More, smaller bacteria in response to ocean's warming?

Abstract

Heterotrophic bacteria play a major role in organic matter cycling in the ocean. Although the high abundances and relatively fast growth rates of coastal surface bacterioplankton make them suitable sentinels of global change, past analyses have largely overlooked this functional group. Here, time series analysis of a decade of monthly observations in temperate Atlantic coastal waters revealed strong seasonal patterns in the abundance, size and biomass of the ubiquitous flow-cytometric groups of low (LNA) and high nucleic acid (HNA) content bacteria. Over this relatively short period, we also found that bacterioplankton cells were significantly smaller, a trend that is consistent with the hypothesized temperature-driven decrease in body size. Although decadal cell shrinking was observed for both groups, it was only LNA cells that were strongly coherent, with ecological theories linking temperature, abundance and individual size on both the seasonal and interannual scale. We explain this finding because, relative to their HNA counterparts, marine LNA bacteria are less diverse, dominated by members of the SAR11 clade. Temperature manipulation experiments in 2012 confirmed a direct effect of warming on bacterial size. Concurrent with rising temperatures in spring, significant decadal trends of increasing standing stocks (3% per year) accompanied by decreasing mean cell size (−1% per year) suggest a major shift in community structure, with a larger contribution of LNA bacteria to total biomass. The increasing prevalence of these typically oligotrophic taxa may severely impact marine food webs and carbon fluxes by an overall decrease in the efficiency of the biological pump.

Major impacts of climate change on deep-sea benthic ecosystems

“…Enhanced warming of the upper ocean is predicted to enhance stratification, reducing nutrient input to the upper euphotic zone and causing a shift in phytoplankton assemblages from large, fast-sinking diatoms (with low surface area:volume [SA:V] ratios) to slow-sinking picoplankton (with high SA:V ratios; Bopp et al, 2005). This shift is likely to reduce export flux to the seafloor, as well as transfer efficiency (Buesseler et al, 2007;Morán et al, 2010Morán et al, , 2015Steinacher et al, 2010). Furthermore, freshening of Arctic regions by sea-ice meltwater and episodic input of large river runoff have been shown to reduce phytoplankton size and, by inference, export flux, a trend that has been projected to continue into the future (Li et al, 2009(Li et al, , 2013.…”

“…Ocean acidification is also predicted to reduce microbial production of nitrate from ammonium (Beman et al, 2011), which could have major consequences for oceanic primary production because a significant fraction of the nitrate used by phytoplankton is generated by nitrification at the ocean surface (Yool et al, 2007). Major consequences of such changes over regional scales will probably include (1) reductions in primary production combined with (2) shifts from diatom-dominated (low SA:V ratio) phytoplankton assemblages with high POC-export efficiencies to picoplankton communities (high SA:V ratio) characterized by low export efficiencies Morán et al, 2010;Morán et al, 2015). In addition, reductions in calcification from lowered pH in surface waters could reduce phytoplankton sinking rates through loss of ballast (Hofmann and Schellnhuber, 2009), though this effect will depend on the ratio of the fraction of ballasted vs. un-ballasted fractions of the sinking POC.…”

“…While few continental margin systems have been investigated directly in terms of the consequences of climate change, their strong gradients and regional variations have allowed significant understanding of the effects of temperature, pH, O 2 and POC flux on deep-sea benthic ecosystems. Warming of surface waters along continental margins, and increased thermal stratification and reduced nutrient supply to the surface are likely to reduce both productivity and phytoplankton type and size Morán et al, 2010Morán et al, , 2015, yielding reduced phytodetrital flux to the seabed Buesseler et al, 2008;. Model outputs suggest that bathyal areas particularly prone to declining POC flux lie in the Norwegian and Caribbean Seas, NW and NE Atlantic, the eastern tropical Pacific, and bathyal Indian and Southern Oceans, which could experience as much as a 55% decline in POC flux by 2100 (Tables 2, 3; Figures 2, 3).…”

Assessment of the bimodality in the distribution of bacterial genome sizes

“…The authors speculated that the observation of two distinct groups of bacteria, those with 'small' and those with 'large' genomes, directly reflects the balance between the opposing trends of genome expansion through gene duplication, horizontal gene transfer and replication, and genome contraction caused by genome streamlining and degradation (Koonin and Wolf, 2008). The observed bimodality in the database was the first empirical evidence to show the two forces at work in bacterial genomes, and the bimodalilty in the distribution has since attracted numerous citations in both peerreviewed articles (Lane, 2011;Mock and Kirkham, 2012;Giovannoni et al, 2014;Morán et al, 2015) and textbooks (Bergman, 2011;Koonin, 2011;Kirchman, 2012;Saitou, 2014;Seshasayee, 2015).…”

“…Some interesting observations with a potential link to the nature of distribution have been emerging in recent years. For example, (i) the bimodality in flow cytometric analysis of bacterial DNA content has been implicated with the bimodal genome size distribution (Schattenhofer et al, 2011;Morán et al, 2015); (ii) there may be other factors such as physical cell space constraints having a role in genome size selection (Kempes et al, 2016) and (iii) perhaps most intriguingly, numerous studies Assessment of the bimodality in the distribution of bacterial genome sizes HS Gweon et al from metagenomics are indicating that species with small genomes are more common than previously thought (Giovannoni et al, 2014;Morán et al, 2015). With the rise of single-cell genomics and improved bioinformatic assembly methods coupled with the continual reduction in genome sequencing, we are currently witnessing rapid growth in the number of sequenced genomes.…”