Rachel E. Sipler

Phytoplankton–bacterial interactions mediate micronutrient colimitation at the coastal Antarctic sea ice edge

Significance The coastal Southern Ocean is a critical climate system component and home to high rates of photosynthesis. Here we show that cobalamin (vitamin B12) and iron availability can simultaneously limit phytoplankton growth in late Austral summer coastal Antarctic sea ice edge communities. Unlike other growth-limiting nutrients, the sole cobalamin source is production by bacteria and archaea. By identifying microbial gene expression changes in response to altered micronutrient availability, we describe the molecular underpinnings of limitation by both cobalamin and iron and offer evidence that this limitation is driven by multiple delicately balanced phytoplankton–bacterial interactions. These results support a growing body of research suggesting that relationships between bacteria and phytoplankton are key to understanding controls on marine primary productivity. Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton–bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore- related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton–bacterial interactions in both positive and negative feedback loops.

Chapter 4 – Dynamics of Dissolved Organic Nitrogen

Dissolved organic nitrogen (DON) is that subset of the dissolved organic carbon (DOC) pool that also contains N. This chapter reviews four major areas of DON research. First, DON concentrations and distributions in aquatic environments are reviewed with respect to the methods used to analyze DON concentrations, global distributions, comparison of DON concentration across a range of aquatic systems and seasonal variations. Second, the composition of the characterizable and uncharacterized DON is reviewed with respect to pools of specific DON forms and the methods used to study and chemical characteristics of the fraction of the DON pool that is not defined. Third, sources of DON to the water column are reviewed with respect to autochthonous sources, allochthonous sources, methods used to measure rates of DON release and release rates of bulk DON and individual DON forms in the literature. The final section reviews sinks for DON with respect to DON bioavailability, methods used to estimate DON uptake rates, mechanisms that contribute to DON bioavailability, and DON uptake rates published in the literature. Recommendations for future research are presented for each research area. This chapter focuses on work published largely after 2001 and topics not included in the earlier review (Bronk, D.A., 2002. Dynamics of DON. In: Hansell, D.A., Carlson, C.A. (eds.) Biogeochemistry of Marine Dissolved Organic Matter. Academic Press, San Diego).

Effect of temperature on rates of ammonium uptake and nitrification in the western coastal Arctic during winter, spring, and summer

Biogeochemical rate processes in the Arctic are not currently well constrained, and there is very limited information on how rates may change as the region warms. Here we present data on the sensitivity of ammonium (NH4+) uptake and nitrification rates to short-term warming. Samples were collected from the Chukchi Sea off the coast of Barrow, Alaska, during winter, spring, and summer and incubated for 24 h in the dark with additions of 15NH4+ at −1.5, 6, 13, and 20°C. Rates of NH4+ uptake and nitrification were measured in conjunction with bacterial production. In all seasons, NH4+ uptake rates were highest at temperatures similar to current summertime conditions but dropped off with increased warming, indicative of psychrophilic (i.e., cold-loving) microbial communities. In contrast, nitrification rates were less sensitive to temperature and were higher in winter and spring compared to summer. These findings suggest that as the Arctic coastal ecosystem continues to warm, NH4+ assimilation may become increasingly important, relative to nitrification, although the magnitude of NH4+ assimilation would be still be lower than nitrification.

Microbial Community Response to Terrestrially Derived Dissolved Organic Matter in the Coastal Arctic

Warming at nearly twice the global rate, higher than average air temperatures are the new ‘normal’ for Arctic ecosystems. This rise in temperature has triggered hydrological and geochemical changes that increasingly release carbon-rich water into the coastal ocean via increased riverine discharge, coastal erosion, and the thawing of the semi-permanent permafrost ubiquitous in the region. To determine the biogeochemical impacts of terrestrially derived dissolved organic matter (tDOM) on marine ecosystems we compared the nutrient stocks and bacterial communities present under ice-covered and ice-free conditions, assessed the lability of Arctic tDOM to coastal microbial communities from the Chukchi Sea, and identified bacterial taxa that respond to rapid increases in tDOM. Once thought to be predominantly refractory, we found that ∼7% of dissolved organic carbon and ∼38% of dissolved organic nitrogen from tDOM was bioavailable to receiving marine microbial communities on short 4 – 6 day time scales. The addition of tDOM shifted bacterial community structure toward more copiotrophic taxa and away from more oligotrophic taxa. Although no single order was found to respond universally (positively or negatively) to the tDOM addition, this study identified 20 indicator species as possible sentinels for increased tDOM. These data suggest the true ecological impact of tDOM will be widespread across many bacterial taxa and that shifts in coastal microbial community composition should be anticipated.

Interactive effects of temperature, CO2 and nitrogen source on a coastal California diatom assemblage

ENVIRONMENTAL GENOMICS, J. CRAIG VENTER INSTITUTE, LA JOLLA, CA , USA, INTEGRATIVE OCEANOGRAPHY DIVISION, SCRIPPS INSTITUTION OF OCEANOGRAPHY, UC SAN DIEGO, LA JOLLA, CA , USA, STATE KEY LABORATORY OF MARINE ENVIRONMENTAL SCIENCE, XIAMEN UNIVERSITY, XIAMEN, FUJIAN , PR CHINA AND COLLEGE OF MARINE AND ENVIRONMENTAL SCIENCES, TIANJIN UNIVERSITY OF SCIENCE AND TECHNOLOGY, TIANJIN ,