4.3: Brown Algae
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Secondary Endosymbiosis in Heterokonts
Brown algae are a photosynthetic lineage of heterokonts. They derived their golden brown chloroplasts from secondary endosymbiosis. In this event, an ancestral oomycete engulfed a red alga. As in primary endosymbiosis, instead of being digested, overtime the red alga degenerated into a chloroplast, this time with 4 membranes -- the engulfing membrane from the oomycete, the red alga’s plasma membrane, and the two membranes of the original chloroplast within the red alga. In many groups derived from secondary endosymbiosis, the chloroplast has lost one of these membranes.
Brown Algae, Phylum Phaeophyta
Brown algae are brown due to the large amounts of carotenoids they produce, primarily one called fucoxanthin. These organisms are exclusively multicellular and can get so large that they require special conductive cells to transport photosynthates from their blades down to the rest of their tissues. These conductive cells are called trumpet hyphae and have sieve plates and resemble sieve tubes found in flowering plants.
Much like Saprolegnia, the body of an alga is termed a thallus because it is not differentiated into specialized tissues. The general morphology of a brown alga includes a holdfast, stipe, gas bladder(s), and blade(s).
Kelp
Figure \(\PageIndex{4}\): A piece of feather boa kelp with several small gas bladders. The four gas bladders shown all attach to the edge of the stipe. If the full thallus were visible, gas bladders would be attached down the entire length of both sides of the flattened stipe. Do you see any blades present? Photo by Maria Morrow, CC-BY-NC.
Fucus
A model organism for the Phaeophyta life cycle is Fucus (rockweed), which, like its relative Saprolegnia, has a diplontic life cycle.
The Fucus thallus has dichotomous branching (forking into two equal branches) and swollen, heart-shaped reproductive tips of the branches. These swollen branch tips are called receptacles.
The receptacles are covered in small bumps, each with a pore at the center of the bump called an ostiole. The bumps are conceptacles, chambers that house the male and female gametangia.
Full Life Cycle Diagram
