| In the Anthropocene, in which we now live, climate change is impacting most life on Earth. Microorganisms support the existence of all higher trophic life forms. To understand how humans and other life forms on Earth (including those we are yet to discover) can withstand anthropogenic climate change, it is vital to incorporate knowledge of the microbial 'unseen majority'. We must learn not just how microorganisms affect climate change (including production and consumption of greenhouse gases) but also how they will be affected by climate change and other human activities. This Consensus Statement documents the central role and global importance of microorganisms in climate change biology. It also puts humanity on notice that the impact of climate change will depend heavily on responses of microorganisms, which are essential for achieving an environmentally sustainable future.

Obtaining information on the genetic capabilities and phylogenetic affinities of individual prokaryotic cells within natural communities is a high priority in the fields of microbial ecology, microbial biogeochemistry, and applied microbiology, among others. A method for prokaryotic in situ PCR (PI-PCR), a technique which will allow single cells within complex mixtures to be identified and characterized genetically, is presented here. The method involves amplification of specific nucleic acid sequences inside intact prokaryotic cells followed by color or fluorescence detection of the localized PCR product via bright-field or epifluorescence microscopy. Prokaryotic DNA and mRNA were both used successfully as targets for PI-PCR. We demonstrate the use of PI-PCR to identify nahA-positive cells in mixtures of bacterial isolates and in model marine bacterial communities.

1. We examined standing-senescing, standing-dead and recently fallen leaf blades of Carex walteriana in fens of the Okefenokee Swamp to determine the nature of the microbial decomposers in the early stages of decomposition, measuring both standing crops and productivities ([^H]leucine->protein method for bacteria, [^^C]acetate-^ergosterol for fungi). 2. Fungal standing crops (ergosterol) became detectable at the mid-senescence stage (leaves about half yellow-brown) and rose to 14-31 mg living-fungal C g"^ organic mass of the decaying system; bacterial standing crops (direct nucroscopy) were =s 0.2 mgC g"' imtil the fallen-leaf stage, when they rose to as high as 0.9 mgC g"^.3. Potential microbial specific growth rates were similar between fungi and bacteria, at about 0.03-0.06 day"', but potential production of fungal mass was 115-512 )j.gC g"ô rganic mass day-\ compared with 0-22 ^gC g'^ day-^ for bacteria. Rates of fungal production were about 6-foId lower on average than previously found for a saltmarsh grass, perhaps because much lower phosphorus concentrations in the freshwater fen limit fungal activity. 4. There was little change in lignocellulose (LC) percentage of decaying leaves, although net loss of organic mass at the fallen, broken stage was estimated to be 59%, suggesting that LC was lost at rates proportional to those for total organics during decay. Monomers of fungal-wall polymers (glucosamine and mannose) accumulated 2-to 4-fold during leaf decay. This may indicate that an increase found for proximate (aciddetergent) lignin could be at least partially due to accumulation of refractory fungal-wall material, including melanin. 5. A common sequence in decaying aquatic grasses is suggested: principally fungal alteration of LC during standing decay, followed by a trend toward bacterial decomposition of the LC after leaves fall and break into particles.

The trophic linkage between marine bacteria and phytoplankton in the surface ocean is a key step in the global carbon cycle, with almost half of marine primary production transformed by heterotrophic bacterioplankton within hours to weeks of fixation. Early studies conceptualized this link as the passive addition and removal of organic compounds from a shared seawater reservoir. Here, we analysed transcript and intracellular metabolite patterns in a two-member model system and found that the presence of a heterotrophic bacterium induced a potential recognition cascade in a marine phytoplankton species that parallels better-understood vascular plant response systems. Bacterium Ruegeria pomeroyi DSS-3 triggered differential expression of >80 genes in diatom Thalassiosira pseudonana CCMP1335 that are homologs to those used by plants to recognize external stimuli, including proteins putatively involved in leucine-rich repeat recognition activity, second messenger production and protein kinase cascades. Co-cultured diatoms also downregulated lipid biosynthesis genes and upregulated chitin metabolism genes. From differential expression of bacterial transporter systems, we hypothesize that nine diatom metabolites supported the majority of bacterial growth, among them sulfonates, sugar derivatives and organic nitrogen compounds. Similar recognition responses and metabolic linkages as observed in this model system may influence carbon transformations by ocean plankton.

ABSTRACT-The photochemical conversion of the nitrogen fraction of dissolved humic substances into more biologically available compounds was studied in 2 estuarine sites in the southeastern U.S. Marine humic substances were isolated using an XAD-8 resin and used in bacterial bioassays and chemical studies. The bioassays demonstrated that humic substances irradiated with natural sunlight supported enhanced bacterial growth, measured as cell accumulation and protein production, due to increased availability of both the carbon and nitrogen components. Chemical analyses demonstrated the photoproduction of ammonium and dissolved primary amines from the coastal hurnic substances. The total biologically available nitrogen [ammonium, &ssolved primary amines, and other unidentified compounds) formed during a day-long irradiation at natural solar radiat~on levels accounted for about 6 % of the original nitrogen associated with the humic substances. Photochemical modification of marine humic substances may provide a source of labile nitrogen to estuanne and coastal ecosystems that has not previously been considered.