Temperature and nutrient stoichiometry interactively modulate organic matter cycling in a pelagic algal–bacterial community

Abstract

A microcosm experiment was conducted to investigate the interactive effects of rising sea-surface temperature and altered nutrient stoichiometry on the biogeochemical cycling of organic matter in a pelagic algal-bacterial assemblage. Natural seawater, containing a mixed bacterial community, was inoculated with an axenic culture of the bloom-forming diatom species Skeletonema costatum. A factorial combination of three temperatures, simulating weak to strong warming as projected for the end of the 21st century, and either nitrogen (N)-replete or -deficient growth conditions were applied. Depending on the type of nutrient limitation, the mixed algal-bacterial communities displayed pronounced differences in the accumulation and microbial utilization of organic matter in response to warming. Under N-deficient conditions, the build-up of organic matter occurred, irrespective of temperature, dominantly in the particulate pool, and only small amounts of dissolved material accumulated. The subsequent bacterial consumption of organic matter was low, as indicated by measurements of bacterial secondary production and extracellular enzyme activities, and remained also largely unaffected by an increase in temperature from 4uC up to 12uC. Contrastingly, warming resulted in a distinct temperature-dependent increase in the accumulation of dissolved organic carbon compounds under N-replete growth conditions. Moreover, rising temperature notably stimulated the bacterial activity, indicating an enhanced flow of organic matter through the microbial loop. These findings suggest that there will be strong shifts in the biogeochemical cycling of organic matter in the upper ocean in response to increased temperature and nutrient loading that will affect pelagic foodweb structures and the biological sequestration of organic matter.

Alterations in microbial community composition with increasing <i>f</i>CO<sub>2</sub>: a mesocosm study in the eastern Baltic Sea

“…Model simulations reveal that whilst near-cell CO 2 and pH conditions are close to those of the bulk water for cells < 5 µm in diameter, they diverge as cell diameters increase (Flynn et al, 2012). This is due to the size-dependent thickness of the diffusive boundary layer, which determines the diffusional transport across the boundary layer and to the cell surface (Wolf-Gladrow and Riebesell, 1997;Flynn et al, 2012). It is suggested that larger cells may be more able to cope with f CO 2 variability as their carbon acquisition is more geared towards handling low CO 2 concentrations in their diffusive boundary layer, e.g.…”

“…Specifically, a negative relationship between the abundances of HNA prokaryotes and f CO 2 was detected and corresponded to reduced bacterial production and respiration at higher f CO 2 (Hornick et al, 2017;Spilling et al, 2016). Although CO 2 enrichment may not directly affect bacterial growth, a co-occurring global rise in temperature can increase enzyme activities, affecting bacterial production and respiration rates (Piontek et al, 2009;Wohlers et al, 2009;Wohlers-Zöllner et al, 2011). The enhanced bacterial remineralization of organic matter may stimulate autotrophic production by the small-sized phytoplankton Riebesell and Tortell, 2011;Engel et al, 2013), intensifying the selection of small cell sizes.…”

“…It is suggested that larger cells may be more able to cope with f CO 2 variability as their carbon acquisition is more geared towards handling low CO 2 concentrations in their diffusive boundary layer, e.g. by means of active carbon acquisition and bicarbonate utilization (Wolf-Gladrow and Riebesell, 1997;Flynn et al, 2012). Moreover, as the Baltic Sea experiences particularly large seasonal fluctuations in pH and f CO 2 (Jansson et al, 2013) due to the low buffering capacity of the waters, phytoplankton here are expected to have a higher degree of physiological plasticity.…”

Effects of rising temperature on pelagic biogeochemistry in mesocosm systems: a comparative analysis of the AQUASHIFT Kiel experiments

“…This decrease in the average ratio of POC to POP may, in fact, rather suggest a temperature-induced shift in the turnover dynamics of organic phosphorus compounds. Thus, results from an AQUASHIFT-related microcosm experiment have shown that experimental warming can substantially stimulate phosphorus cycling through the activity of the organic phosphorus-cleaving enzyme alkaline phosphatase (APA; Wohlers-Zöllner et al 2011), hence enabling a faster replenishment of the POP pool at elevated temperature and reducing the C:P ratio of POM.…”

The biological pump in a high CO<sub>2 world

“…Several mesocosm simulation experiments have examined the potential impacts of rising temperatures alone (Sommer & Lengfellner 2008, Lassen et al 2010, Taucher et al 2012, or in combination with perturbations of pCO 2 (Kim et al 2011), light (Lewandowska & Sommer 2010), nutrients (Wohlers-Zöllner et al 2011) or grazing (Sommer & Lewandowska 2011), on phytoplankton blooms. All of these studies observed an early onset and peak of the spring phytoplankton blooms, which has also been observed in situ (Edwards & Richardson 2004).…”