random sequencing. Fungal biomass was estimated by ergosterol analysis. No change in the ECM fungal community structure was seen after 5 years of apatite exposure, although the fungal biomass increased threefold upon apatite amendment. Our results indicate that host trees enhance carbon allocation to ECM fungi colonizing P sources in P-poor forests but the lack of change in the composition of the ECM fungal community suggests that P transfer rates were similar among the species. Alternatively, higher P transfer among certain species was not rewarded with higher carbon transfer from the host.

Response of Microbial Communities to Changing Climate Conditions During Summer Cyanobacterial Blooms in the Baltic Sea.

Frequencies and biomass of Baltic Sea cyanobacterial blooms are expected to be higher in future climate conditions, but also of longer duration as a result of increased sea surface temperature. Concurrently, climate predictions indicate a reduced salinity in the Baltic Sea. These climate-driven changes are expected to alter not solely the phytoplankton community but also the role of microbial communities for nutrient remineralization. Here, we present the response of summer plankton communities (filamentous cyanobacteria, picocyanobacteria, and heterotrophic bacteria) to the interplay of increasing temperature (from 16 to 18°C and 20°C) and reduced salinity (from salinity 6.9 to 5.9) in the Baltic Proper (NW Gotland Sea) using a microcosm approach. Warmer temperatures led to an earlier peak of cyanobacterial biomass, while yields were reduced. These conditions caused a decrease of nitrogen-fixers (Dolichospermum sp.) biomass, while non nitrogen-fixers (Pseudanabaena sp.) increased. Salinity reduction did not affect cyanobacterial growth nor community composition. Among heterotrophic bacteria, Actinobacteria showed preference for high temperature, while Gammaproteobacteria thrived at in situ temperature. Heterotrophic bacteria community changed drastically at lower salinity and resembled communities at high temperature. Picocyanobacteria and heterotrophic bacterial biomass had a pronounced increase associated with the decay of filamentous cyanobacteria. This suggests that shifts in community composition of heterotrophic bacteria are influenced both directly by abiotic factors (temperature and salinity) and potentially indirectly by cyanobacteria. Our findings suggest that at warmer temperature, lower yield of photosynthetic cyanobacteria combined with lower proportion of nitrogen-fixers in the community could result in lower carbon export to the marine food web with consequences for the decomposer community of heterotrophic bacteria.