Author: Publisher:
ISBN: 9780438629073
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Languages : en
Pages : 171
Book Description
Kelp forest ecosystems are distributed on the rocky reefs of coastal regions worldwide. Kelps (order Laminariales) are a diverse group of brown macroalgae containing numerous species including giant kelp, Macrocystis pyrifera. The giant kelp dominates the kelp forests of southern California, engineering complex three-dimensional habitat which provides foundational shelter, nursery, and nutrients to marine organisms including invertebrates, fish, and marine mammals. While the kelp forests of southern California are well-regarded for their ecological and economic importance, traditional ecological surveys have ignored the most abundant fraction within the ecosystem - microbes. Microbes, including bacteria, archaea, and micro-eukaryotes, are present on all submerged surfaces in marine ecosystems, including microbiomes on macroorganisms. Microbes serve key ecological roles, including cycling of nutrients such as carbon and nitrogen to higher trophic levels within the ecosystem. Host-associated microbes have a symbiotic relationship with the host, providing environmentally-limited nutrients and protection from pathogen invasion in exchange for settlement substrate and host-derived metabolic exudates. Despite the known importance of microbes in marine ecosystems, microbes are absent from classic ecological models describing the top-down and bottom-up regulating forces structuring kelp populations. Thus, for my dissertation I aimed to address the lack of knowledge on the microbial ecology of southern California kelp forests, including the interactions between the predominant macroalgae and the associated microbiomes. In Chapter 1, I established a baseline description of the taxonomic structure and functional potential of microbial communities residing within the Macrocystis pyrifera-dominated kelp forest of Point Loma, CA, and identified sources of variation in microbiome profiles. The Point Loma kelp forest is subject to fluctuations in environmental conditions resulting from seasonality and stratification, which has an influence on kelp forest productivity. However, the potential direct and indirect effects of altered kelp forest environmental conditions on the microbial community structure and function has yet to be described, and as such I aimed to address this for Chapter 1. I sampled microbiomes from both the M. pyrifera biofilm and the adjacent water column seasonally over a three year investigation (2013-2016). I described the microbiomes in great depth using culture-independent whole genome shotgun metagenomics, and assessed the spatial and temporal variability in microbiome composition, function, and diversity. The microbiomes of the kelp and water were distinct in both taxonomic composition and functional potential. Kelp microbiomes remained stable across vertical depth, did not change significantly across season, and were not influenced by biophysical measurements of the surrounding environmental conditions. In contrast, the water-associated microbiomes varied significantly across depth with distinct community profiles above and below the thermocline stratification, showed significant changes across season, and seasonal changes within microbiome structure were strongly correlated with biophysical measurements of kelp forest productivity. Overall, my results showed that while free-living microbiomes were structured by the surrounding environmental conditions, host factors outweighed environmental factors in structuring hostassociated microbiomes. In Chapter 2, I examined the potential shifts in the baseline kelp forest microbiomes resulting from disturbance caused by the spread of an invasive alga, and the potential microbial influence in the alga's invasion strategy. This investigation took place at Santa Catalina Island within the Channel Islands 40 km offshore southern California, throughout the progression of the species invasion (2014-2018). Catalina Island has historically boasted dense populations of the native alga Macrocystis pyrifera, but has undergone disturbance due to the invasion and spread of invasive alga, Sargassum horneri. Invasive species cause significant alterations to ecosystems with both physical and chemical influences which deter and inhibit recovery of native species; however, whether there is a microbial influence in the invasion strategy of alga S. horneri is unknown. First, I described the microbiome of the invasive S. horneri and compared it to the native M. pyrifera, and found the microbiomes of the two macroalgal species to be distinct; specifically, the S. horneri microbiome was enriched in potentially pathogenic Vibrios. Next, I identified the potential for S. horneri presence to induce changes in the surrounding microbiomes. In 2018 as the native alga M. pyrifera was attempting to recover from disturbance, M. pyrifera and S. horneri existed in an interface at some sites. Where the two were in direct contact, I observed evidence of tissue bleaching and deterioration of the native alga, and investigated whether this detriment was a direct result of microbial pathogens. I did not find evidence of S. horneri microbiome inoculation onto the adjacent M. pyrifera; rather, the bleached M. pyrifera showed dysbiosis where the microbiome was lost completely compared to healthy M. pyrifera individuals nearby. However, I did find evidence of S. horneri microbiome inoculation onto the benthic substrate directly below the invasive alga, which resulted in an enrichment of Vibrios compared to the microbiome from benthic substrate beneath native M. pyrifera, and the enrichment of Vibrios on the benthic substrate occurred concurrently with a lack of native algal recovery at those locations. My findings suggest that the invasive species S. horneri has altered the baseline microbiome structure in the kelp forest, and the induced microbial changes may have an ongoing influence on the native species as they attempt to recover from disturbance. In Chapter 3, I investigated the microbial influence on a key stage of recruitment of the giant kelp, Macrocystis pyrifera. Macroalgae, including kelps, rely on recruitment processes to maintain adult populations and recover from disturbances. During kelp recruitment processes, microscopic propagules are released into the kelp forest water column where they are suspended until settlement onto the benthic substrate. Microscopic propagules are highly susceptible to abiotic conditions including UV irradiation, temperature, and nutrients, and biotic conditions including grazing. However, the influence of microbes on kelp propagule success has not been widely studied. Given that microbes are abundant in marine ecosystems (106 cells per ml of seawater) and are present on every submerged surface, microbes are likely to interact with kelp propagules during recruitment processes. For this investigation I reared M. pyrifera microscopic propagules in laboratory microcosms and exposed them to environmentally-sourced microbial communities within treatments. First, I investigated whether the presence of microbes influenced M. pyrifera propagules, and found that removing microbes in seawater increased propagule recruitment success. I then assessed the propagule success when exposed to a nearshore (Point Loma, CA) microbial community compared to an offshore (Santa Catalina Island, CA) microbial community, at multiple levels of microbial abundance. At the time the experiment was conducted, Catalina Island fostered a pristine kelp forest with lower anthropogenic influence compared to the nearshore Point Loma kelp forest. The nearshore (Point Loma) microbial community treatments yielded similar results to the first experiment, where removing microbes resulted in higher kelp propagule success. In contrast, kelp propagules exposed to offshore (Catalina) microbes had the greatest success when microbes were present at intermediate abundances, rather than removed completely. In both treatments, a microbialization of the seawater resulted in observed morphological detriment to kelp propagules. This study suggests that kelp forest microbes have an influence on a key stage of kelp recruitment, and the composition of the microbial community is important in recruitment success. Collectively, my dissertation shows that microbes influence kelp forest ecosystem dynamics and must be incorporated into future population and community models for a more holistic description of the ecosystem.
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