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1. Interactive effects of temperature and nitrogen on the physiology of kelps (Nereocystis luetkeana and Saccharina latissima)

   https://doi.org/10.3389/fmars.2023.1281104  

   Fales, Robin J. ; Weigel, Brooke L. ; Carrington, Emily ; Berry, Helen D. ; Dethier, Megan N. ( November 2023 , Frontiers in Marine Science)

   Thanos Dailianis (Ed.)

   Kelp forest declines have been linked to warming ocean temperatures worldwide. Ocean warming rarely occurs in isolation, so multiple stressor studies are necessary to understand the physiological responses of kelp to climate change. The canopy-forming bull kelp, Nereocystis luetkeana, is going locally extinct in areas of the Salish Sea that are seasonally warm and nutrient poor, while the understory kelp, Saccharina latissima, persists at those sites. Further, nitrogen availability can alter physiological responses of kelps to temperature stress, including alleviating warming stress. We compared the physiological responses of kelp sporophytes to high temperature stress and nitrogen limitation between two populations of N. luetkeana with different environmental histories (warm and nutrient poor vs. cold and nutrient rich) and between two species, N. luetkeana and S. latissima. Using laboratory mesocosms, we tested the interactive effects of short term (8-9 day) exposure of kelp blades to different temperatures: low (9, 13°C), moderate (15, 16°C), and warm (21°C) at two different nitrogen concentrations: low (1-3 μM) vs. high (>10 μM). We examined a wide array of physiological responses: blade growth, photosynthesis, respiration, photosynthetic yield, nutrient uptake, and tissue C:N. Both kelp species responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of both species showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that N. luetkeana and S. latissima are susceptible to thermal stress over short time periods. Populations of N. luetkeana from warm, nutrient poor and cool, nutrient rich areas were equally susceptible to the effects of ocean warming. Our results suggest that nutrient additions may actually reduce kelp performance at supra-optimal temperatures, and a thorough understanding of kelp responses to coastal temperature and nutrient dynamics is needed to guide conservation and restoration actions. 

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2. Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

   https://doi.org/10.1111/jpy.13291  

   Bell, Lauren E. ; Kroeker, Kristy J. ( November 2022 , Journal of Phycology)

   Production rates reported for canopy‐forming kelps have highlighted the potential contributions of these foundational macroalgal species to carbon cycling and sequestration on a globally relevant scale. Yet, the production dynamics of many kelp species remain poorly resolved. For example, productivity estimates for the widely distributed giant kelpMacrocystis pyriferaare based on a few studies from the center of this species' range. To address this geospatial bias, we surveyed giant kelp beds in their high latitude fringe habitat in southeast Alaska to quantify foliar standing crop, growth and loss rates, and productivity ofM. pyriferaand co‐occurring understory kelpsHedophyllum nigripesandNeoagarum fimbriatum. We found that giant kelp beds at the poleward edge of their range produce ~150 g C · m⁻²· year⁻¹from a standing biomass that turns over an estimated 2.1 times per year, substantially lower rates than have been observed at lower latitudes. Although the productivity of high latitudeM. pyriferadwarfs production by associated understory kelps in both winter and summer seasons, phenological differences in growth and relative carbon and nitrogen content among the three kelp species suggests their complementary value as nutritional resources to consumers. This work represents the highest latitude consideration ofM. pyriferaforest production to date, providing a valuable quantification of kelp carbon cycling in this highly seasonal environment.

    

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3. Spatial organization of the kelp microbiome at micron scales

   https://doi.org/10.1186/s40168-022-01235-w  

   Ramírez-Puebla, S. Tabita ; Weigel, Brooke L. ; Jack, Loretha ; Schlundt, Cathleen ; Pfister, Catherine A. ; Mark Welch, Jessica L. ( March 2022 , Microbiome)

   Elucidating the spatial structure of host-associated microbial communities is essential for understanding taxon-taxon interactions within the microbiota and between microbiota and host. Macroalgae are colonized by complex microbial communities, suggesting intimate symbioses that likely play key roles in both macroalgal and bacterial biology, yet little is known about the spatial organization of microbes associated with macroalgae. Canopy-forming kelp are ecologically significant, fixing teragrams of carbon per year in coastal kelp forest ecosystems. We characterized the micron-scale spatial organization of bacterial communities on blades of the kelpNereocystis luetkeanausing fluorescence in situ hybridization and spectral imaging with a probe set combining phylum-, class-, and genus-level probes to localize and identify > 90% of the microbial community.

   We show that kelp blades host a dense microbial biofilm composed of disparate microbial taxa in close contact with one another. The biofilm is spatially differentiated, with clustered cells of the dominant symbiontGranulosicoccussp. (Gammaproteobacteria) close to the kelp surface and filamentousBacteroidetesandAlphaproteobacteriarelatively more abundant near the biofilm-seawater interface. A community rich inBacteroidetescolonized the interior of kelp tissues. Microbial cell density increased markedly along the length of the kelp blade, from sparse microbial colonization of newly produced tissues at the meristematic base of the blade to an abundant microbial biofilm on older tissues at the blade tip. Kelp from a declining population hosted fewer microbial cells compared to kelp from a stable population.

   Imaging revealed close association, at micrometer scales, of different microbial taxa with one another and with the host. This spatial organization creates the conditions necessary for metabolic exchange among microbes and between host and microbiota, such as provisioning of organic carbon to the microbiota and impacts of microbial nitrogen metabolisms on host kelp. The biofilm coating the surface of the kelp blade is well-positioned to mediate interactions between the host and surrounding organisms and to modulate the chemistry of the surrounding water column. The high density of microbial cells on kelp blades (10⁵–10⁷cells/cm²), combined with the immense surface area of kelp forests, indicates that biogeochemical functions of the kelp microbiome may play an important role in coastal ecosystems.

    

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4. Carbon content, carbon fixation yield and dissolved organic carbon release from diverse marine nitrifiers

   https://doi.org/10.1002/lno.12252  

   Bayer, Barbara ; McBeain, Kelsey ; Carlson, Craig A. ; Santoro, Alyson E. ( October 2022 , Limnology and Oceanography)

   Nitrifying microorganisms, including ammonia‐oxidizing archaea, ammonia‐oxidizing bacteria, and nitrite‐oxidizing bacteria, are the most abundant chemoautotrophs in the ocean and play an important role in the global carbon cycle by fixing dissolved inorganic carbon (DIC) into biomass. The release of organic compounds by these microbes is not well quantified, but may represent an as‐yet unaccounted source of dissolved organic carbon (DOC) available to marine food webs. Here, we provide measurements of cellular carbon and nitrogen quotas, DIC fixation yields and DOC release of 10 phylogenetically diverse marine nitrifiers. All investigated strains released DOC during growth, representing on average 5–15% of the fixed DIC. Changes in substrate concentration and temperature did not affect the proportion of fixed DIC released as DOC, but release rates varied between closely related species. Our results also indicate previous studies may have underestimated DIC fixation yields of marine nitrite oxidizers due to partial decoupling of nitrite oxidation from CO₂fixation, and due to lower observed yields in artificial compared to natural seawater medium. The results of this study provide critical values for biogeochemical models of the global carbon cycle, and help to further constrain the implications of nitrification‐fueled chemoautotrophy for marine food‐web functioning and the biological sequestration of carbon in the ocean.

    

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5. Factors influencing urea use by giant kelp ( Macrocystis pyrifera , Phaeophyceae)

   https://doi.org/10.1002/lno.11674  

   Smith, Jason M. ; Blasco, Gordon ; Brzezinski, Mark A. ; Melack, John M. ; Reed, Daniel C. ; Miller, Robert J. ( December 2020 , Limnology and Oceanography)

   Urea is an available and readily used source of nitrogen for giant kelp,Macrocystis pyrifera, but little is known about its potential importance for sustaining growth. Results of kinetic experiments indicate urea uptake saturates at an average maximum rate (V_max) of 0.73–0.92 μmol N g dw⁻¹h⁻¹with a half saturation constant (K_s) of 1.02–1.08 μM. The affinity of giant kelp for urea was high relative to that reported for other seaweeds. However, results of similar kinetics experiments with natural, co‐occurring phytoplankton communities indicate that the rate of urea uptake by phytoplankton was > 10‐fold higher than that of giant kelp. Urea uptake by giant kelp decreased 3–12% in darkness (relative to in light) compared to a 66–85% decline for phytoplankton. Similar differences were observed for ammonium and nitrate, suggesting that light intensity and photocycles influence the outcome of competition for N between giant kelp and phytoplankton. Monthly measures of urease in kelp tissues revealed persistent activity at levels that were 100‐fold higher than rates of urea uptake (0.13–0.35 μmol N g fw⁻¹min⁻¹). This finding, coupled with unsuccessful efforts to induce additional urease activity through substrate additions, suggests that urease plays a role in giant kelp physiology beyond that of processing urea taken up from the environment. Collectively, our results suggest giant kelp uses multiple forms of N including urea to sustain year‐round growth. Its consistent capacity to acquire N during both day and night may help offset its low uptake rates relative to phytoplankton and increase its ability to compete for N during periods of low N availability.

    

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