Plant microbiomes are shaped by forces working at different spatial scales. Environmental factors determine a pool of potential symbionts while host physiochemical factors influence how those microbes associate with distinct plant tissues. These scales are seldom considered simultaneously, despite their potential to interact. Here, we analyze epiphytic microbes from nine Hibiscus tiliaceus trees across a steep, but short, environmental gradient within a single Hawaiian watershed. At each location, we sampled eight microhabitats: leaves, petioles, axils, stems, roots, and litter from the plant, as well as surrounding air and soil. The composition of bacterial communities is better explained by microhabitat, while location better predicted compositional variance for fungi. Fungal community compositional dissimilarity increased more rapidly along the gradient than did bacterial composition. Additionally, the rates of fungal community compositional dissimilarity along the gradient differed among plant parts, and these differences influenced the distribution patterns and range size of individual taxa. Within plants, microbes were compositionally nested such that aboveground communities contained a subset of the diversity found belowground. Our findings indicate that both environmental context and microhabitat contribute to microbial compositional variance in our study, but that these contributions are influenced by the domain of microbe and the specific microhabitat in question, suggesting a complicated and potentially interacting dynamic.

Molecular phylogenetic and morphological analyses of Hawaiian members of the red algal family Dumontiaceae were used to clarify the species diversity of Dudresnaya and Gibsmithia from Hawaiʻi. Although no new species of Dudresnaya were detected in the analyses, D. babbittiana is newly recorded from Lalo, Manawai, and Oʻahu. A new species of Gibsmithia, G. punonomaewa A.R.Sherwood, is described here and reported from mesophotic depths (79-104 m) of the Papahānaumokuākea Marine National Monument, Hawaiʻi. This new species differs from others in the genus based on the following combination of characters: moderate thallus size (up to 11 cm), smooth and terete gelatinous lobes, presence of a stipe (which is often branched), globose carposporangia, and a non-isodiametric shaped cell subtending the tetrasporangia. This new taxon increases the number of Gibsmithia species recorded from Hawaiʻi to three. Phylogenetically, G. pun onomaewa is most closely related to G. dotyi and G. larkumii, which are both widespread in distribution. The relatively dark habitat of the mesophotic in Papahānaumokuākea Marine National Monument contrasts with the surf ace waters of tropical and subtropical habitats where most Gibsmithia species are found, further highlighting the uniqueness of the species.

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Small red algal morphologically variable blades have been extensively collected from Hawaiian reefs, but for many specimens their taxonomy remains poorly understood. In surveys of the Papahānaumokuākea Marine National Monument (PMNM) and Main Hawaiian Islands (MHI), we discovered two taxa of undescribed small (< 5 cm) red blades that matched the genera Psaromenia and Meredithia, based on morphology and molecular analyses. Neither genus has been previously recorded in the Hawaiian Islands, and neither group of specimens matched currently described species in these two genera. Accordingly, these specimens are described here as new species within the family Kallymeniaceae. Psaromenia laulamaula sp. nov., exclusively found at mesophotic depths (83-94 m) in PMNM, is easily distinguished from other members of the genus by its comparatively large, procarpic carpogonial branch system and solitary obovate pink-tomagenta blades. Conversely, Meredithia hawaiiensis sp. nov., occurring in both shallow (0-17 m) and mesophotic depths (55 m), has high morphological plasticity, with characters that overlap with other Meredithia species, and can only be distinguished based on DNA sequences. This study provides additional evidence of the extent of diversity in the Kallymeniaceae that is poorly characterized from mesophotic depths and provides further evidence that members of the macroalgal flora contain overlooked biodiversity.

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