Biology subjects

Steward, G. F.

Publications and source records attributed to Steward, G. F..

4 recordsLinked to original sources

bioRxiv2022

Trophic strategies explain the ocean niches of small eukaryotic phytoplankton

A large fraction of marine primary production is performed by diverse small protists, and many of these phytoplankton are mixotrophs that also consume bacterial prey. However, the mechanisms structuring this diversity and its biogeochemical consequences remain poorly understood. Here we use isolates from seven major taxa to demonstrate tradeoffs between phototrophic and phagotrophic abilities. We then show that trophic strategy along the autotrophy-mixotrophy spectrum correlates strongly with global niche differences, across depths and across gradients of stratification and chlorophyll a. A model of competition shows that community shifts can be explained by greater fitness of faster-grazing mixotrophs when nutrients are scarce and light is plentiful. Our results illustrate how basic physiological constraints and principles of resource competition can organize complexity in the surface ocean ecosystem.

ecology↗

bioRxiv2021

Variable freshwater influences on the abundance of Vibrio vulnificus in a tropical urban estuary

To better understand the controls on the opportunistic human pathogen Vibrio vulnificus in warm tropical waters, we conducted a year-long investigation in the Ala Wai Canal, a channelized estuary in Honolulu, HI. The abundance of V. vulnificus as determined by qPCR of the hemolysin gene (vvhA), varied spatially and temporally nearly four orders of magnitude ([≤] 3 to 14,000 mL-1). Unlike in temperate and subtropical systems, temperatures were persistently warm (19-31{degrees}C) and explained little of the variability in V. vulnificus abundance. Salinity (1-36 ppt) had a significant, but non-linear, relationship with V. vulnificus abundance with highest vvhA concentrations (> 2,500 mL-1) observed only at salinities from 7 to 22 ppt. V. vulnificus abundances were lower on average in the summer dry season when waters were warmer but more saline. Highest canal-wide average abundances were observed during a time of modest rainfall when moderate salinities and elevated concentrations of reduced nitrogen species and silica suggested a groundwater influence. Distinguishing the abundances of two genotypes of V. vulnificus (C-type and E-type) suggest that C-type strains, which are responsible for most human infections, were usually less abundant (25% on average), but their relative contribution was greater at higher salinities, suggesting a broader salinity tolerance. Generalized regression models suggested up to 67% of sample-to-sample variation in log-transformed V. vulnificus abundance was explained (n = 202) using the measured environmental variables, and up to 97% of the monthly variation in canal-wide average concentrations (n = 13) was explained with the best subset of four variables. IMPORTANCEOur data illustrate that, in the absence of strong seasonal variation in water temperature in the tropics, variation in salinity driven by rainfall becomes a primary controlling variable on V. vulnificus abundance. There is thus a tendency for a rainfall-driven seasonal cycle in V. vulnificus abundance that is inverted from the temperature-driven seasonal cycle at higher latitudes. However, stochasticity in rainfall and its non-linear, indirect effects on V. vulnificus concentration means that high abundances can occur at any location in the canal at any time of year, making it challenging to predict concentrations of this pathogen at high temporal or spatial resolution. Much of the variability in canal-wide average concentrations, on the other hand, was explained by a few variables that reflect the magnitude of freshwater input to the system, suggesting that relative risk of exposure to this pathogen could be predicted for the system as a whole.

microbiology↗

bioRxiv2021

Broad phylogenetic and functional diversity among mixotrophic consumers of Prochlorococcus

Small eukaryotic phytoplankton are major contributors to global primary production and marine biogeochemical cycles. Many taxa are thought to be mixotrophic, but quantitative studies of phagotrophy exist for very few. In addition, little is known about consumers of Prochlorococcus, the abundant cyanobacterium at the base of oligotrophic ocean food webs. Here we describe thirty-nine new phytoplankton isolates from the North Pacific Subtropical Gyre (Station ALOHA), all flagellates [~]2-5 um diameter, and we quantify their ability to graze Prochlorococcus. The mixotrophs are from diverse classes (dictyochophytes, haptophytes, chrysophytes, bolidophytes, a dinoflagellate, and a chlorarachniophyte), many from previously uncultured clades. Grazing ability varied substantially, with specific clearance rate (volume cleared per body volume) varying over ten-fold across isolates and six-fold across genera. Slower grazers tend to create more biovolume per prey biovolume consumed. Using qPCR we found that the haptophyte Chrysochromulina was most abundant among the isolated mixotrophs at Station ALOHA, with 76-250 cells mL-1 across depths in the upper euphotic zone. Our results show that within a single ecosystem the phototrophs that ingest bacteria come from many branches of the eukaryotic tree, and are functionally diverse, indicating a broad range of strategies along the spectrum from phototrophy to phagotrophy.

ecology↗

bioRxiv2020

Making sense of virus size and the tradeoffs shaping viral fitness

Viruses span an impressive size range, with genome length varying more than a thousandfold and capsid volume nearly a millionfold. Physical constraints suggest that smaller viruses may have multiple fitness advantages, because a greater number of viral offspring can be created from limited host resources, and because smaller particles diffuse to encounter new hosts more rapidly. At the same time, a larger genome size allows for numerous additional functions that may increase fitness, such as better control of replication, transcription, translation, and host metabolism, and neutralization of host defenses. However, it is unclear whether important viral traits correlate with size, and whether this causes size to vary among host types or environmental contexts. Here we focus on viruses of aquatic unicellular organisms, which exhibit the greatest known range of virus size. We develop and synthesize theory, and analyze data where available, to consider how size affects the primary components of viral fitness. We suggest that the costs of larger size (lower burst size and diffusivity) are mitigated by the role of a larger genome in increasing infection efficiency, broadening host range, and potentially increasing attachment success and decreasing decay rate. These countervailing selective pressures may explain why such a breadth of sizes exist and can even coexist when infecting the same host populations. We argue that oligotrophic environments may be particularly enriched in unusually large or "giant" viruses, because environments with diverse, resource-limited phagotrophic eukaryotes at persistently low concentrations may select for broader host range, better control of host metabolism, lower decay rate, and a physical size that mimics bacterial prey. Finally, we describe areas where further research is needed to understand the ecology and evolution of viral size diversity.

ecology↗