Abstract

   Histone methylation and acetylation play a crucial role in response to
   developmental cues and environmental changes. Previously, we employed
   mass spectrometry to identify histone modifications such as H3K27ac and
   H3K36me3 in the model diatom Phaeodactylum tricornutum, which have been
   shown to be important for transcriptional activation in animal and
   plant species. To further investigate their evolutionary implications,
   we utilized chromatin immunoprecipitation followed by deep sequencing
   (ChIP-Seq) and explored their genome-wide distribution in P.
   tricornutum. Our study aimed to determine their role in transcriptional
   regulation of genes and transposable elements (TEs) and their
   co-occurrence with other histone marks. Our results revealed that
   H3K27ac and H3K36me3 were predominantly localized in promoters and
   genic regions indicating a high conservation pattern with studies of
   the same marks in plants and animals. Furthermore, we report the
   diversity of genes encoding H3 lysine 36 (H3K36)
   trimethylation–specific methyltransferase in microalgae leveraging
   diverse sequencing resources including the Marine Microbial Eukaryote
   Transcriptome Sequencing Project database (MMETSP). Our study expands
   the repertoire of epigenetic marks in a model microalga and provides
   valuable insights into the evolutionary context of epigenetic-mediated
   gene regulation. These findings shed light on the intricate interplay
   between histone modifications and gene expression in microalgae,
   contributing to our understanding of the broader epigenetic landscape
   in eukaryotic organisms.

   Keywords: epigenetics, microalgae, evolution, post translational
   modifications of histones

1. Introduction

   Diatoms, a group of unicellular photosynthetic organisms belonging to
   the stramenopile lineage, are characterized by their remarkable
   diversity, over 100,000 extant species known to date [[26]1]. The
   evolutionary history of diatoms is closely linked to the biogeochemical
   cycles of the Earth’s oceans and atmosphere. As primary producers,
   diatoms play a pivotal role in the global carbon cycle, sequestering
   carbon dioxide and contributing significantly to marine primary
   productivity, thereby supporting complex aquatic food webs [[27]2].
   Their distinctive silica-frustule structure confers remarkable
   mechanical strength and resilience, enabling them to withstand
   predation and environmental stresses, and facilitating their adaptation
   to diverse aquatic environments [[28]3]. With the advent of microscopy
   in the 19th century, diatoms were divided into mainly two groups based
   on the symmetry: centric diatoms, which are usually circular or
   triangular in shape and have radial symmetry, and pennate diatoms,
   which are elongated and have bilateral symmetry [[29]4]. These two
   groups also differ in their ecological preferences and patterns of