Large Plankton Enhance Heterotrophy Under Experimental Warming in a Temperate Coastal Ecosystem

Abstract

Microbes are key players in oceanic carbon fluxes. Temperate ecosystems are seasonally variable and thus suitable for testing the effect of warming on microbial carbon fluxes at contrasting oceanographic conditions. In four experiments conducted in February, April, August and October 2013 in coastal NE Atlantic waters, we monitored microbial plankton stocks and daily rates of primary production, bacterial heterotrophic production and respiration at in situ temperature and at 2 and 4°C over ambient values during 4-day incubations.Ambient total primary production (TPP) exceeded total community respiration (< 200 lm, TR) in winter and fall but not in spring and summer. The bacterial contribution to ecosystem carbon fluxes was low, with bacterial production representing on average 6.9 ± 3.2% of TPP and bacterial respiration (between 0.8 and 0.2 lm) contributing on average 35 ± 7% to TR. Warming did not result in a uniform increase in the variables considered, and most significant effects were found only for the 4°C increase. In the summer and fall experiments, under warm and nutrient-deficient conditions, the net TPP/TR ratio decreased by 39 and 34% in the 4°C treatment, mainly due to the increase in respiration of large organisms rather than bacteria. Our results indicate that the interaction of temperature and substrate availability in determining microbial carbon fluxes has a strong seasonal component in temperate planktonic ecosystems, with temperature having a more pronounced effect and generating a shift toward net heterotrophy under more oligotrophic conditions as found in summer and early fall.

Low Abundances but High Growth Rates of Coastal Heterotrophic Bacteria in the Red Sea

“…The fact that bacterial respiration was negatively correlated with in situ temperature is at odds with other studies (López-Urrutia, 2007; Robinson, 2008). However, other seasonal studies (e.g., Lemée et al, 2002; Alonso-Sáez et al, 2008) have failed to find significant correlations between both variables, while bacterial respiration was also negatively correlated with in situ temperature in the NE Atlantic (Huete-Stauffer et al, 2017), indicating that other factors could regulate respiration from a seasonal perspective. Seasonal temperature variability also covaries with bacterial community composition, DOM lability, inorganic nutrient concentrations or incident UV light, all possibly affecting bacterial metabolism.…”

“…Since this increase in Synechococcus biomass corresponded to particulate primary production (PPP), we subsequently estimated the dissolved primary production (DPP) potentially contributing to DOC fluxes. For that, we assumed a very conservative value of 50% percent extracellular release (PER) [PER = DPP/(DPP + PPP)], much higher than the expected PER for exponentially growing phytoplankton cells (e.g., Morán et al, 2002; Huete-Stauffer et al, 2017). Even with this unrealistically high DPP values, this input would only represent on average 4.8% of the changes in DOC during the exponential phase of bacterial growth.…”

Temperature sensitivities of microbial plankton net growth rates are seasonally coherent and linked to nutrient availability

“…The finding by Huete‐Stauffer and colleagues () that only in the late winter–spring period did bacteria specific growth rates approach the theoretical value of 0.65 eV was explained by elevated organic matter concentrations made available by phytoplankton blooms in that period (Bode et al, ), while the low summer E values were attributed to strong substrate limitation (López‐Urrutia and Morán, ; Sinsabaugh and Shah, ), in line with previous studies carried out at the study site using different approaches (Morán et al, ; Calvo‐Díaz et al, ). In the experiments reported here, extant DOM could have been supplemented by dissolved primary production (8%–47% of the total in four experiments carried out in 2013, Huete‐Stauffer et al, ), microzooplankton grazing (Nagata, ) and/or algal mortality (Agustí and Duarte, ).…”

Warming the phycosphere: Differential effect of temperature on the use of diatom‐derived carbon by two copiotrophic bacterial taxa

“…However, our results are consistent with a previous study where picophytoplankton dominated the algal communities at the same coastal site (Arandia‐Gorostidi et al ., 2017b) and other studies showing a lack of increase in particulate primary production with warming (Huete‐Stauffer et al ., 2017). On the contrary, warming has been related with a relative increase in the release of newly fixed C as dissolved compounds (Morán et al ., 2006; Engel et al ., 2011; Yvon‐Durocher et al ., 2012; Huete‐Stauffer et al ., 2017), which may explain the clear effect of temperature on the C uptake activity of microphytoplankton‐associated bacteria. Altogether, warming may result in important imbalances of the carbon budget in coastal waters, ultimately transferring more carbon to heterotrophic bacteria in relation to autotrophic cells.…”